Morphological diversity of the gastrointestinal tract in fishes

Comparative Analysis of Intestinal Morphology and Gut Microbiota of Spinibarbus sinensis Under Different Aquaculture Systems

Fish gut health is influenced by various factors, with the environment being a significant one. S. sinensis is a key aquaculture species in China, yet research on the impact of different aquaculture systems on its intestinal health remains limited. This study aims to explore the changes in intestinal morphology and gut microbiota of S. sinensis under two aquaculture systems. The juveniles of S. sinensis were divided into two groups and cultured in traditional ponds (CT) and an in-pond tank culture system (JY), with equal amounts of feed provided daily over a 72-day experimental period. The results showed no significant differences in growth performance metrics, including the specific growth rate, weight gain rate, hepatosomatic index, and viscerosomatic index between the two groups. In terms of intestinal morphology, the JY group villus width was significantly wider than the CT group, and the number of goblet cells in the CT group was significantly higher than that of the JY group (p < 0.05), which suggested that the fish in the JY group may have better intestinal nutrient absorption capacity, while the water quality in the CT group may be worse. The 16S rRNA gene sequencing analysis of the gut microbiota showed that the JY group had a significantly higher Shannon index compared to the CT group (p < 0.05), indicating greater species richness and evenness. Principal Coordinates Analysis (PCoA) revealed a distinct clustering of gut microbiota between the two groups. At the phylum level, the relative abundance of Fusobacteriota was significantly higher in the CT group, whereas Bacteroidota and Proteobacteria were significantly higher in the JY group (p < 0.05). Furthermore, KEGG pathway predictions indicated differences in the potential metabolic capabilities of the gut microbiota between the two groups (p < 0.05). Overall, this study is the first to conduct a comparative analysis of the growth performance, intestinal tissue morphology, and gut microbiota of S. sinensis under two different aquaculture systems, which has valuable implications for the further optimization of aquaculture practices.

Morphology of the Digestive Tube of the Amazonian Freshwater Stingray Potamotrygon wallacei (Elasmobranchii: Potamotrygonidae): A Stereological Approach

This work aimed to describe and quantify the tissue components of the digestive tube of the neotropical freshwater stingray, Potamotrygon wallacei. For this, conventional histology and stereological methods were used to estimate tissue volume. The volumes of the four fundamental layers and the tissue components in the stomach (cardiac and pyloric) and spiral intestine were also estimated. In the cardiac stomach, the mucosa layer occupies 44.7% of the total volume of the organ wall. The gastric glands are the main components, and these structures alone represent 49.7% of this layer. This large number of gastric glands suggests a high potential for processing food items with a high protein content. The stereological methods were sensitive enough to show a reduction in the volume of the gastric glands from the cardiac region toward the pyloric region. Gastric glands are absent in the pyloric region of the stomach. However, the muscularis becomes thicker towards the pyloric region. The increase in smooth muscle thickness is due to the thickening of the inner muscular layer. This suggests that the role of the pyloric stomach may be related to the mixing of the chyme and assisting its passage to the spiral intestine. In the spiral intestine, data on the volume of the mucosa layer (and epithelial lining) suggest that the spiral valve has a large absorptive area. In several respects, the morphology of the digestive tube of P. wallacei is similar to that of other batoids. However, its slight morphological variations may be related to the habitat specificity of this species.

Pseudogenization of NK3 homeobox 2 (Nkx3.2) in monotremes provides insight into unique gastric anatomy and physiology

The enzymatic breakdown and regulation of food passage through the vertebrate antral stomach and pyloric sphincter (antropyloric region) is a trait conserved over 450 million years. Development of the structures involved is underpinned by a highly conserved signalling pathway involving the hedgehog, bone morphogenetic protein and Wingless/Int-1 (Wnt) protein families. Monotremes are one of the few vertebrate lineages where acid-based digestion has been lost, and this is consistent with the lack of genes for hydrochloric acid secretion and gastric enzymes in the genomes of the platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus) . Furthermore, these species feature unique gastric phenotypes, both with truncated and aglandular antral stomachs and the platypus with no pylorus. Here, we explore the genetic underpinning of monotreme gastric phenotypes, investigating genes important in antropyloric development using the newest monotreme genomes (mOrnAna1.pri.v4 and mTacAcu1) together with RNA-seq data. We found that the pathway constituents are generally conserved, but surprisingly, NK3 homeobox 2 (Nkx3.2) was pseudogenized in both platypus and echidna. We speculate that the unique sequence evolution of Grem1 and Bmp4 sequences in the echidna lineage may correlate with their pyloric-like restriction and that the convergent loss of gastric acid and stomach size genotypes and phenotypes in teleost and monotreme lineages may be a result of eco-evolutionary dynamics. These findings reflect the effects of gene loss on phenotypic evolution and further elucidate the genetic control of monotreme stomach anatomy and physiology.

Sensitivity of different organs and tissues as biomarkers of oxidative stress in juvenile tambaqui (Colossoma macropomum) submitted to fasting

The present study was conducted to evaluate the effects of fasting on responses of oxidative biomarkers and antioxidant defenses using different organs and tissues of Colossoma macropomum. The fish were divided into two groups: fed (control) and fasting (7 days). After 7 days, the fish were sampled for assessment of oxidative stress biomarkers (MDA-lipid peroxidation and PCO-protein carbonyl) and antioxidant defenses (SOD-superoxide dismutase; CAT-catalase; GPX-glutathione peroxidase; and GST-glutathione-S -transferase) in the liver, intestine, gills, muscle, brain, and plasma. The results showed an increase in MDA, PCO, SOD, and GPX concentrations in the liver and intestine of fasting fish. In contrast, in the branchial tissue, there was a reduction in the activity of SOD and CAT enzymes in fasting fish. There was also a reduction in CAT activity in the muscle of fasting fish, while in the brain, there were no changes in oxidative stress biomarkers. Plasma showed a relatively low antioxidant response. In conclusion, our results confirm that a 7-day fasting period induced tissue-specific antioxidant responses, but the increase in antioxidant responses was only for the SOD and GPX enzymes of the liver and intestine. Additionally, the liver and intestine were the most responsive tissues, whereas the plasma was the least sensitive to oxidative stress.

Linking anatomical and histological traits of the digestive tract to resource consumption and assimilation of omnivorous tetra fishes

This study explores the interplay between digestive tract traits, food intake, and assimilation in omnivorous tetra fishes (Psalidodon bifasciatus, P. aff. gymnodontus, and Bryconamericus ikaa) from the Iguaçu River basin, an ecologically significant region known for high endemism. We hypothesize that variations in digestive tracts across species would be associated with differences in diet, isotopic composition in fish tissues, and overall diet assimilation. To test this, we employed stereoscopic and light microscopy to characterize the gross anatomy, histomorphology, and histochemistry of fish digestive tracts. Additionally, we used stomach content and stable isotope analyses to trace fish diets. While these tetra fishes shared histological structures, disparities were noted in anatomical digestive traits and diet preferences. The smallest species, B. ikaa, with a shorter intestine, had fewer pyloric caeca and primarily consumed animal-based diets. Conversely, P. bifasciatus and P. aff. gymnodontus, with longer intestines, displayed numerous pyloric caeca and consumed a balanced mix of animal and plant items. Despite anatomical and dietary differences, all three species predominantly assimilated animal-origin food. The tetra fishes had histological variations among digestive tract segments, with the esophagus having the thickest muscular layer, gradually thinning towards the posterior intestine. The final portion of the intestine exhibited a significant expansion in the lumen perimeter, while the esophagus had the smallest lumen area. Goblet cells were most concentrated in the posterior intestine for all species. The gross anatomy of these tetra fishes aligns with their omnivorous habit, while diet assimilation was dominated by animal-origin food. These findings provide crucial insights into the structural and tissue characteristics of their digestive systems, laying the groundwork for deeper exploration into the physiological aspects of their digestive tracts and enhancing our understanding of their feeding strategies.

The Nutritional Value of Biowaste Bovine Slaughterhouse Meals for Monogastric Species Feeding: The Guinea Pig as an Animal Model

Biowaste from slaughterhouses can be recovered to benefit food security and reduce contamination potential. More than 3 billion heads of livestock are consumed worldwide, which will increase by 17% by 2028, generating more biowaste, increasing infectious agents, and causing economic losses due to circular economy principles not being applied. This work evaluated the nutritional quality of four types of biowaste from bovine slaughter which were transformed into a meal for guinea pigs (rumen content (RCM), ears (EaM), blood (BM), and cheeks (CM)) according to their chemical composition, digestible components, energy contribution, and voluntary consumption. For the animal model, adult male guinea pigs were arranged in metabolic cages for feces collection without urinary contamination. Nine guinea pigs were used in each digestibility test. First, a direct digestibility test was conducted using a meal of barley as a reference diet (RD), the indigestibility coefficient of which allowed for the estimation of the digestibility of biowaste meals through indirect calculations; for this, diets composed of 80% of the RD and 20% of the corresponding biowaste meals were evaluated. The difference method was suitable for determining the digestibility of beef biowaste using the indigestibility coefficients of the reference diet to calculate the digestibility of ingredients which could not be offered as 100% of the meal but were incorporated as 20%. The digestible protein and metabolizable energy contents of RCM, EaM, BM, and CM were 10.2% and 2853 kcal/kg, 44.5% and 3325 kcal/kg, 70.7% and 2583 kcal/kg, and 80.8% and 3386 kcal/kg, respectively. The CM and BM feeds had the highest contributions of digestible protein due to their higher nitrogen content, and the CM and EaM feeds had the highest ME contents due to their higher fat contents. The biowaste meal consumption in descending order was CM > RCM > EaM > BM, which were consumed without problems. These results are indicative that these components can be part of guinea pigs' diets, and it is recommended to continue studies into guinea pig growth and fattening diets with different levels of these biowaste meals.

Convergent gene losses and pseudogenizations in multiple lineages of stomachless fishes

The regressive evolution of independent lineages often results in convergent phenotypes. Several teleost groups display secondary loss of the stomach, and four gastric genes, atp4a, atp4b, pgc, and pga2 have been co-deleted in agastric (stomachless) fish. Analyses of genotypic convergence among agastric fishes showed that four genes, slc26a9, kcne2, cldn18a, and vsig1, were co-deleted or pseudogenized in most agastric fishes of the four major groups. kcne2 and vsig1 were also deleted or pseudogenized in the agastric monotreme echidna and platypus, respectively. In the stomachs of sticklebacks, these genes are expressed in gastric gland cells or surface epithelial cells. An ohnolog of cldn18 was retained in some agastric teleosts but exhibited an increased non-synonymous substitution when compared with gastric species. These results revealed novel convergent gene losses at multiple loci among the four major groups of agastric fish, as well as a single gene loss in the echidna and platypus.

Comparative digestive biology between the ponyfishes from the Pranburi River estuary, Thailand

We investigated the digestive biology of two prevalent leiognathid species in Pranburi River estuary, Thailand: the decorated ponyfish (Nuchequula gerreoides) and the splendid polyfish (Eubleekeria splendens). A total of 632 samples collected from February to April and September to November 2017 were analysed using morphological and histological approaches. The overall structures were similar between the species: a short mucous-cell-rich oesophagus region, a well-developed gastric gland uniformly present across the stomach's mucosal layer, and three finger-like pyloric caeca between the stomach and intestine. However, there were marked differences in the mouth, gill raker, and intestinal coefficient (IC). N. gerreoides had a relatively longer mouth, smoother gill rakers, and an IC of 1.08 ± 0.01, similar to those of other carnivorous fish. In contrast, the gill raker of E. splendens had more villiform teeth that can filter-feed better, and their IC was 2.16 ± 0.02 (i.e., longer intestine). Although digestive structures were generally similar between the ponyfishes, these differences suggest that N. gerreoides is relatively carnivorous with stronger suction, whereas E. splendens may be an omnivorous or herbivorous filter-feeder.

Immunohistochemical characterisation of the adult Nothobranchius furzeri intestine

Nothobranchius furzeri is emerging as an exciting vertebrate organism in the field of biomedicine, developmental biology and ecotoxicology research. Its short generation time, compressed lifespan and accelerated ageing make it a versatile model for longitudinal studies with high traceability. Although in recent years the use of this model has increased enormously, there is still little information on the anatomy, morphology and histology of its main organs. In this paper, we present a description of the digestive system of N. furzeri, with emphasis on the intestine. We note that the general architecture of the intestinal tissue is shared with other vertebrates, and includes a folding mucosa, an outer muscle layer and a myenteric plexus. By immunohistochemical analysis, we reveal that the mucosa harbours the same type of epithelial cells observed in mammals, including enterocytes, goblet cells and enteroendocrine cells, and that the myenteric neurons express neurotransmitters common to other species, such as serotonin, substance P and tyrosine hydroxylase. In addition, we detect the presence of a proliferative compartment at the base of the intestinal folds. The description of the normal intestinal morphology provided here constitutes a baseline information to contrast with tissue alterations in future lines of research assessing pathologies, ageing-related diseases or damage caused by toxic agents.

Anatomy of the endocrine pancreas in actinopterygian fishes and its phylogenetic implications

The anatomy and organisation of the endocrine pancreas in ray-finned fishes vary widely. The two main morphoanatomical character states are diffuse versus compact pancreatic tissue. The latter are called Brockmann Bodies (BBs), or principal islets. The present study is the first comprehensive survey on the anatomy of the endocrine pancreas (diffuse versus compact) across 322 actinopterygian species in 39 orders and 135 families based on literature, specimen dissections, and Magnetic Resonance Imaging (MRI). The data obtained show that large endocrine pancreatic islets (BB) have appeared several times in teleost evolution: in some ostariophysian clades and within the Salmoniformes and Neoteleostei. Acanthomorpha (spiny-rayed fishes) is the largest clade of the Neoteleostei. Within this clade, an absence of BBs is only observed in flying fishes (Exocoetidae), parrotfishes (Scarinae), and some of the scarine relatives, the Labridae. The presence of BBs in examined jellynose fish species from the Ateleopodiformes indicates support for its sister-group relationship to the Ctenosquamata (Myctophiformes + Acanthomorpha). More investigations are still needed to corroborate the presence or absence of BBs as a putative synapomorphy for a clade comprising Ateleopodiformes and Ctenosquamata.

Analysis of cell-specific transcriptional responses in human colon tissue using CIBERSORTx

Diet is an important determinant of overall health, and has been linked to the risk of various cancers. To understand the mechanisms involved, transcriptomic responses from human intervention studies are very informative. However, gene expression analysis of human biopsy material only represents the average profile of a mixture of cell types that can mask more subtle, but relevant cell-specific changes. Here, we use the CIBERSORTx algorithm to generate single-cell gene expression from human multicellular colon tissue. We applied the CIBERSORTx to microarray data from the PHYTOME study, which investigated the effects of different types of meat on transcriptional and biomarker changes relevant to colorectal cancer (CRC) risk. First, we used single-cell mRNA sequencing data from healthy colon tissue to generate a novel signature matrix in CIBERSORTx, then we determined the proportions and gene expression of each separate cell type. After comparison, cell proportion analysis showed a continuous upward trend in the abundance of goblet cells and stem cells, and a continuous downward trend in transit amplifying cells after the addition of phytochemicals in red meat products. The dietary intervention influenced the expression of genes involved in the growth and division of stem cells, the metabolism and detoxification of enterocytes, the translation and glycosylation of goblet cells, and the inflammatory response of innate lymphoid cells. These results show that our approach offers novel insights into the heterogeneous gene expression responses of different cell types in colon tissue during a dietary intervention.

Identification of hub genes in digestive system of mandarin fish (Siniperca chuatsi) fed with artificial diet by weighted gene co-expression network analysis

Mandarin fish (Siniperca chuatsi) is a carnivorous freshwater fish and an economically important species. The digestive system (liver, stomach, intestine, pyloric caecum, esophagus, and gallbladder) is an important site for studying fish domestication. In our previous study, we found that mandarin fish undergoes adaptive changes in histological morphology and gene expression levels of the digestive system when subjected to artificial diet domestication. However, we are not clear which hub genes are highly associated with domestication. In this study, we performed WGCNA on the transcriptomes of 17 tissues and 9 developmental stages and combined differentially expressed genes analysis in the digestive system to identify the hub genes that may play important functions in the adaptation of mandarin fish to bait conversion. A total of 31,657 genes in 26 samples were classified into 23 color modules via WGCNA. The modules midnightblue, darkred, lightyellow, and darkgreen highly associated with the liver, stomach, esophagus, and gallbladder were extracted, respectively. Tan module was highly related to both intestine and pyloric caecum. The hub genes in liver were cp, vtgc, c1in, c9, lect2, and klkb1. The hub genes in stomach were ghrl, atp4a, gjb3, muc5ac, duox2, and chia2. The hub genes in esophagus were mybpc1, myl2, and tpm3. The hub genes in gallbladder were dyst, npy2r, slc13a1, and slc39a4. The hub genes in the intestine and pyloric caecum were slc15a1, cdhr5, btn3a1, anpep, slc34a2, cdhr2, and ace2. Through pathway analysis, modules highly related to the digestive system were mainly enriched in digestion and absorption, metabolism, and immune-related pathways. After domestication, the hub genes vtgc and lect2 were significantly upregulated in the liver. Chia2 was significantly downregulated in the stomach. Slc15a1, anpep, and slc34a2 were significantly upregulated in the intestine. This study identified the hub genes that may play an important role in the adaptation of the digestive system to artificial diet, which provided novel evidence and ideas for further research on the domestication of mandarin fish from molecular level.

Tissue-specific transcriptomes reveal potential mechanisms of microbiome heterogeneity in an ancient fish

The lake sturgeon (Acipenser fulvescens) is an ancient, octoploid fish faced with conservation challenges across its range in North America, but a lack of genomic resources has hindered molecular research in the species. To support such research, we created a transcriptomic database from 13 tissues: brain, esophagus, gill, head kidney, heart, white muscle, liver, glandular stomach, muscular stomach, anterior intestine, pyloric cecum, spiral valve and rectum. The transcriptomes for each tissue were sequenced and assembled individually from a mean of 98.3 million (±38.9 million SD) reads each. In addition, an overall transcriptome was assembled and annotated with all data used for each tissue-specific transcriptome. All assembled transcriptomes and their annotations were made publicly available as a scientific resource. The non-gut transcriptomes provide important resources for many research avenues. However, we focused our analysis on messenger ribonucleic acid (mRNA) observations in the gut because the gut represents a compartmentalized organ system with compartmentalized functions, and seven of the sequenced tissues were from each of these portions. These gut-specific analyses were used to probe evidence of microbiome regulation by studying heterogeneity in microbial genes and genera identified from mRNA annotations. Gene set enrichment analyses were used to reveal the presence of photoperiod and circadian-related transcripts in the pyloric cecum, which may support periodicity in lake sturgeon digestion. Similar analyses were used to identify different types of innate immune regulation across the gut, while analyses of unique transcripts annotated to microbes revealed heterogeneous genera and genes among different gut tissues. The present results provide a scientific resource and information about the mechanisms of compartmentalized function across gut tissues in a phylogenetically ancient vertebrate. Database URL: https://figshare.com/projects/Lake_Sturgeon_Transcriptomes/133143.

The development of the digestive system and the fate of the yolk syncytial layer in postembryogenesis of Stenodus leucichthys nelma (Teleostei)

Stenodus leucichthys nelma is an economically important species for cold-water aquaculture. Unlike other Coregoninae, S. leucichthys nelma is a piscivore. Here, we describe in detail the development of the digestive system and the yolk syncytial layer from hatching to early juvenile stage using histological and histochemical methods to determine their common and specific characteristics and to test the hypothesis that the digestive system of S. leucichthys nelma rapidly acquires adult features. The digestive tract differentiates at hatching and starts to function before the transition to mixed feeding. The mouth and anus are open, mucous cells and taste buds are present in the buccopharyngeal cavity and esophagus, pharyngeal teeth have erupted, the stomach primordium is seen, the intestinal epithelium with mucous cells is folded and the intestinal valve is observed; the epithelial cells of the postvalvular intestine contain supranuclear vacuoles. The liver blood vessels are filled with blood. The cells of exocrine pancreas are loaded with zymogen granules, and at least two islets of Langerhans are present. However, the larvae remain dependent on maternal yolk and lipids for a long time. The adult features of the digestive system develop gradually, the most significant changes take place approximately from 31 to 42 days posthatching. Then, the gastric glands and pyloric caeca buds appear, the U-shaped stomach with glandular and aglandular regions develops, the swim bladder inflates, the number of islets of Langerhans increases, the pancreas becomes scattered, and the yolk syncytial layer undergoes programmed death during the larval-to-juvenile transition. During postembryonic development, the mucous cells of the digestive system contain neutral mucosubstances.

How Do Alternative Protein Resources Affect the Intestine Morphology and Microbiota of Atlantic Salmon?

The availability and cost of fishmeal constitute a bottleneck in Atlantic salmon production expansion. Fishmeal is produced from wild fish species and constitutes the major feed ingredient in carnivorous species such as the Atlantic salmon. These natural stocks are at risk of depletion and it is therefore of major importance to find alternative protein sources that meet the nutritional requirements of the Atlantic salmon, without compromising the animals' health. Terrestrial animal by-products have been used in aquaculture feed, but their use is limited by the lack of several essential amino acids and consumer acceptance. In the case of plant ingredients, it is necessary to take into account both their concentration and the extraction methodologies, since, if not dosed correctly, they can cause macro- and microscopic alterations of the structure of the gastrointestinal tract and can also negatively modulate the microbiota composition. These alterations may compromise the digestive functions, growth of the animal, and, ultimately, its well-being. An updated revision of alternative protein sources is provided, with the respective impact on the intestine health in terms of both morphology and microbiota composition. Such information may constitute the premise for the choice and development of Atlantic salmon feeds that guarantee fish health and growth performance without having a significant impact on the surrounding environment, both in terms of depletion of the fish's natural stocks and in terms of pressure on the terrestrial agriculture. The sustainability of aquaculture should be a priority when choosing next-generation ingredients.

Smooth muscle "microsphincters" in the gastric mucosa of stingrays (Elasmobranchii, Dasyatidae)

The occurrence of smooth muscle sphincters around gastric gland tubules in the cardiac stomach of some elasmobranch species is reported for the first time. These "microsphincters" took the form of a twisted torus, approximately 12-16 muscle fibres thick, that could fully constrict the gland tubules. However, their inconsistent positioning does not suggest a role in partitioning the tubules from the stomach lumen or in modulating secretory activity. Further research is required to ascertain the full taxonomic occurrence of these structures across the Elasmobranchii and to elucidate their function.

A Morphological and Ultrastructural Study of the Anterior Digestive Tract of Adult Nile Tilapia Oreochromis niloticus

Among the most-used fish species in aquaculture is the Nile tilapia, due to its rapid growth rate and its adaptation to a wide range of farming conditions. A careful description of the morphology of the digestive tract, particularly the esophagus and stomach, allows a better understanding of the relationship between structure and function. Combining scanning and light microscopy we highlighted the presence of five different zones in the stomach (1: esophagus-gastric lumen passage; 2: descending glandular portion; 3: fundic portion; 4: ascending glandular portion; 5: gastric-pyloric transition portion). Histochemical investigation showed a secretion of carboxylates mucopolysaccharides along the esophagus and sulphated complex carbohydrates in the stomach. These results suggest that mucins play a protective role of the epithelial lining, which is essential for a correct digestive process. Finally, the characterization of the main cellular structures may be inspiring for more advanced studies aiming to decipher the role of specific molecules, such as neuropeptides, involved in the physiological digestive process.

Immunohistochemistry of the Gut-Associated Lymphoid Tissue (GALT) in African Bonytongue (Heterotis niloticus, Cuvier 1829)

Heterotis niloticus is a basal teleost, belonging to the Osteoglossidae family, which is widespread in many parts of Africa. The digestive tract of H. niloticus presents similar characteristics to those of higher vertebrates, exhibiting a gizzard-like stomach and lymphoid aggregates in the intestinal lamina propria. The adaptive immune system of teleost fish is linked with each of their mucosal body surfaces. In fish, the gut-associated lymphoid tissue (GALT) is generally a diffuse immune system that represents an important line of defense against those pathogens inhabiting the external environment that can enter through food. The GALT comprises intraepithelial lymphocytes, which reside in the epithelial layer, and lamina propria leukocytes, which consist of lymphocytes, macrophages, granulocytes, and dendritic-like cells. This study aims to characterize, for the first time, the leukocytes present in the GALT of H. niloticus, by confocal immuno- fluorescence techniques, using specific antibodies: toll-like receptor 2, major histocompatibility complex class II, S100 protein, serotonin, CD4, langerin, and inducible nitric oxide synthetase. Our results show massive aggregates of immune cells in the thickness of the submucosa, arranged in circumscribed oval-shaped structures that are morphologically similar to the isolated lymphoid follicles present in birds and mammals, thus expanding our knowledge about the intestinal immunity shown by this fish.

Morphological and Molecular Functional Evidence of the Pharyngeal Sac in the Digestive Tract of Silver Pomfret, Pampus argenteus

The pharyngeal sac is a comparatively rare organ in the digestive tract among teleost fishes. However, our understanding of this remarkable organ in the silver pomfret (Pampus argenteus) is limited. In the present study, we examined the various morphological and histological characteristics of the pharyngeal sac using histochemical techniques and electron microscopy. The pharyngeal sac showed unique characteristics such as well-developed muscular walls, weakly keratinized epithelium, numerous goblet cells, and needle-like processes on the papillae. The porous cavity of the papillae contained numerous adipocytes and was tightly enveloped by type I collagen fibers. These structures might provide mechanical protection and excellent biomechanical properties for grinding and shredding prey. A comparison of gene expression levels between the pharyngeal sac and esophagus using RNA-seq showed that phenotype-associated genes (epithelial genes and muscle genes) were upregulated, whereas genes related to nutrient digestion and absorption were downregulated in the pharyngeal sac. These results support the role of the pharyngeal sac in shredding and predigesting food. Overall, these findings provide a clearer understanding of the pharyngeal sac morphology and explain the morphological adaptations of the digestive tract for feeding on gelatinous prey. To our knowledge, this is the first report on pharyngeal sac gene expression in P. argenteus.

Comparative Research on Intestinal Functions of Wild and Cultured Hemibarbus maculatus in Jialing River

Hemibarbus maculatus is a common economic fish in the midstream and downstream of the Jialing River. In order to resolve the difficulties in aquacultural cultivation, we tested the intestinal and liver digestive function of wild and cultured Hemibarbus maculatus. Histological methods and special biochemical staining methods were used to compare the differences of morphological structure, goblet cells, argyrophil cells, lymphocytes and Na+/K+ATPase in the intestine, and the morphological structure, glycogen and lipid in the liver between the two kinds of Hemibarbus maculatus. The results showed that higher amount of fat was found to attached to the gut, lower Na+/K+ATPase vitality in the foregut and hidgut (p < 0.01) and lower number of goblet cells in the hindgut (p < 0.01) of the cultured Hemibarbus maculatus when compared to the wild ones. The number of the argyrophilic cells did not show significant differences between the two kinds, but the number of lymphocytes was significantly lower in the segments of gut in cultured. This suggests the absorptive function and intestinal immunity are weaker in the cultured Hemibarbus maculatus. In addition, more glycogen and lipid were found in the liver of cultured fishes, which indicates the decreased digestive function of the cultured Hemibarbus maculatus. In conclusion, the intestinal digestion, absorption and lymphocytes level of the wild are generally better than those of the cultured, and more hepatic lipopexia and glycogen are present in the cultured ones. Future aquacultural activities should consider these changes when facing pragmatic problems.

Hidden diversity: comparative functional morphology of humans and other species

Gastrointestinal (GI) morphology plays an important role in nutrition, health, and epidemiology; yet limited data on GI variation have been collected since 1885. Here we demonstrate that students can collect reliable data sets on gut morphology; when they do, they reveal greater morphological variation for some structures in the GI tract than has been documented in the published literature. We discuss trait variability both within and among species, and the implications of that variability for evolution and epidemiology. Our results show that morphological variation in the GI tract is associated with each organ's role in food processing. For example, the length of many structures was found to vary significantly with feeding strategy. Within species, the variability illustrated by the coefficients of variation suggests that selective constraints may vary with function. Within humans, we detected significant Pearson correlations between the volume of the liver and the length of the appendix (t-value = 2.5278, df = 28, p = 0.0174, corr = 0.4311) and colon (t-value = 2.0991, df = 19, p = 0.0494, corr = 0.4339), as well as between the lengths of the small intestine and colon (t-value = 2.1699, df = 17, p = 0.0445, corr = 0.4657), which are arguably the most vital organs in the gut for nutrient absorption. Notably, intraspecific variation in the small intestine can be associated with life history traits. In humans, females demonstrated consistently and significantly longer small intestines than males (t-value₁₅ = 2.245, p = 0.0403). This finding supports the female canalization hypothesis, specifically, increased female investment in the digestion and absorption of lipids.

Host biology, ecology and the environment influence microbial biomass and diversity in 101 marine fish species

Fish are the most diverse and widely distributed vertebrates, yet little is known about the microbial ecology of fishes nor the biological and environmental factors that influence fish microbiota. To identify factors that explain microbial diversity patterns in a geographical subset of marine fish, we analyzed the microbiota (gill tissue, skin mucus, midgut digesta and hindgut digesta) from 101 species of Southern California marine fishes, spanning 22 orders, 55 families and 83 genera, representing ~25% of local marine fish diversity. We compare alpha, beta and gamma diversity while establishing a method to estimate microbial biomass associated with these host surfaces. We show that body site is the strongest driver of microbial diversity while microbial biomass and diversity is lowest in the gill of larger, pelagic fishes. Patterns of phylosymbiosis are observed across the gill, skin and hindgut. In a quantitative synthesis of vertebrate hindguts (569 species), we also show that mammals have the highest gamma diversity when controlling for host species number while fishes have the highest percent of unique microbial taxa. The composite dataset will be useful to vertebrate microbiota researchers and fish biologists interested in microbial ecology, with applications in aquaculture and fisheries management.

Inhibition of gastric acid secretion with omeprazole affects fish specific dynamic action and growth rate: Implications for the development of phenotypic stomach loss

An acid-secreting stomach provides many selective advantages to fish and other vertebrates; however, phenotypic stomach loss has occurred independently multiple times and is linked to loss of expression of both the gastric proton pump and the protease pepsin. Reasons underpinning stomach loss remain uncertain. Understanding the importance of gastric acid-secretion to the metabolic costs of digestion and growth will provide information about the metabolic expense of acid-production and performance. In this study, omeprazole, a well characterized gastric proton pump inhibitor, was used to simulate the agastric phenotype by significantly inhibiting gastric acidification in Nile tilapia. The effects on post-prandial metabolic rate and growth were assessed using intermittent flow respirometry and growth trials, respectively. Omeprazole reduced the duration (34.4%) and magnitude (34.5%) of the specific dynamic action and specific growth rate (21.3%) suggesting a decrease in digestion and assimilation of the meal. Gastric pH was measured in control and omeprazole treated fish to confirm that gastric acid secretion was inhibited for up to 12 h post-treatment (p &lt; 0.05). Gastric evacuation measurements confirm a more rapid emptying of the stomach in omeprazole treated fish. These findings reinforce the importance of stomach acidification in digestion and growth and present a novel way of determining costs of gastric digestion.

Effects of Dietary Shifts on Ontogenetic Development of Metabolic Rates in Age 0 Lake Sturgeon (Acipenser fulvescens)

AbstractIn many fish species, ontogenetic dietary shifts cause changes in both quantitative and qualitative intake of energy, and these transitions can act as significant bottlenecks in survival within a given year class. In the present study, we estimated routine metabolic rate (RMR) and forced maximum metabolic rate (FMR) in age 0 lake sturgeon (Acipenser fulvescens) on a weekly basis from 6 to 76 days posthatch (dph) within the same cohort of fish. We were particularly interested in the period of dietary transition from yolk to exogenous feeding between 6 and 17 dph and as the fish transitioned from an artemia-based diet to a predominantly bloodworm diet between 49 and 67 dph. Measurement of growth rate and energy density throughout indicated that there was a brief period of growth arrest during the transition from artemia to bloodworm. The highest mass-specific RMR (mg O₂ kg^-1 h^-1) recorded throughout the first 76 d of development occurred during the yolk sac phase and during transition from artemia to bloodworm. Similarly, diet transition from artemia to bloodworm-when growth arrest was observed-increased scaled RMR (i.e., mg O₂ kg^-0.89 h^-1), and it did not significantly differ from scaled FMR. Log-log relationships between non-mass-specific RMR or FMR (i.e., mg O₂ h^-1) and body mass significantly changed as the growing fish adapted to the nutritional differences of their primary diet. We demonstrate that dietary change during early ontogeny has consequences for growth that may reflect altered metabolic performance. Results have implications for understanding cohort and population dynamics during early life and effective management for conservation fish hatcheries.

Performance of Co-Housed Neon Tetras (Paracheirodon innesi) and Glowlight Rasboras (Trigonostigma hengeli) Fed Commercial Flakes and Lyophilized Natural Food

Little to no research has been conducted thus far regarding aquarium fish nutrition. In order to ensure the welfare of house-kept ornamentals, such studies should take into account that there are distinct biological differences occurring between different fish species/taxa, especially in regard to the structure of their digestive organs. Accordingly, a 12-week trial was executed to assess the effects of two commercial flakes and a mix of lyophilized natural food on the condition of co-reared neon tetras, Paracheirodon innesi (Characidae), and glowlight rasboras, Trigonostigma hengeli (Danionidae). The four feeding groups were as follows: (T)-Tetra flakes; (O)-Omega flakes; (TO)-Tetra + Omega; (TOL)-Tetra + Omega + Lyophilizate (twice a week). There were no differences in final body weight (FBW) between the feeding groups of either species, but in the case of neon tetras, FBW increased significantly from the initial value only for the T group. However, histological observations and measurements of digestive organs (livers, intestines) showed pronounced differences between the two species. The supplementation with natural food in group TOL caused lipoid hepatic degeneration only in the rasboras. The healthiest histological structure of livers and longest intestinal folds were found in group T of the tetras and group TO of the rasboras. Whole-mount staining for bone and cartilage did not reveal any significant deformities or differences in terms of bone mineralization. In conclusion, it was outlined that concurrent feeding of co-housed, anatomically diverse ornamental fish species is a highly ambiguous task, because the nutritional strategy applied for a community tank may yield radically divergent effects, most of which may remain unnoticed when depending only on external body observations and measurements. Most emphatically, this was highlighted in regard to the dietary supplementation with natural food-although no significant effects were observed in neon tetras, severe lipoid liver degeneration occurred in glowlight rasboras.

Comparative analysis of the small and large intestines of Acará Geophagus brasiliensis (Quoy & Gaimard, 1824) (Pisces: Cichlidae)

Geophagus brasiliensis, popularly known as acará, is a common fish in lentic freshwater environments in South America. This species has a detritivorous-iliophagous or omnivorous feeding habit, with high food plasticity; however, there are no studies describing its intestinal tract histologically. Therefore, the present study analysed through histological and histochemical techniques the intestines of the acará. Adult specimens were collected with gillnets, anaesthetized and euthanized. Then, the fish were submitted to biometry and dissection to remove fragments of intestines. The samples were fixed in Bouin liquid for 12 hours and subjected to histological and histochemical techniques. Histologically, all samples of intestines were organized into four layers: mucosa, submucosa, muscular and serosa. The small intestine (foregut and midgut) was characterized by the presence of intestinal villi covered by simple prismatic epithelium with a striated border and goblet cells supported by the connective lamina propria. In the large intestine (hindgut), there was an absence of villi and an abundance of goblet cells. Positive reaction to Periodic Acid-Schiff (PAS) and Alcian Blue (AB) pH 2.5 reactions were detected in goblet cells, indicating the presence of mucosubstances. No lipids were detected in the intestine cells due to the negative reaction to the Sudan Black B. The results of the present study provide subsidies for a better understanding of the intestinal morphology of teleosts and provide valuable information for phylogenetic studies.

Morphological and histochemical features of the digestive tract of Leiarius marmoratus (Gill, 1870)

Leiarius marmoratus, a freshwater catfish from Pimelodidae family, shows great biological and commercial relevance because of its geographic distribution and adaptation to fish-farm. The knowledge of the morphological characteristics of the digestive tract is fundamental to the understanding of fish physiology and nutrition, which helps in the planning of diets to provide better management and success in fish farming. Thus, this work described the morphology and histochemistry of the digestive tract of L. marmoratus adults. After euthanasia, the animals were dissected for analysis of the digestive tract. The oesophagus is a short and distensive organ with longitudinal folds that allow the passage of large food, e.g., other fishes. Oesophageal mucosa layer shows a stratified epithelium with goblet cells and club cells. The secretion of goblet cells is composed of neutral and acidic mucins that are anchored in the epithelium luminal face by epithelial cells fingerprint-like microridges, lubricating the surface to facilitate the food sliding. Club cells have protein secretion that can be involved in alarm signals when epithelium is damaged and in immunological defence. The saccular stomach is highly distensible to store large food. Gastric mucosa layer is composed of epithelial cells with intense secretion of neutral mucin to protect against self-digestion of gastric juice. Cardiac and fundic regions of stomach show well-developed gastric glands composed of oxynticopeptic cells. These cells have numerous mitochondria, highlighting their intense activity in the synthesis of acid and enzymes. The intestine is divided into three regions: anterior, middle and posterior. Although it is a short tube, intestine shows longitudinal folds and microvilli of enterocytes to increase the contact surface. These folds are higher in the anterior region of the intestine, highlighting their function in digestion and absorption. Intestinal goblet cells have acidic and neutral mucins that lubricate the epithelium and aid in digestive processes. These cells increase in number towards aboral, and they are related to the protection and lubrication to expulsion of faecal bolus.

Anatomy of the digestive system of lumpfish (Cyclopterus lumpus) as an adaptation to puffing behavior

The first description of the histology of the digestive tract of the lumpfish Cyclopterus lumpus showed an adaptation for collecting and storing water. The morphology of this species' tract is mostly typical of carnivorous fishes, having a sac-like stomach for storage and a relatively short intestine of approximately 1.2× the standard body length. The esophagus is relatively short (0.13× standard body length) and thick-walled; it has a thick muscular coat, and its thickness changes from 0.4 to 1 mm during contraction. The stomach has well-developed gastric glands and is divided into the histologically distinguishable cardiac, fundus, and pyloric parts. The intestine has the same structure throughout its entire length. The water-holding ability of lumpfish is provided by well-developed muscles in the esophagus and stomach distensibility, which differs morphologically from the method used by Tetradontidae and Diodontidae species, who store water in the caudal part of the esophagus. The morphology of the digestive tract proved that this species is an intermediate between Tetradontidae and Diodontidae, which could inflate their bodies by swallowing certain amounts of water, and other fishes which are not able to do so. Results of the work extend current knowledge on the morphological capability of the digestive system of teleost fish, providing water-blowing behavior.

Phylogeny, body morphology, and trophic level shape intestinal traits in coral reef fishes

Trait-based approaches are increasingly used to study species assemblages and understand ecosystem functioning. The strength of these approaches lies in the appropriate choice of functional traits that relate to the functions of interest. However, trait-function relationships are often supported by weak empirical evidence.Processes related to digestion and nutrient assimilation are particularly challenging to integrate into trait-based approaches. In fishes, intestinal length is commonly used to describe these functions. Although there is broad consensus concerning the relationship between fish intestinal length and diet, evolutionary and environmental forces have shaped a diversity of intestinal morphologies that is not captured by length alone.Focusing on coral reef fishes, we investigate how evolutionary history and ecology shape intestinal morphology. Using a large dataset encompassing 142 species across 31 families collected in French Polynesia, we test how phylogeny, body morphology, and diet relate to three intestinal morphological traits: intestinal length, diameter, and surface area.We demonstrate that phylogeny, body morphology, and trophic level explain most of the interspecific variability in fish intestinal morphology. Despite the high degree of phylogenetic conservatism, taxonomically unrelated herbivorous fishes exhibit similar intestinal morphology due to adaptive convergent evolution. Furthermore, we show that stomachless, durophagous species have the widest intestines to compensate for the lack of a stomach and allow passage of relatively large undigested food particles.Rather than traditionally applied metrics of intestinal length, intestinal surface area may be the most appropriate trait to characterize intestinal morphology in functional studies.

Morphology of the digestive system in carnivorous freshwater dourado Salminus brasiliensis

The digestive system of teleost shows remarkable functional and morphological diversity. In this study, the digestive tract and accessory organs of dourado Salminus brasiliensis are characterized using anatomical, histological, histochemical and immunohistochemical analyses. The existence of taste buds bordered by microridges in the oesophagus of dourado was recorded for the first time, thus showing that the species drives food intake by either swallowing or rejecting the food item. The Y-shaped stomach of dourado consisted of cardiac, cecal and pyloric regions with tubular gastric glands registered solely in the cardiac and cecal segments. The intestine is a short N-shaped tube with two loops, an intestinal coefficient of 0.73. The structure of pyloric caeca is similar to that of the intestine wall, comprising tunica mucosa, tela submucosa, tunica muscularis and tunica serosa layers. Histochemical analyses revealed an increased incidence of goblet cells from the midgut to the hindgut segment. A well-developed enteric plexus of scattered nerve cell and fibres are found along the digestive tract, and the calcitonin gene-related peptide (CGRP) immunoreactive neurons and fibres were identified in the myenteric plexus from the oesophagus to the hindgut. The exocrine pancreas appears diffuse in the mesentery around the stomach, intestine and also reaches the liver, and the endocrine pancreas is organized as a few islets of Langerhans. The liver comprises three distinct, asymmetric lobes, and the portal triad arrangement was registered in this tissue.

Digestive tube of an herbivorous fish (Rhytiodus microlepis) from the Amazonian floodplain lakes: A morphological and histochemical study

This study describes the morphology and histochemistry of the digestive tube of the herbivorous fish Rhytiodus microlepis, which is commonly found in the Amazonian floodplain lakes, Brazil. Thirty-eight fish were used in this study. The digestive tube was measured and dissected for anatomical description, and stained with (i) haematoxylin and eosin, for histological analyses; (ii) periodic acid-Schiff, for detected neutral mucins; and (iii) Alcian Blue (AB) pH 1.0 and 2.5 for acid and sulphated acid mucins. The oesophagus, with its secretory cells and taste buds, is adapted for lubrication of vegetable matter, resistance to friction and reception of stimuli. The stomach is U-shaped and rich in gastric glands, particularly in cardiac and fundic regions, which are adapted to receive large amounts of food, and promotes the digestion. The intestine comprises more than 70% of the digestive tube. All portions of the intestine, except the rectum, have the same pattern of mucosal folds, and the goblet cells reacted positively to all histochemical methods (PAS, AB 1.0 and 2.5), while rodlet cells reacted only to periodic acid-Schiff (PAS) staining. Neutral mucins and sulphated acid mucins predominated in the anterior portion of the intestine and a high concentration of carboxylated acid mucosubstances were present in the other portions. The rectum showed a strong reaction to all histochemical methods. The muscular layer of the intestine consists of three layers, which showed features presumably related to the rigid nature of the food. A simple squamous epithelium constitutes the serous layer over the entire length of the tube.

Assessing the impacts of in-feed probiotic on the growth performance and health condition of pangasius (Pangasianodon hypophthalmus) in a farm trial

The effects of in-feed probiotics on growth performance, haematological parameters, gut microbial content, and morphological changes to pangasius fish were assessed. The trial had three phases, i.e., larvae to fry (20 days), fry to fingerlings (45 days), and grow-out phase from fingerlings to marketing (90 days). The stocking densities were 400 m^-3, 200 m^-3, and 12 m^-3 for phases 1, 2, and 3, respectively. Phases 1 and 2 were conducted in hapas in the same pond, whereas phase 3 was performed in concrete tanks. The in-feed probiotic was administered at a rate of 0.2 g kg^-1 of feed three times per day in phases 1 and 2 only. In phase 3, in-feed probiotics was not applied to any groups. The treated group exhibited higher growth performances (p < 0.05) than the control in all three phases of experiment. The survival % in phase 1 and 2 were found significantly (p < 0.05) higher in treatment groups. This indicates that pangasius nurserers would benefit from using probiotics as a safeguard to increase fry survival to a greater extent. Two haematological parameters including red blood cells (RBC) and white blood cells (WBC) levels were found significantly (p < 0.05) higher in treated groups in phase 2 and 3, while glucose and hemoglobin level were found significantly (p < 0.05) higher in the treated groups during phases 2 and 3, respectively. The gut microbiota content was relatively higher in the treated groups in phase 2 and 3. Histological findings indicate that the use of probiotics during the nursing phases of pangasius induced a positive change in the intestinal morphological structures. The positive impacts of probiotics on the phase 3 confirmed an immediate and long-term growth performance and health of pangasius.

Screwed up: Spirality of segments and other iterated structures suggest an underlying principle of seriality in bilaterians

This review deals with helicomery, that is, the specific malformation of a spiral arrangement of segments and other serial structures. Helicomery was first described in annelid and arthropod body segments. However, corresponding patterns occur in arthropod appendages and other bilaterians with serially arranged body parts, such as tapeworms, nematodes, vertebrates, and probably chitons. The specifics of the spirals such as length, orientation, and handedness are described. Most spirals are dorsal and comprise only a few loops. Helicomery is formed by a shift of cells during development or in adults caused by changes in cell adhesion or mechanical impacts such as lesions. A model for the formation of helicomery is proposed, which is based on medieval church labyrinths. These complex spiral structures are derived from concentric lines by the shift of relatively few tiles. This principle of "small causes, great effect" also applies to "spiral segments," because helicomery dissolves segmental patterns and questions the concept of segments as distinct structures. The widespread occurrence of helicomery in nonhomologous serial structures might indirectly indicate an underlying principle of seriality among Bilateria.

Temporal changes in the gut microbiota in farmed Atlantic cod (Gadus morhua) outweigh the response to diet supplementation with macroalgae

BACKGROUND

Aquaculture successfully meets global food demands for many fish species. However, aquaculture production of Atlantic cod (Gadus morhua) is just 2.5% of total market production. For cod farming to be a viable economic venture specific challenges on how to increase growth, health and farming productivity need to be addressed. Feed ingredients play a key role here. Macroalgae (seaweeds) have been suggested as a functional feed supplement with both health and economic benefits for terrestrial farmed animals and fish. The impact of such dietary supplements to cod gut integrity and microbiota, which contribute to overall fish robustness is unknown. The objective of this study was to supplement the diet of juvenile Atlantic cod with macroalgae and determine the impacts on fish condition and growth, gut morphology and hindgut microbiota composition (16S rRNA amplicon sequencing). Fish were fed one of three diets: control (no macroalgal inclusion), 10% inclusion of either egg wrack (Ascophyllum nodosum) or sea lettuce (Ulva rigida) macroalgae in a 12-week trial.

RESULTS

The results demonstrated there was no significant difference in fish condition, gut morphology or hindgut microbiota between the U. rigida supplemented fish group and the control group at any time-point. This trend was not observed with the A. nodosum treatment. Fish within this group were further categorised as either 'Normal' or 'Lower Growth'. 'Lower Growth' individuals found the diet unpalatable resulting in reduced weight and condition factor combined with an altered gut morphology and microbiome relative to the other treatments. Excluding this group, our results show that the hindgut microbiota was largely driven by temporal pressures with the microbial communities becoming more similar over time irrespective of dietary treatment. The core microbiome at the final time-point consisted of the orders Vibrionales (Vibrio and Photobacterium), Bacteroidales (Bacteroidetes and Macellibacteroides) and Clostridiales (Lachnoclostridium).

CONCLUSIONS

Our study indicates that U. rigida macroalgae can be supplemented at 10% inclusion levels in the diet of juvenile farmed Atlantic cod without any impact on fish condition or hindgut microbial community structure. We also conclude that 10% dietary inclusion of A. nodosum is not a suitable feed supplement in a farmed cod diet.

Gut immunity in European sea bass (Dicentrarchus labrax): a review

In this review, we summarize and discuss the trends and supporting findings in scientific literature on the gut mucosa immune role in European sea bass (Dicentrarchus labrax L.). Overall, the purpose is to provide an updated overview of the gastrointestinal tract functional regionalization and defence barriers. A description of the available information regarding immune cells found in two immunologically-relevant intestinal compartments, namely epithelium and lamina propria, is provided. Attention has been also paid to mucosal immunoglobulins and to the latest research investigating gut microbiota and dietary manipulation impacts. Finally, we review oral vaccination strategies, as a safe method for sea bass vaccine delivery.

Morphology, histology and histochemistry of the digestive tract of the Banded tilapia, Tilapia sparrmanii (Perciformes: Cichlidae)

This study described anatomical, histological and histochemical features of the mucosal layer of the digestive tract of Tilapia sparrmanii Smith, 1840, an omnivorous freshwater fish endemic to Southern Africa. This species exhibited a short thick oesophagus with long deep longitudinal folds (466.68 ± 16.91 µm), and a thick (173.50 ± 10.92 µm) muscular layer that allow the passage of large food items. The mucosa was lined with stratified secretory epithelium rich in goblet cells that secreted neutral and acid mucins. The stomach was a sac-like structure with simple tubular glands surrounded by connective tissue. The mucosa was lined with simple columnar epithelium and the lamina propria exhibited a well-developed layer of gastric glands that occupied the entire length of the cardio-fundic region. The stomach mucosa consisted of epithelial cells with intense neutral mucin secretion which protects against gastric juice. Neck cells of gastric glands synthesized neutral and acid mucins. The intestine was highly coiled and presented a complex pattern of transversal folds internally (villi). Villi length decreased progressively from the anterior to the posterior intestine (p &lt; 0.0001). Tunica muscularis of the mid-intestine had the thinnest thickness among all parts of the intestine (p &lt; 0.0001). Goblet cells whose numbers increased towards the rectum secreted both acid and neutral mucins. The results indicate structural similarities of T. sparrmaniiGIT with other tilapia species and will be useful for understanding the physiology of the digestive systems as well as functional components of the GIT.

Description of the unusual digestive tract of Platax orbicularis and the potential impact of Tenacibaculum maritimum infection

Background

Ephippidae fish are characterized by a discoid shape with a very small visceral cavity. Among them Platax orbicularis has a high economic potential due to its flesh quality and flesh to carcass ratio. Nonetheless, the development of its aquaculture is limited by high mortality rates, especially due to Tenacibaculum maritimum infection, occurring one to three weeks after the transfer of fishes from bio-secure land-based aquaculture system to the lagoon cages for growth. Among the lines of defense against this microbial infection, the gastrointestinal tract (GIT) is less studied. The knowledge about the morphofunctional anatomy of this organ in P. orbicularis is still scarce. Therefore, the aims of this study are to characterize the GIT in non-infected P. orbicularis juveniles to then investigate the impact of T. maritimum on this multifunctional organ.

Methods

In the first place, the morpho-anatomy of the GIT in non-infected individuals was characterized using various histological techniques. Then, infected individuals, experimentally challenged by T. maritimum were analysed and compared to the previously established GIT reference.

Results

The overlapped shape of the GIT of P. orbicularis is probably due to its constrained compaction in a narrow visceral cavity. Firstly, the GIT was divided into 10 sections, from the esophagus to the rectum. For each section, the structure of the walls was characterized, with a focus on mucus secretions and the presence of the Na⁺/K⁺ ATPase pump. An identification key allowing the characterization of the GIT sections using in toto histology is given. Secondly, individuals challenged with T. maritimum exhibited differences in mucus type and proportion and, modifications in the mucosal and muscle layers. These changes could induce an imbalance in the trade-off between the GIT functions which may be in favour of protection and immunity to the disadvantage of nutrition capacities.

Mucosal Hallmarks in the Alimentary Canal of Northern Pike Esox lucius (Linnaeus)

Simple Summary

In vertebrates, mucous cells are one of the main cellular components of the gut mucosal system, which secrete different mucin types involved in several functions. Endocrine cells are scattered in the epithelium of the gut mucosa, and they produce and release regulatory molecules affecting food intake and nutrition. The goal of this study was to obtain data on quantitative distribution of mucous and endocrine cell types in the alimentary canal of the northern pike (Esox lucius), using histochemistry and immunofluorescence. In the stomach of pike, there is a high abundance of mixed mucins, with the acid component contributing to the lubrication of mucosae, where they are associated with the rapid passage of digesta through the intestine. Neutral mucins increase in the intestine aborally. The distribution of endocrine cells of the diffuse endocrine system shows the presence of somatostatin and catecholamine-secreting endocrine cells and the lack of gastrin-secreting endocrine cells. We show a close regulatory relation between endocrine and mucous cells of the gut mucosal system involved in the physiology of fish nutrition. Results confirmed the relationship between the carnivorous diet and the gut mucins distribution of northern pike; indeed, our data provide very important information to ichthyologists who study dietary behavior of species.

Abstract

On the basis of trophic behavior, fish are classified as herbivores, carnivores, omnivores, or detritivores. Epithelial mucous cells secrete mucin types specific to diet and digestive function. Mucus secretion is regulated mainly by molecular modulators produced by epithelial endocrine cells in response to luminal or tissue stimuli. These modulators are involved in control of food intake and digestive functions. Immunohistochemical and immunofluorescence studies were conducted on 10 adult northern pike (Esox lucius Linnaeus, 1758) from Lake Piediluco (Central Italy) to quantify distribution of sub-types of mucous and endocrine cells in alimentary mucosal epithelium. Neutral mucins predominated in the esophagus, and mixed and acidic mucins predominated in stomach and intestine. The gastric epithelium contained endocrine cells secreting somatostatin, tyrosine hydroxylase, and substance P. Mucous cells secreting neutral mucins increased in number from proximal to distal intestine, with endocrine cells containing substance P in the proximal intestine and those containing Leu-enkephalin throughout the intestine. Lectin histochemistry of gut sections revealed an abundance of N-acetyl-glucosamine and N-acetyl-galactosamine as carbohydrate residues on the mucin chain. The quantity and content of endocrine and mucous cells in the alimentary canal of E. lucius showed a direct relationship with its diet.

Extra-gastric expression of the proton pump H+/K+-ATPase in the gills and kidney of the teleost Oreochromis niloticus

Potassium regulation is essential for the proper functioning of excitable tissues in vertebrates. The H+/K+-ATPase (HKA), which is composed of the HKα1 (gene: atp4a) and HKβ (gene: atp4b) subunits, has an established role in potassium and acid-base regulation in mammals and is well known for its role in gastric acidification. However, the role of HKA in extra-gastric organs such as the gill and kidney is less clear, especially in fishes. In the present study in Nile tilapia, Oreochromis niloticus, uptake of the K+ surrogate flux marker rubidium (Rb+) was demonstrated in vivo; however, this uptake was not inhibited with omeprazole, a potent inhibitor of the gastric HKA. This contrasts with gill and kidney ex vivo preparations, where tissue Rb+ uptake was significantly inhibited by omeprazole and SCH28080, another gastric HKA inhibitor. The cellular localization of this pump in both the gill and kidney was demonstrated using immunohistochemical techniques with custom-made antibodies specific for Atp4a and Atp4b. Antibodies against the two subunits showed the same apical ionocyte distribution pattern in the gill and collecting tubules/ducts in the kidney. Atp4a antibody specificity was confirmed by western blotting. RT-PCT was used to confirm the expression of both subunits in the gill and kidney. Taken together, these results indicate for the first time K+ (Rb+) uptake in O. niloticus and that HKA is implicated, as shown through the ex vivo uptake inhibition by omeprazole and SCH28080, verifying a role for HKA in K+ absorption in the gill's ionocytes and collecting tubule/duct segments of the kidney.

Endogenous cortisol production and its relationship with feeding transitions in larval lake sturgeon (Acipenser fulvescens)

Our understanding of the importance of cortisol in the development of fishes largely stems from teleosts and in particular the zebrafish, Danio rerio. However, studies examining the ontogeny of the cortisol endocrine axis in acipenseriformes (sturgeon and paddlefish) have demonstrated similar general patterns during early development. Beginning with maternal deposition of cortisol in the egg, followed by development of de novo synthesis, a hypo-responsive period, and finally the ability of the fish to appropriately increase whole-body levels of cortisol in response to a stressor. In the present study, we demonstrate a similar pattern of ontogeny in the cortisol response in lake sturgeon over two-year classes. Whole-body levels of cortisol were examined over two cohorts and found to be different in both concentration and timing of endogenous production. The 2016 cohort were found to have relatively high levels of cortisol and developed to first feeding approximately six days faster than the 2017 cohort with lower levels of cortisol. In the 2017 cohort, mRNA expression of steroidogenic acute regulatory protein (StAR) and glucocorticoid receptor 1 (GR1) increased just prior to the increase in cortisol and associated onset of exogenous feeding. Treatment in metyrapone, an inhibitor of 11β-hydroxylase, significantly inhibited cortisol production and resulted in the inability of the fish to appropriately transition to exogenous feeding. Data suggest a potential key role for cortisol in lake sturgeon as they transition between diets during early life history.

Morphological and functional development of the spiral intestine in cloudy catshark (Scyliorhinus torazame)

Cartilaginous fish have a comparatively short intestine known as the spiral intestine that consists of a helical spiral of intestinal mucosa. However, morphological and functional development of the spiral intestine has not been fully described. Unlike teleosts, cartilaginous fish are characterized by an extremely long developmental period in ovo or in utero; for example, in the oviparous cloudy catshark (Scyliorhinus torazame), the developing fish remains inside the egg capsule for up to 6 months, suggesting that the embryonic intestine may become functional prior to hatching. In the present study, we describe the morphological and functional development of the spiral intestine in the developing catshark embryo. Spiral formation of embryonic intestine was completed at the middle of stage 31, prior to 'pre-hatching', which is a developmental event characterized by the opening of the egg case at the end of the first third of development. Within 48 h of the pre-hatching event, egg yolk began to flow from the external yolk sac into the embryonic intestine via the yolk stalk. At the same time, there was a rapid increase in mRNA expression of the peptide transporter pept1 and neutral amino acid transporter slc6a19 Secondary folds in the intestinal mucosa and microvilli on the apical membrane appeared after pre-hatching, further supporting the onset of nutrient absorption in the developing intestine at this time. We demonstrate the acquisition of intestinal nutrient absorption at the pre-hatching stage of an oviparous elasmobranch.

Antibiotic effect and microbiome persistence vary along the European seabass gut

The constant increase in aquaculture production has led to extensive use of antibiotics as a means to prevent and treat diseases, with adverse implications on the environment, animal health and commensal microbes. Gut microbes are important for the host proper functioning, thus evaluating such impacts is highly crucial. Examining the antibiotic impact on gut segments with different physiological roles may provide insight into their effects on these microhabitats. Hence, we evaluated the effect of feed-administrated antibiotics on the composition and metabolic potential of the gut microbiome in the European seabass, an economically important aquaculture species. We used quantitative PCR to measure bacterial copy numbers, and amplicon sequencing of the 16S rRNA gene to describe the composition along the gut, after 7-days administration of two broad-range antibiotic mixtures at two concentrations. While positive correlation was found between antibiotic concentration and bacterial abundance, we showed a differential effect of antibiotics on the composition along the gut, highlighting distinct impacts on these microbial niches. Moreover, we found an increase in abundance of predicted pathways related to antibiotic-resistance. Overall, we show that a high portion of the European seabass gut microbiome persisted, despite the examined antibiotic intake, indicating high stability to perturbations.

Gut and intestinal biometrics of the giant trevally, Caranx ignobilis, fed an experimental diet with difference sources of activated charcoal

Background: The giant trevally, Caranx ignobilis, is a commercially important marine fish in Indonesia. This species was initially cultured in Aceh Province. Previous reports showed that charcoal has a positive effect on survival and feed utilization of the giant trevally. However, the effects of adding charcoal to the diet on gut and intestine biometrics has, to our knowledge, never been described.

Methods: Four activated charcoal sources were tested in this study using a completely randomized experimental design; coconut shell charcoal, mangrove wood charcoal, rice husk charcoal, and kernel palm shell charcoal. All treatments were performed with four replications. Juvenile giant trevally (average body weight, 16.52 ± 3.12 g; and average total length, 10.26 ± 0.64 cm) were stocked into the experimental tank at a density of 15 fish per tank. The fish were fed an experimental diet twice daily at 7 AM and 5 PM ad satiation for 42 days.

Results: Analysis of variance showed that adding charcoal to the diet had significant effects on the length and width of the foveola gastrica and villous intestine (P < 0.05). The greatest length and width of the foveola gastrica was recorded in fish fed an experimental diet of rice husk charcoal with average values of 311.811 ± 9.869 µm and 241.786 ± 10.394 µm, respectively. The greatest length of intestinal villous was found in fish fed the mangrove wood charcoal diet, with a value of 135.012 ± 5.147 µm, but this length was not significantly different to that in fish fed rice charcoal and kernel palm shell charcoal. However, the greatest width of intestinal villous was recorded in fish fed the control diet (without charcoal; P < 0.05).

Conclusion: The optimal sizes of the foveola gastrica and villous intestine were found in fish fed an experimental diet with rice husk charcoal.

Gut and intestinal biometrics of the giant trevally, Caranx ignobilis, fed an experimental diet with difference sources of activated charcoal

Background: The giant trevally, Caranx ignobilis, is a commercially important marine fish in Indonesia. This species was initially cultured in Aceh Province. Previous reports showed that charcoal has a positive effect on survival and feed utilization of the giant trevally. However, the effects of adding charcoal to the diet on gut and intestine biometrics has, to our knowledge, never been described. Methods: Four activated charcoal sources were tested in this study using a completely randomized experimental design; coconut shell charcoal, mangrove wood charcoal, rice husk charcoal, and kernel palm shell charcoal. All treatments were performed with four replications. Juvenile giant trevally (average body weight, 16.52 ± 3.12 g; and average total length, 10.26 ± 0.64 cm) were stocked into the experimental tank at a density of 15 fish per tank. The fish were fed an experimental diet twice daily at 7 AM and 5 PM ad satiation for 42 days. Results: Analysis of variance showed that adding charcoal to the diet had significant effects on the length and width of the foveola gastrica and villous intestine (P < 0.05). The greatest length and width of the foveola gastrica was recorded in fish fed an experimental diet of rice husk charcoal with average values of 311.811 ± 9.869 µm and 241.786 ± 10.394 µm, respectively. The greatest length of intestinal villous was found in fish fed the mangrove wood charcoal diet, with a value of 135.012 ± 5.147 µm, but this length was not significantly different to that in fish fed rice charcoal and kernel palm shell charcoal. However, the greatest width of intestinal villous was recorded in fish fed the control diet (without charcoal; P < 0.05). Conclusion: The optimal sizes of the foveola gastrica and villous intestine were found in fish fed an experimental diet with rice husk charcoal.

Ecological and Ontogenetic Components of Larval Lake Sturgeon Gut Microbiota Assembly, Successional Dynamics, and Ecological Evaluation of Neutral Community Processes

Gastrointestinal (GI) or gut microbiotas play essential roles in host development and physiology. These roles are influenced partly by the microbial community composition. During early developmental stages, the ecological processes underlying the assembly and successional changes in host GI community composition are influenced by numerous factors, including dispersal from the surrounding environment, age-dependent changes in the gut environment, and changes in dietary regimes. However, the relative importance of these factors to the gut microbiota is not well understood. We examined the effects of environmental (diet and water sources) and host early ontogenetic development on the diversity of and the compositional changes in the gut microbiota of a primitive teleost fish, the lake sturgeon (Acipenser fulvescens), based on massively parallel sequencing of the 16S rRNA gene. Fish larvae were raised in environments that differed in water source (stream versus filtered groundwater) and diet (supplemented versus nonsupplemented Artemia fish). We quantified the gut microbial community structure at three stages (prefeeding and 1 and 2 weeks after exogenous feeding began). The diversity declined and the community composition differed significantly among stages; however, only modest differences associated with dietary or water source treatments were documented. Many taxa present in the gut were over- or underrepresented relative to neutral expectations in each sampling period. The findings indicate dynamic relationships between the gut microbiota composition and host gastrointestinal physiology, with comparatively smaller influences being associated with the rearing environments. Neutral models of community assembly could not be rejected, but selectivity associated with microbe-host GI tract interactions through early ontogenetic stages was evident. The results have implications for sturgeon conservation and aquaculture production specifically and applications of microbe-based management in teleost fish generally.IMPORTANCE We quantified the effects of environment (diet and water sources) and host early ontogenetic development on the diversity of and compositional changes in gut microbial communities based on massively parallel sequencing of the 16S rRNA genes from the GI tracts of larval lake sturgeon (Acipenser fulvescens). The gut microbial community diversity declined and the community composition differed significantly among ontogenetic stages; however, only modest differences associated with dietary or water source treatments were documented. Selectivity associated with microbe-host GI tract interactions through early ontogenetic stages was evident. The results have implications for lake sturgeon and early larval ecology and survival in their natural habitat and for conservation and aquaculture production specifically, as well as applications of microbe-based management in teleost fish generally.

Edwardsiella tarda induces enteritis in farmed seahorses (Hippocampus erectus): An experimental model and its evaluation

Bacterial enteritis is an important deadly threat to farmed seahorses. However, its pathogenesis is obscure because of the paucity of reproducible experimental intestinal inflammation models. Herein, a strain of Edwardsiella tarda YT1 from farmed seahorse Hippocampus erectus was isolated and identified by morphological, phylogenetic, and biochemical analysis, and confirmed as a pathogen of enteritis for the first time by challenge experiment. Two E. tarda concentrations (1 × 10⁵ and 1 × 10⁷ colony forming units [cfu] ml^-1) were confirmed suitable for an enteritis model by intraperitoneal injection. To develop and evaluate the experimental model, we challenged seahorses with E. tarda and found that (1) the infection inhibited body length increase, significantly decreased body weight (P < 0.05), and induced typical pathological features including anorexia, anal inflammation, and intestinal fluid retention; (2) 19 external (weight, height, anal inflammation, feeding status, and intestinal fluid retention), histological (goblet and inflammatory cell numbers and thickening of lamina propria and muscularis mucosae), and molecular (hepcidin, liver-expressed antimicrobial peptide, lysozyme, piscidin, interleukin [IL]-1β, IL-1β receptor, IL-2, IL-10, interferon1, tumor necrosis factor [TNF]-α, and toll-like receptor 5 [TLR5]) indicators were suitable for model evaluation, as they could sensitively respond and varied similarly throughout the experiment, indicating the high sensitivity of seahorses against pathogen invasion; (3) TLR5 may play an essential role in triggering host immune responses during E. tarda-induced chronic enteritis, and (4) the evaluating system could reflect the pattern and intensity of disease progression. Thus, we developed an experimental model and an evaluating system of bacterial enteritis in farmed seahorses, helping us to reveal the pathogenesis of bacterial enteritis, identify potential therapeutic drugs, and search suitable genetic markers for seahorse molecular breeding.

Chronic dietary exposure to polystyrene microplastics in maturing Japanese medaka (Oryzias latipes)

Fish studies report consumption of microplastics (MPs) in the field, and concern exists over associated risks. However, laboratory studies with adult fish are scarce. In this study, outbred and see-through Japanese medaka (Oryzias latipes) were fed diets amended with 500, 1000, or 2000 μg/g 10 μm fluorescent spherical polystyrene microplastics (MPs) for 10 weeks during their maturation from juveniles to spawning adults. No behavioral changes, growth differences, or mortalities occurred. In vivo examinations and histologic sections showed no evidence of translocation of MPs from the gut to other internal organs. Mature females experienced dose-dependent decreases in egg number. Scanning electron microscopic examination of gills and gut revealed MPs in both areas. Swollen enterocytes were observed on apices of gut folds only in exposed fish. These were particularly apparent in foreguts of the high exposure group. Enterocytes with eroded brush borders were found in foregut of high and medium exposure groups. Increased mucus production, in long strands and sheets, was seen over primary and secondary lamellae of gills. Histological analysis showed alteration in buccal cavity, kidney, and spleen. Thickening and roughening of epithelium in headgut and pharynx and cellular alterations in spleen occurred. Head kidney was the primary site of alteration. Glomerulopathy and nephrogenesis were observed in exposed fish, increasing in severity with exposure level.

Morphological and histochemical characterisation of the mucosa of the digestive tract in matrinxã Brycon amazonicus (Teleostei: Characiformes)

This study describes anatomical, histological and histochemical features of the digestive tract mucosal layer of the matrinxã Brycon amazonicus, an omnivorous freshwater fish endemic from the Amazon basin. This species presents short thick oesophagus with longitudinal folds, that allow the passage of large food items. The mucosa is lined with a stratified secretory epithelium rich in goblet cells that secrete neutral and acid mucins. The two mucin types provide different viscosity in anterior and posterior oesophagus related to the protective and lubricant functions, respectively. The stomach is a highly distensible Y-shaped saccular organ. Here, it is proposed that this anatomical shape plays an essential role in food storage when food availability is abundant. The stomach mucosa is composed of epithelial cells with intense neutral mucin secretion to protects against gastric juice. The intestine is slightly coiled and presents internally a complex pattern of transversal folds that increases the absorption surface and the retention time of food. Goblet cells in the intestine secrete acid and neutral mucins that lubricate the epithelium and aid in the digestive processes. In the rectum, an increase in goblet cells population occurs that may be related to better lubrication.