Digestive enzyme profiles reveal digestive capacity and potential energy sources in fed and starved spiny lobster (Jasus edwardsii) phyllosoma larvae

Dietary effects of vitamin C on antioxidant capacity, intestinal microbiota and the resistance of pathogenic bacteria in cultured Silver pomfret (Pampus argenteus)

As most teleosts are unable to synthesize vitamin C, supplemental diets containing vitamin C diets play a crucial role in fish health. The aim of this study was to investigate the effect of dietary vitamin C on the intestinal enzyme activity and intestinal microbiota of silver pomfre (Pampus argenteus). Four experimental diets were supplemented with basic diets containing 300 mg of vitamin C/kg (group tjl3), 600 mg of vitamin C/kg (group tjl6), and 1200 mg of vitamin C/kg (group tjl12), as well as vitamin C-free supplemental basic diet (group tjl0), respectively. The four diets were fed to juvenile P. argenteus (average initial weight: 4.68 ± 0.93 g) for 6 weeks. The results showed that the activity of SOD (superoxide dismutase) and CAT (catalase) increased significantly while that of MDA (malondialdehyde) decreased significantly in group tjl3 compared to vitamin group tjl0. At the genus level, groups tjl0, tjl6, and tjl12 contained the same dominant microbial community, Stenotrophomonas, Photobacterium, and Vibrio, whereas group tjl3 was dominated by Stenotrophomonas, Delftia, and Bacteroides. Among the fish fed with a basic diet containing 300 mg of vitamin C/kg, the intestines exhibited a notable abundance of probiotic bacteria, including lactic acid bacteria (Lactobacillus) and Bacillus. The abundance of Aeromonas in groups tjl3 and tjl6 was lower than that of the vitamin C-free supplemental basic diet group, whereas Aeromonas was not detected in group tjl12. In addition, a causative agent of the disease outbreak in cultured P. argenteus, Photobacterium damselae subsp. Damselae (PDD) was the dominant microbiota community in groups tjl0, tjl6 and tjl12, whereas the abundance of PDD in group tjl3 was the lowest among the diets. Taken together, the diets supplied with vitamin C could influence the composition microbial community of P. argenteus. The low level of vitamin C (300 mg of vitamin C/kg per basic diet) supplementation could not only improve the antioxidant capacity but also resist the invasion of pathogenic bacteria.

Responses of digestive enzyme profiles in newly-hatched (Zoea I) larvae of the mud crab Scylla serrata to intermittent food availability and food deprivation

Activities (mU larva-1) of enzymes critical to digestion were examined to better understand how newly-hatched (Zoea I) larvae of the mud crab Scylla serrata respond to intermittent food availability and food deprivation. Specifically, this study examined the activities of trypsin-like proteases, nonspecific esterases, and α-amylase across three experiments that simulated scenarios in which larvae hatch and experience rearing conditions where food was either: (1) continuously available or unavailable; (2) initially unavailable, but subsequently available; or (3) initially available, but subsequently unavailable. Results showed that food availability exerts a significant influence on enzyme profiles in newly-hatched larvae, with nutritional history influencing their response to food deprivation. When food was unavailable from hatch, there was no significant change in larval enzyme activities between 6 and 78 h post-hatch. If food became available at any point during this period, however, newly-hatched larvae were capable of rapidly (within 12-24 h) adjusting enzyme activities in response. Furthermore, a short (36 h) period of food availability appears sufficient to permit continuous substrate utilization during subsequent food deprivation of equivalent duration. Such flexibility is an important physiological strategy allowing newly-hatched larvae of S. serrata to adapt and thrive in challenging tropical oceanic environments and provides a basis for optimizing protocols for hatchery production of this species.

An emerging role of vitamin D3 in amino acid absorption in different intestinal segments of on-growing grass carp (Ctenopharyngodon idella)

Vitamin D3 (VD3), an essential nutrient for animals, has been demonstrated to stimulate the uptake of certain amino acids. However, the role of VD3 in the intestine, the primary site for digestion and absorption of nutrients, remains poorly characterized. Here, the grass carp (Ctenopharyngodon idella) was studied to assess the influence of different doses of VD3 (15.2, 364.3, 782.5, 1,167.9, 1,573.8, and 1,980.1 IU/kg) on growth performance, intestinal morphology, digestive absorption, amino acid transport, and potential signaling molecule levels in a feeding experiment. As a result, dietary VD3 improved growth performance, intestinal structure, and digestive and brush border enzyme activities. Additionally, most intestinal free amino acids and their transporters were upregulated after VD3 intake, except for Ala, Lys, Asp, Leu, solute carrier (SLC) 7A7, SLC1A5, and SLC1A3 mRNA in different segments, Leu and SLC6A14 mRNA in the proximal intestine, and SLC7A5 mRNA in the mid and distal intestine. In the crucial target of rapamycin (TOR) signal pathway of amino acid transport, the gene and protein expression of TOR, S6 kinase 1, and activating transcription factor 4 were elevated, whereas 4E-binding protein 1 was decreased, further suggesting an advanced amino acid absorption capacity in the fish due to VD3 supplementation. Based on percentage weight gain, feed efficiency, and trypsin activity, the VD3 requirements of on-growing grass carp were estimated to be 968.33, 1,005.00, and 1,166.67 IU/kg, respectively. Our findings provide novel recommendations for VD3 supplementation to promote digestion and absorption capacities of fish, contributing to the overall productivity of aquaculture.

Responses of digestive enzyme profiles to various scenarios of food availability in newly-hatched Stage I phyllosoma larvae of the tropical spiny lobster Panulirus ornatus

Stage I phyllosoma larvae of the spiny lobster Panulirus ornatus hatch in tropical oceanic waters with limited and variable food resources. To better understand how these larvae cope with food deprivation, this study examined specific (mU mg^-1 protein) and total (mU larva^-1) activities of major digestive enzymes (i.e., α-amylase, non-specific esterase, trypsin-like protease) during a series of ex situ experiments. Specifically, temporal changes in enzyme profiles were examined in three experiments simulating scenarios in which phyllosoma hatch and begin development in an environment where (1) prey is either continuously present or absent; (2) prey is initially present, but subsequently absent; (3) prey is initially absent, but subsequently present. Results indicated that the accessibility of suitable prey provides an overarching influence on digestive enzyme activities and substrate utilisation in Stage I phyllosoma of P. ornatus, with enzymatic responses to both intermittent prey availability and food deprivation being influenced by their nutritional history. In the absence of prey, larval digestive enzyme activities (mU larva^-1) initially remained static from hatch but eventually declined with extended food deprivation. When prey became available, enzyme activities increased, with delayed access to prey having minimal impact on this enzymatic response. Furthermore, phyllosoma were able to adjust α-amylase and non-specific esterase activities (within 8 h) in response to the disappearance of prey, demonstrating adaptive changes to endure periods of food deprivation that were based on their nutritional history. The ability of phyllosoma to regulate enzyme activities from hatch is an important physiological strategy allowing them to survive in an environment characterised by highly variable zooplankton biomass and abundance, and explains why diets that differ greatly in nutrient composition have consistently been found effective for rearing phyllosoma in captivity.

Changes in weight gain, digestive and metabolic enzyme activities in Labeo rohita fingerlings in response to multiple stress exposure and dietary nutraceutical

A feeding trial was conducted for 4 months to study the effect of a nutraceutical conglomerate at different levels (0, 0.1, and 0.5%) on stress-exposed (high stocking density, H and feed restriction, R) Labeo rohita fingerlings. Six isonitrogenous diets with a crude protein of 35% were prepared for the different treatments, viz. THR0, THR1, THR5, TNS0, TNS1 and TNS5. Stress significantly reduced the weight gain and feeding of 0.1% nutraceutical improved it in both stress-exposed and non-exposed groups from 3 months onwards. Two-and three-month stress-exposed groups exhibited lower (p < 0.05) protease activity, while amylase activity was significantly higher in 2-month stress-exposed groups. Higher lipase activity was found in stress-exposed groups irrespective of the duration of stress. Higher serum glutathione peroxidase (GPx), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and hepatic and muscle lactate dehydrogenase (LDH) activities were observed in the THR0 groups; however, THR1 showed a lower activity of these enzymes. Increased malate dehydrogenase (MDH) and glucose-6-phosphate dehydrogenase (G6PDH) activity was recorded in the 1st and 2nd months stress-exposed groups. However, THR1 group exhibited a reduced MDH and G6PDH activity compared to THR0 group in 1st and 2nd months. Hence, it can be concluded that the duration of multiple stress exposure adversely affected the digestive and metabolic enzymes activities and feeding of 0.1% nutraceutical conglomerates could restore the activities of digestive and metabolic enzymes in rohu fingerlings.

Vital Carbohydrate and Lipid Metabolites in Serum Involved in Energy Metabolism during Pubertal Molt of Mud Crab (Scylla paramamosain)

Pubertal molt is a vital stage in the cultivation of mature female crabs in the aquacultural industry of Scylla paramamosain. Since fasting occurs during molting, which requires a large supply of energy, internal energy reserves are critical. However, the dynamics of energy supply during pubertal molt is not clear. This study focuses on the variations of carbohydrates and lipids in serum during the pubertal molt of S. paramamosain via a metabolomics approach. Eleven lipid or carbohydrate metabolic pathways were significantly influenced postmolt. A remarkable decrease in carbohydrates in serum suggested that free sugars were consumed for energy. A significant decrease in glucose and alpha-d-glucosamine 1-phosphate showed that chitin synthesis exhausted glycogen, resulting in insufficient glucose supply. An increase in l-carnitine and acetylcarnitine, and a significant decrease in 15 fatty acyls and 8 glycerophosphocholines in serum indicated that carnitine shuttle was stimulated, and β-oxidation was upregulated postmolt. In addition, astaxanthin, ponasterone A, and riboflavin in serum were significantly decreased postmolt. Eleven potential metabolite biomarkers were identified for pubertal molt. Taken together, carbohydrates and lipids were possibly major energy reserves in pubertal molt. This study suggests that an increase in carbohydrate and lipid levels in crab feed may alleviate the effects of fasting during molt and improve farm productivity in mature female crabs.

The effect of partial replacement of fish meal by soy protein concentrate on growth performance, immune responses, gut morphology and intestinal inflammation for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

An 8-week feeding trial was conducted to investigate the effects of partial replacement of fish meal by soy protein concentrate (SPC) on the growth performance, immune responses, intestine morphology and relation gene expression of intestinal inflammation for juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) (initial weight 12.5 ± 0.00 g). Eight isonitrogenous and isolipidic diets (48.61% protein and 11.17% lipid) were formulated by replacing 0% (the control), 11%, 22%, 33%, 44%, 55%, 66%, and 77% of fish meal (FM) with SPC, respectively (the eight dietary be named FM, S11, S22, S33, S44, S55, S66, and S77, respectively). With the replacement level increased, the final body weight, weight gain ratio (WGR), specific growth rate (SGR), and survival rate of fish were significantly decreased (P < 0.05) compared with the group FM. By contrast, the feed conversion ratio (FCR) of fish was significantly increased (P < 0.05) when the replacement level up to 44%. Partial FM replacement by SPC (ranging from 11% to 77%) substantially reduced (P < 0.05) the serum total protein, albumin, and total cholesterol contents compared with the group FM. Liver total superoxide dismutase, glutathione peroxidase, catalase activities, and total antioxidant capacity showed the same trend of gradual increase first and then decrease. Their highest values were found in the replacement levels of 55%, 33%, 22%, and 55% and were significantly higher (P < 0.05) than the control group. The lowest malondialdehyde content was observed in group S77 and was significantly lower (P < 0.05) than that of the control group. The complements C3 and C4 contents of fish fed with experimental diets (replacement level ranged from 11% to 66%) were significantly higher (P < 0.05) than the group FM. The liver lysozyme activity of the control group was the lowest and was significantly lower than that of other dietary treatments (P < 0.05). Villus length and muscle thickness in the intestine of fish were significantly lower (P < 0.05) than other groups when the replacement level exceeded 44%. With dietary replacement levels increased, the TLR22, MyD88, p65, pro-inflammatory cytokines (IL-1β, TNF-α, IL-12P40 and INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, epinecidin, MHCIIβ and hepcidin) mRNA levels in the proximal intestine were significantly up-regulated (P < 0.05). The TLR22, MyD88, p65, pro-inflammatory cytokines (IL-1β, TNF-α, IL-12P40 and INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, MHCIIβ and hepcidin) mRNA levels in the mid intestine were significantly up-regulated (P < 0.05). The mRNA levels of TLR22, anti-inflammatory cytokines (IL-1β, TNF-α, IL-12P40, INF-γ) and anti-inflammatory cytokines (TGF-β, IL-10, epinecidin, MHCIIβ and hepcidin) in the distal intestine were significantly down-regulated (P < 0.05). The mRNA expression of MyD88 and p65 mRNA were showed a tend increased first and then decreased, and the highest values were observed in group S33 and S55 (P < 0.05), respectively. Based on the present work, the correlation between WGR and FM replacement level with SPC was described using the broken-line model, which estimated the optimum FM replacement to 37.23% for juvenile hybrid grouper dietary.

Effect of water temperature on diel feeding, locomotion behaviour and digestive physiology in the sea cucumber Apostichopus japonicus

This study used controlled laboratory conditions to directly assess the role of water temperature in controlling diel feeding and locomotion behaviours, and digestive physiology, in the sea cucumber Apostichopus japonicus The results revealed that both the proportion of feeding individuals and ingestion rate were highest at 16°C. Regardless of water temperature, sea cucumbers appeared to be nocturnal and their peak feeding activity occurred at 00:00 h to 04:00 h. Tentacle insertion rate was not significantly correlated with water temperature (<24°C). In all temperature treatments except 24°C, the proportion of moving sea cucumbers was also observed to be higher at night than during the day. The water temperature above thermal threshold (24°C) for aestivation may alter the diel locomotion rhythm. The highest lipase and amylase activities were both observed at 20°C. The highest activities of lipase and amylase at all temperature treatments were observed at 22:00 h to 02:00 h, which was slightly earlier than the feeding peak. In conclusion, even in total darkness, A. japonicus showed more active feeding and moving activities, and higher digestive enzyme activities, at night than during the day. These results demonstrated that diel feeding and locomotion behaviours, at least in the short term, were not controlled by light or low water temperature (<24°C) but by an endogenous rhythm, and A. japonicus had the ability to optimize the digestive function for the coming feeding peak. These findings should provide valuable information for the development of the aquaculture of this species.

Temperature Affects the Use of Storage Fatty Acids as Energy Source in a Benthic Copepod (Platychelipus littoralis, Harpacticoida)

The utilization of storage lipids and their associated fatty acids (FA) is an important means for organisms to cope with periods of food shortage, however, little is known about the dynamics and FA mobilization in benthic copepods (order Harpacticoida). Furthermore, lipid depletion and FA mobilization may depend on the ambient temperature. Therefore, we subjected the temperate copepod Platychelipus littoralis to several intervals (3, 6 and 14 days) of food deprivation, under two temperatures in the range of the normal habitat temperature (4, 15 °C) and under an elevated temperature (24 °C), and studied the changes in FA composition of storage and membrane lipids. Although bulk depletion of storage FA occurred after a few days of food deprivation under 4 °C and 15 °C, copepod survival remained high during the experiment, suggesting the catabolization of other energy sources. Ambient temperature affected both the degree of FA depletion and the FA mobilization. In particular, storage FA were more exhausted and FA mobilization was more selective under 15 °C compared with 4 °C. In contrast, depletion of storage FA was limited under an elevated temperature, potentially due to a switch to partial anaerobiosis. Food deprivation induced selective DHA retention in the copepod's membrane, under all temperatures. However, prolonged exposure to heat and nutritional stress eventually depleted DHA in the membranes, and potentially induced high copepod mortality. Storage lipids clearly played an important role in the short-term response of the copepod P. littoralis to food deprivation. However, under elevated temperature, the use of storage FA as an energy source is compromised.

Marine invertebrate lipases: Comparative and functional genomic analysis

Lipases are key enzymes involved in lipid digestion, storage and mobilization of reserves during fasting or heightened metabolic demand. This is a highly conserved process, essential for survival. The genomes of five marine invertebrate species with distinctive digestive system were screened for the six major lipase families. The two most common families in marine invertebrates, the neutral an acid lipases, are also the main families in mammals and insects. The number of lipases varies two-fold across analyzed genomes. A high degree of orthology with mammalian lipases was observed. Interestingly, 19% of the marine invertebrate lipases have lost motifs required for catalysis. Analysis of the lid and loop regions of the neutral lipases suggests that many marine invertebrates have a functional triacylglycerol hydrolytic activity as well as some acid lipases. A revision of the expression profiles and functional activity on sequences in databases and scientific literature provided information regarding the function of these families of enzymes in marine invertebrates.

Digestive enzymes of two brachyuran and two anomuran land crabs from Christmas Island, Indian Ocean

The digestive ability of four sympatric land crabs species (the gecarcinids, Gecarcoidea natalis and Discoplax celeste and the anomurans, Birgus latro and Coenobita perlatus) was examined by determining the activity of their digestive enzymes. The gecarcinids are detritivores that consume mainly leaf litter; the robber crab, B. latro, is an omnivore that preferentially consumes items high in lipid, carbohydrate and/or protein; C. perlatus is also an omnivore/detritivore. All species possess protease, lipase and amylase activity for hydrolysing ubiquitous protein, lipid and storage polysaccharides (glycogen and starch). Similarly all species possess enzymes such as N-acetyl-β-D-glucosaminidase, the cellulases, endo-β-1,4-glucanase and β-glucohydrolase and hemicellulases, lichenase and laminarinase for the respective hydrolysis of structural substrates chitin, cellulose and hemicelluloses, lichenan and laminarin. Except for the enzyme activities of C. perlatus, enzyme activity could not be correlated to dietary preference. Perhaps others factors such as olfactory and locomotor ability and metabolic status may determine the observed dietary preferences. The digestive fluid of C. perlatus possessed higher endo-β-1,4-glucanase, lichenase and laminarinase activities compared to that of the other species. Thus, C. perlatus may be efficient at digestion of cellulose and hemicellulose within plant material. Zymography indicated that the majority of protease, lipase, phosphatase, amylase, endo-β-1,4-glucanase, β-glucohydrolase and N-acetyl-β-D-glucosaminidase isozymes were common to all species, and hence were inherited from a common aquatic ancestor. Differences were observed for the phosphatase, lipase and endo-β-1,4-glucanase isozymes. These differences are discussed in relation to phylogeny and possible evolution to cope with the adoption of a terrestrial diet.

Effect of cardiac glycosides from Nerium indicum on feeding rate, digestive enzymes activity and ultrastructural alterations of hepatopancreas in Pomacea canaliculata

Cardiac glycosides from Nerium indicum showed potent molluscicide activity against Pomacea canaliculata (GAS), but the toxicological mechanism is still far less understood. Effects of sublethal treatments of cardiac glycosides on feeding rate, digestive enzymes and ultrastructural alterations of the hepatopancreas in GAS were evaluated in this study. Exposure of GAS to sublethal concentrations of cardiac glycosides resulted in a significant reduction of feeding rate of GAS. The amylase, cellulose and protease activity were increase significantly at the end of 24 h followed by significant inhibition after 48 h of exposure while lipase activity was not affected significantly at the end of 24 h followed by a significant inhibition after 48 h of exposure during experimental period. The main ultrastructural alterations of hepatopancreas observed in snails under cardiac glycosides treatment comprised disruption of nuclear membrane, increased vesiculation and dilatation of endoplasmic reticulum, and vacuolization and swelling of mitochondrial compared to the untreated GAS. These results, for the first time, provide systematic evidences showing that cardiac glycosides seriously impairs the hepatopancreas tissues of GAS, resulting in inhibition of digestive enzymes activity and feeding rate and cause GAS death in the end.

Determining the diet of larvae of western rock lobster (Panulirus cygnus) using high-throughput DNA sequencing techniques

The Western Australian rock lobster fishery has been both a highly productive and sustainable fishery. However, a recent dramatic and unexplained decline in post-larval recruitment threatens this sustainability. Our lack of knowledge of key processes in lobster larval ecology, such as their position in the food web, limits our ability to determine what underpins this decline. The present study uses a high-throughput amplicon sequencing approach on DNA obtained from the hepatopancreas of larvae to discover significant prey items. Two short regions of the 18S rRNA gene were amplified under the presence of lobster specific PNA to prevent lobster amplification and to improve prey amplification. In the resulting sequences either little prey was recovered, indicating that the larval gut was empty, or there was a high number of reads originating from multiple zooplankton taxa. The most abundant reads included colonial Radiolaria, Thaliacea, Actinopterygii, Hydrozoa and Sagittoidea, which supports the hypothesis that the larvae feed on multiple groups of mostly transparent gelatinous zooplankton. This hypothesis has prevailed as it has been tentatively inferred from the physiology of larvae, captive feeding trials and co-occurrence in situ. However, these prey have not been observed in the larval gut as traditional microscopic techniques cannot discern between transparent and gelatinous prey items in the gut. High-throughput amplicon sequencing of gut DNA has enabled us to classify these otherwise undetectable prey. The dominance of the colonial radiolarians among the gut contents is intriguing in that this group has been historically difficult to quantify in the water column, which may explain why they have not been connected to larval diet previously. Our results indicate that a PCR based technique is a very successful approach to identify the most abundant taxa in the natural diet of lobster larvae.

Effect of fasting on digestive gland lipase transcripts expression in Penaeus vannamei

Digestive and intracellular lipases were studied in the digestive gland of whiteleg shrimp Penaeus vannamei. A partial sequence of the intracellular lipase was obtained from the digestive gland cDNA. The digestive and intracellular lipase mRNAs were detected differentially in different body parts of shrimp; digestive lipase mRNA is exclusively found in the digestive gland, suggesting a function as a digestive enzyme. Intracellular lipase mRNA was found in pleopods, digestive tube, uropods, hemocytes, muscle and gonad and its function was related to mobilization of energy reserves. The lipase transcripts in the digestive gland of shrimp, showed a dynamic expression at 120 h of fasting causing significant changes of digestive and intracellular mRNA, intracellular lipase mRNA were 3.33-fold higher than digestive lipase mRNA after fasting period, suggesting an alternate expression to maintain the lipid homeostasis under stress conditions.

Phenotypic plasticity of gut structure and function during periods of inactivity in Apostichopus japonicus

Apostichopus japonicus is a common sea cucumber that undergoes seasonal inactivity phases and ceases feeding during the summer months. We used this sea cucumber species as a model in which to examine phenotypic plasticity of the digestive tract in response to food deprivation. We measured the body mass, gross gut morphology and digestive enzyme activities of A. japonicus before, during, and after the period of inactivity to examine the effects of food deprivation on the gut structure and function of this animal. Individuals were sampled semi-monthly from June to November (10 sampling intervals over 178 days) across temperature changes of more than 18 degrees C. On 5 September, which represented the peak of inactivity and lack of feeding, A. japonicus decreased its body mass, gut mass and gut length by 50%, 85%, and 70%, respectively, in comparison to values for these parameters preceding the inactive period. The activities of amylase, cellulase and lipase decreased by 77%, 98%, and 35% respectively, in comparison to mean values for these enzymes in June, whereas pepsin activity increased two-fold during the inactive phase. Alginase and trypsin activities were variable and did not change significantly across the 178-day experiment. With the exception of amylase and cellulase, all body size indices and digestive enzyme activities recovered and even surpassed the mean values preceding the inactive phase during the latter part of the experiment (October-November). Principal Component Analysis (PCA) utilizing the digestive enzyme activity and body size index data divided the physiological state of this cucumber into four phases: an active stage, prophase of inactivity, peak inactivity, and a reversion phase. These phases are all consistent with previously suggested life stages for this species, but our data provide more defined characteristics of each phase. A. japonicus clearly exhibits phenotypic plasticity (or life-cycle staging) of the digestive tract during its annual inactive period.