Enzymatic capacities of metabolic fuel use in cuttlefish (Sepia officinalis) and responses to food deprivation: insight into the metabolic organization and starvation survival strategy of cephalopods

Increased seawater temperature triggers thermal, oxidative and metabolic response of Ostrea edulis, leading to anaerobiosis

Bivalves are among the marine organisms most influenced by climate change. Despite the flat oyster's Ostrea edulis high economic value, its culture is developed on a very small scale, since this species possesses a strong susceptibility to abiotic stressors. Due to climate change, temperature is one of the most critical environmental parameters for the welfare of the Mediterranean basin's marine inhabitants. The present study's purpose was to investigate the physiological performance of the Mediterranean's native O. edulis as it faces exposure to different temperatures. Since juveniles are more susceptible to abiotic stressors, this experimental procedure was focused on young individuals. The seawater temperatures studied included a standard control temperature of 21 °C (often observed in several marine areas throughout the Mediterranean), as well as increased seawater temperatures of 25 °C and 28 °C, occasionally occurring in shallow Mediterranean waters inhabited by bivalve spat. These were selected since the tissues of O. edulis becomes partly anaerobic in temperatures exceeding 26 °C, while cardiac dysfunction (arrhythmia) emerges at 28 °C. The results demonstrate that temperatures above 25 °C trigger both the transcriptional upregulation of hsp70 and hsp90, and the antioxidant genes Cu/Zn sod and catalase. Enhancement of thermal tolerance and increased defense against increased ROS production during thermal stress, were observed. As the intensity and duration of thermal stress increases, apoptotic damage may also occur. The increased oxidative and thermal stress incurred at the highest temperature of 28 °C, seemed to trigger the switch from aerobic to anaerobic metabolism, reflected by higher pepck mRNA expressions and lower ETS activity.

The impact of ascidian biofouling on the farmed Mediterranean mussel Mytilus galloprovincialis physiology and welfare, revealed by stress biomarkers

In biofouling communities, ascidians are among the most damaging species, presenting severe threats, such as depressed growth rates and decreased chances of lower survival, to shellfish aquaculture. However, little is known concerning the fouled shellfish physiology. In an effort to obtain information for the magnitude of stress caused by ascidians to farmed Mytilus galloprovincialis, five seasonal samplings took place in a mussel aquaculture farm suffering from ascidian biofoulants, in Vistonicos Bay, Greece. The dominant ascidian species were recorded and several stress biomarkers, including Hsp gene expression at both mRNA and protein levels, as well as MAPKs levels, and enzymatic activities of intermediate metabolism were examined. Almost all investigated biomarkers revealed elevated stress levels in fouled mussels compared to non-fouled. This enhanced physiological stress seems to be season-independent and can be attributed to the oxidative stress and/or feed deprivation caused by ascidian biofouling, thus illuminating the biological impact of this phenomenon.

Effect of starvation and pesticide exposure on neutral lipid composition of the digestive gland of males of the apple snails Pomacea canaliculata

Pollutants as well as starvation usually modify homeostasis of neutral lipids in aquatic organisms. However, studies on the simultaneous effects of both stressors are scarce. The aim of this study was to evaluate the effect of toxicant exposure under starvation conditions on neutral lipids of the freshwater snail Pomacea canaliculata, selected as the model organism. Starved adult male snails were exposed to sublethal concentration of the pesticide cypermethrin (100 µg/L) during 4 and 10 days. Fed snails were sacrificed at the onset of the experiment (T₀), along with starved snails exposed to the pesticide vehicle (ethanol) and another group without solvent served as controls. Total lipid content, neutral lipid classes, fatty acid composition, and pesticide accumulation were determined in the digestive gland of snails. The ethanol concentration used was not an additional stressful agent. As expected, starvation caused a decrease in neutral lipid content in the digestive gland of snails with respect to T₀ snails. Pesticide exposure caused, on the other hand, an increase in triacylglycerol content compared to ethanol exposure at day 10 of the bioassay. This increment correlated with the bioconcentration of cypermethrin, which was 47% higher by day 10 than by day 4. The fatty acid profile of triacylglycerols in the digestive gland was significantly altered under starvation and pesticide exposure. Stressed male snails showed the ability to preserve polyunsaturated fatty acids, as evidenced by their significant increase with respect to T₀ snails. These results suggest that the alteration of lipid homeostasis could be involved in an adaptive mechanism of aquatic organisms to lipophilic and obesogenic pollutants.

Sensitivity of histopathological and histochemical parameters in the digestive gland of the apple snail Pomacea canaliculata exposed to cypermethrin

The aim of this study was to evaluate the toxic effects of the pesticide cypermethrin (CYP) in the digestive gland of the apple snail, Pomacea canaliculata, analysing histological and histochemical alterations. Adult snails were exposed to sublethal CYP concentrations (10, 25, and 100 µg/L) under acute (1 day) and sub-chronic (14 days) conditions. Histological analyses of the morphology of the digestive gland were performed and a histopathological condition index (HI) was calculated. Also, both intracellular accumulation of lipofuscins (LF) and neutral lipids (NL) were evaluated. CYP exposure induced tissue damage to this organ, such as disorganisation of the connective tissue, fibrosis, haemocytic infiltration, atrophy, and necrosis under acute and sub-chronic conditions. These alterations, integrated into a single HI value, revealed notable CYP effects during both acute and sub-chronic exposures. Cell type replacement, measured as Vv_BAS, was only observed in the sub-chronic treatment. Under acute conditions, the pyrethroid affected NL accumulation at the highest concentration, while in sub-chronic conditions NL accumulation was only observed at the lowest concentrations. P. canaliculata also showed a dose-dependent response of LF under acute CYP exposure conditions. However, under sub-chronic conditions, this parameter was not sensitive to pesticide exposure. All these relevant structural lesions may affect the normal function of the digestive gland, even though the species presented additional mechanisms, as infiltration of hemocyte and basophilic cell hyperplasia, that help it to tolerate the exposure to pollutants. This study showed that some histological and histochemical parameters are sensitive in P. canaliculata at CYP concentrations to which the snail could be exposed in the environments it inhabits.

Proteomic changes in various organs of Haemaphysalis longicornis under long-term starvation

Haemaphysalis longicornis (Neumann), a tick of public health and veterinary importance, spend the major part of their life cycle off-host, especially the adult host-seeking period. Thus, they have to contend with prolonged starvation. Here, we investigated the underlying molecular mechanism of tick starvation endurance in the salivary glands, midguts, ovaries, and Malpighian tubules of starved H. longicornis ticks using the data-independent acquisition quantitative proteomic approach to study the proteome changes. Essential synthases such as glutamate synthase, citrate synthase, and ATP synthase were up-regulated probably due to increased proteolysis and amino acid catabolism during starvation. The up-regulation of succinate dehydrogenase, ATP synthase, cytochrome c oxidase, and ADP/ATP translocase closely fits with an increased oxidative phosphorylation function during starvation. The differential expression of superoxide dismutase, glutathione reductase, glutathione S-transferase, thioredoxin, and peroxiredoxin indicated fasting-induced oxidative stress. The up-regulation of heat shock proteins could imply the activation of a protective mechanism that checks excessive protein breakdown during starvation stress. The results of this study could provide useful information about the vulnerabilities of ticks that could aid in tick control efforts.

Ticks are a common blood-sucking parasite, which spread many pathogens that cause serious diseases such as Lyme disease to people. Ixodid ticks can take up to three blood meals in their life. During the long process of waiting for their host in the wild, they have evolved a strong ability to tolerate hunger, which should not take more than a year. To study these tenacious molecular regulatory mechanisms, we conducted the DIA quantitative proteomics technology to perform large-scale protein quantitative research on various tissues of Haemaphysalis longicornis starved for a long time. Through the analysis of thousands of proteins produced by the performed research, the results showed that many proteins in the ticks starved for a long time had expressed quantitative changes such as the increased expression of some synthase enzymes. The large amount of data provided by this study can help to better understand the molecular mechanism of ticks’ long-term hunger tolerance. Although this study focuses on finding possible mechanisms for tick starvation resistance at the protein level, the current findings may well have a bearing on research about special activities such as ultra long-distance space travel in the dormant state of the human body in the future.

Transcriptome Profiling Based on Larvae at Different Time Points After Hatching Provides a Core Set of Gene Resource for Understanding the Metabolic Mechanisms of the Brood-Care Behavior in Octopus ocellatus

The metabolic processes of organisms are very complex. Each process is crucial and affects the growth, development, and reproduction of organisms. Metabolism-related mechanisms in Octopus ocellatus behaviors have not been widely studied. Brood-care is a common behavior in most organisms, which can improve the survival rate and constitution of larvae. Octopus ocellatus carried out this behavior, but it was rarely noticed by researchers before. In our study, 3,486 differentially expressed genes (DEGs) were identified based on transcriptome analysis of O. ocellatus. We identify metabolism-related DEGs using GO and KEGG enrichment analyses. Then, we construct protein-protein interaction networks to search the functional relationships between metabolism-related DEGs. Finally, we identified 10 hub genes related to multiple gene functions or involved in multiple signal pathways and verified them using quantitative real-time polymerase chain reaction (qRT-PCR). Protein-protein interaction networks were first used to study the effects of brood-care behavior on metabolism in the process of growing of O. ocellatus larvae, and the results provide us valuable genetic resources for understanding the metabolic processes of invertebrate larvae. The data lay a foundation for further study the brood-care behavior and metabolic mechanisms of invertebrates.

Combined metabolomics and histological analysis of the tissues from cuttlefish Sepia pharaonis exposed to inking stress

Inking is part of a defensive stress response in cephalopods (cuttlefish, squid, and octopus). Some individual cuttlefish (Sepia pharaonis) die after continued stress and inking; however, the physiological effects of cephalopods in response to stress and inking remain unknown. The present study investigated the metabolic profile and discussed the physiological roles of S. pharaonis tissues in response to continuous inking using the ¹H NMR spectroscopy coupled with multivariate data analysis. A total of 50 metabolites, including amino acids, organic osmolytes, nucleotides, energy storage compounds, and obvious tissue-specific metabolites induced by inking stress, were identified in S. pharaonis tissues. Exposure to inking stress had different effects on the levels of the studied metabolites, for example, the levels of isoleucine, trimethylamine-N-oxide, and betaine increased, but those of arginine and ATP decreased in the liver; inosine and lactate were accumulated whereas glutamate and choline were depleted in the gill; the levels of lactate and isoleucine were elevated but those of arginine and glycogen were depleted in the muscle tissue. Furthermore, the corresponding metabolic pathways of the characteristic metabolites indicated major changes in the functions of these metabolites. Histological changes in the studied tissues revealed liver lobule damage immediately after inking, with the presence of disordered epithelial cells and partial cell necrosis in the gill. Our results demonstrated that a combination of metabolomics and histological analyses could provide molecular-level insights for elucidating the defense response of cuttlefish against predators.

Continuous Inking Affects the Biological and Biochemical Responses of Cuttlefish Sepia pharaonis

Several marine mollusks, including cephalopods (cuttlefish, squid, and octopus) and gastropods (e.g., sea hares), can release a colored ink secretion when chased by predators or stimulated. Ink release is part of a defensive response, but the threshold for the biochemical responses caused by stimulation is unknown. The present study aimed to reveal antipredator responses of cuttlefish, such as escaping via inking and/or jetting, and to investigate its biological and biochemical responses to continuous ink release. Results showed that the behavioral responses to continuous ink release mainly manifested as blazing body pattern changes. Cuttlefish escaped from predators covered by jetting/inking and warned the potential threats by displaying a unique body pattern. Moreover, persistent inking in the presence of an overt stimulus caused uncontrollable ink release from the ink duct/anal canal (loss of control). This study first verified the characteristics of the cuttlefish ink solution, prepared a standard curve of ink solution concentrations, and fitted the relationship function between the release frequency and the released ink weight. Biological statistics indicated that cuttlefish has the ability to continuously release ink (releasing ∼90% of the ink from the ink sac) and that the individuals adapted well during the recovery period. However, re-releasing ink would result in "overexploitation" and high mortality. Hexokinase (HK), pyruvate kinase (PK), and superoxide dismutase (SOD) activities, as well as malondialdehyde (MDA) concentration increased or remained stable in different tissues after releasing ink. The expression of heat shock protein 90 and arginine kinase (AK) were upregulated by stimuli in all tissues. Biochemical changes indicated that continuous inking not only consumed considerable energy but also damaged the tissues. In summary, cuttlefish released almost 90% of their ink for active defense against predators, and it took ∼30 days for the ink sac to be refilled, but "overexploitation" resulted in serious physiological damage. These findings will be helpful to further study the defense and ink release mechanisms and to consider animal health and welfare when using cephalopods as experimental animals and for aquaculture practices.

Interrelationship Between Contractility, Protein Synthesis and Metabolism in Mantle of Juvenile Cuttlefish (Sepia officinalis)

Young juvenile cuttlefish (Sepia officinalis) can grow at rates as high as 12% body weight per day. How the metabolic demands of such a massive growth rate impacts muscle performance that competes for ATP is unknown. Here, we integrate aspects of contractility, protein synthesis, and energy metabolism in mantle of specimens weighing 1.1 g to lend insight into the processes. Isolated mantle muscle preparations were electrically stimulated and isometric force development monitored. Preparations were forced to contract at 3 Hz for 30 s to simulate a jetting event. We then measured oxygen consumption, glucose uptake and protein synthesis in the hour following the stimulation. Protein synthesis was inhibited with cycloheximide and glycolysis was inhibited with iodoacetic acid in a subset of samples. Inhibition of protein synthesis impaired contractility and decreased oxygen consumption. An intact protein synthesis is required to maintain contractility possibly due to rapidly turning over proteins. At least, 41% of whole animal ṀO₂ is used to support protein synthesis in mantle, while the cost of protein synthesis (50 μmol O₂ mg protein^–1) in mantle was in the range reported for other aquatic ectotherms. A single jetting challenge stimulated protein synthesis by approximately 25% (2.51–3.12% day^–1) over a 1 h post contractile period, a similar response to that which occurs in mammalian skeletal muscle. Aerobic metabolism was not supported by extracellular glucose leading to the contention that at this life stage either glycogen or amino acids are catabolized. Regardless, an intact glycolysis is required to support contractile performance and protein synthesis in resting muscle. It is proposed that glycolysis is needed to maintain intracellular ionic gradients. Intracellular glucose at approximately 3 mmol L^–1 was higher than the 1 mmol L^–1 glucose in the bathing medium suggesting an active glucose transport mechanism. Octopine did not accumulate during a single physiologically relevant jetting challenge; however, octopine accumulation increased following a stress that is sufficient to lower Arg-P and increase free arginine.

Global impact of diet and temperature over aquaculture of Octopus vulgaris paralarvae from a transcriptomic approach

Common octopus, Octopus vulgaris, is an economically important cephalopod species. However, its rearing under captivity is currently challenged by massive mortalities previous to their juvenile stage due to nutritional and environmental factors. Dissecting the genetic basis and regulatory mechanism behind this mortality requires genomic background knowledge. A transcriptomic sequencing of 10 dph octopus paralarvae from different experimental conditions was constructed via RNA-seq. A total of 613,767,530 raw reads were filtered and de novo assembled into 363,527 contigs of which 82,513 were annotated in UniProt carrying also their GO and KEGG information. Differential gene expression analysis was carried out on paralarvae reared under different diet regimes and temperatures, also including wild paralarvae. Genes related to lipid metabolism exhibited higher transcriptional levels in individuals whose diet includes crustacean zoeas, which had an impact over their development and immune response capability. High temperature induces acclimation processes at the time that increase metabolic demands and oxidative stress. Wild individuals show an expression profile unexpectedly similar to Artemia fed individuals. Proteomic results support the hypothesis revealed by transcriptional analysis. The comparative study of the O. vulgaris transcriptomic profiles allowed the identification of genes that deserve to be further studied as candidates for biomarkers of development and health. The results obtained here on the transcriptional variations of genes caused by diet and temperature will provide new perspectives in understanding the molecular mechanisms behind nutritional and temperature requirements of common octopus that will open new opportunities to deepen in paralarvae rearing requirements.

Acute-Stress Biomarkers in Three Octopodidae Species After Bottom Trawling

Several Octopodidae species have a great potential for the diversification of worldwide aquaculture. Unfortunately, the lack of stress-related biomarkers in this taxon results an obstacle for its maintenance in conditions where animal welfare is of paramount relevance. In this study, we made a first approach to uncover physiological responses related to fishing capture in Eledone moschata, Eledone cirrhosa, and Octopus vulgaris. Captured octopus from all three species were individually maintained in an aquaculture system onboard of oceanographic vessel in south-western waters of Europe. Haemolymph plasma and muscle were collected in animals at the moment of capture, and recovery was evaluated along a time-course of 48 h in Eledone spp., and 24 h for O. vulgaris. Survival rates of these species captured in spring and autumn were evaluated. Physiological parameters such as plasma pH, total CO2, peroxidase activity, lysozyme, hemocyanin, proteases, pro-phenoloxidase, anti-proteases, free amino acids, lactate and glucose levels, as well as muscle water percentage, free amino acids, lactate, glycogen and glucose values were analyzed. The immune system appears to be compromised in these species due to capture processes, while energy metabolites were mobilized to face the acute-stress situation, but recovery of all described parameters occurs within the first 24 h after capture. Moreover, this situation exerts hydric balance changes, as observed in the muscle water, being these responses depending on the species assessed. In conclusion, three Octopodidae species from south-western waters of Europe have been evaluated for stress-related biomarkers resulting in differentiated mechanisms between species. This study may pave the way to further study the physiology of stress in adult octopuses and develop new methodologies for their growth in aquaculture conditions.

Do squid breathe through their skin?

Squid are thought to obtain a large portion of their oxygen via simple diffusion across the skin in addition to uptake at the gills. Although this hypothesis has support from indirect evidence and is widely accepted, no empirical examinations have been conducted to assess the validity of this hypothesis. In this study, we examined cutaneous respiration in two squid species, Doryteuthis pealeii and Lolliguncula brevis, using a divided chamber to physically separate the mantle cavity and gills from the outer mantle surface. We measured oxygen consumption and ammonia excretion rates in the two compartments and found that, at rest, squid only obtain enough oxygen cutaneously to meet the demand of the skin tissue locally (12% of total) and excrete little ammonia across the skin. The majority of oxygen is obtained via the traditional branchial pathway. In light of these findings, we re-examine and discuss the indirect evidence that has supported the cutaneous respiration hypothesis.

The Digestive Tract of Cephalopods: a Neglected Topic of Relevance to Animal Welfare in the Laboratory and Aquaculture

Maintenance of health and welfare of a cephalopod is essential whether it is in a research, aquaculture or public display. The inclusion of cephalopods in the European Union legislation (Directive 2010/63/EU) regulating the use of animals for scientific purposes has prompted detailed consideration and review of all aspects of the care and welfare of cephalopods in the laboratory but the information generated will be of utility in other settings. We overview a wide range of topics of relevance to cephalopod digestive tract physiology and their relationship to the health and welfare of these animals. Major topics reviewed include: (i) Feeding cephalopods in captivity which deals with live food and prepared diets, feeding frequency (ad libitum vs. intermittent) and the amount of food provided; (ii) The particular challenges in feeding hatchlings and paralarvae, as feeding and survival of paralarvae remain major bottlenecks for aquaculture e.g., Octopus vulgaris; (iii) Digestive tract parasites and ingested toxins are discussed not only from the perspective of the impact on digestive function and welfare but also as potential confounding factors in research studies; (iv) Food deprivation is sometimes necessary (e.g., prior to anesthesia and surgery, to investigate metabolic control) but what is the impact on a cephalopod, how can it be assessed and how does the duration relate to regulatory threshold and severity assessment? Reduced food intake is also reviewed in the context of setting humane end-points in experimental procedures; (v) A range of experimental procedures are reviewed for their potential impact on digestive tract function and welfare including anesthesia and surgery, pain and stress, drug administration and induced developmental abnormalities. The review concludes by making some specific recommendations regarding reporting of feeding data and identifies a number of areas for further investigation. The answer to many of the questions raised here will rely on studies of the physiology of the digestive tract.

Time Course of Metabolic Capacities in Paralarvae of the Common Octopus, Octopus vulgaris, in the First Stages of Life. Searching Biomarkers of Nutritional Imbalance

The culture of the common octopus (Octopus vulgaris) is promising since the species has a relatively short lifecycle, rapid growth, and high food conversion ratios. However, recent attempts at successful paralarvae culture have failed due to slow growth and high mortality rates. Establishing an optimal nutritional regime for the paralarvae seems to be the impeding step in successful culture methods. Gaining a thorough knowledge of food regulation and assimilation is essential for paralarvae survival and longevity under culture conditions. The aim of this study, then, was to elucidate the characteristic metabolic organization of octopus paralarvae throughout an ontogenic period of 12 days post-hatching, as well as assess the effect of diet enrichment with live prey containing abundant marine phospholipids. Our results showed that throughout the ontogenic period studied, an increase in anaerobic metabolism took place largely due to an increased dependence of paralarvae on exogenous food. Our studies showed that this activity was supported by octopine dehydrogenase activity, with a less significant contribution of lactate dehydrogenase activity. Regarding aerobic metabolism, the use of amino acids was maintained for the duration of the experiment. Our studies also showed a significant increase in the rate of oxidation of fatty acids from 6 days after-hatching. A low, although sustained, capacity for de novo synthesis of glucose from amino acids and glycerol was also observed. Regardless of the composition of the food, glycerol kinase activity significantly increased a few days prior to a massive mortality event. This could be related to a metabolic imbalance in the redox state responsible for the high mortality. Thus, glycerol kinase might be used as an effective nutritional and welfare biomarker. The studies in this report also revealed the important finding that feeding larvae with phospholipid-enriched Artemia improved animal viability and welfare, significantly increasing the rate of survival and growth of paralarvae.

The Digestive Tract of Cephalopods: Toward Non-invasive In vivo Monitoring of Its Physiology

Ensuring the health and welfare of animals in research is paramount, and the normal functioning of the digestive tract is essential for both. Here we critically assess non- or minimally-invasive techniques which may be used to assess a cephalopod's digestive tract functionality to inform health monitoring. We focus on: (i) predatory response as an indication of appetitive drive; (ii) body weight assessment and interpretation of deviations (e.g., digestive gland weight loss is disproportionate to body weight loss in starvation); (iii) oro-anal transit time requiring novel, standardized techniques to facilitate comparative studies of species and diets; (iv) defecation frequency and analysis of fecal color (diet dependent) and composition (parasites, biomarkers, and cytology); (v) digestive tract endoscopy, but passage of the esophagus through the brain is a technical challenge; (vi) high resolution ultrasound that offers the possibility of imaging the morphology of the digestive tract (e.g., food distribution, indigestible residues, obstruction) and recording contractile activity; (vii) needle biopsy (with ultrasound guidance) as a technique for investigating digestive gland biochemistry and pathology without the death of the animal. These techniques will inform the development of physiologically based assessments of health and the impact of experimental procedures. Although intended for use in the laboratory they are equally applicable to cephalopods in public display and aquaculture.

Hypoxic Induced Decrease in Oxygen Consumption in Cuttlefish (Sepia officinalis) Is Associated with Minor Increases in Mantle Octopine but No Changes in Markers of Protein Turnover

The common cuttlefish (Sepia officinalis), a dominant species in the north-east Atlantic ocean and Mediterranean Sea, is potentially subject to hypoxic conditions due to eutrophication of coastal waters and intensive aquaculture. Here we initiate studies on the biochemical response to an anticipated level of hypoxia. Cuttlefish challenged for 1 h at an oxygen level of 50% dissolved oxygen saturation showed a decrease in oxygen consumption of 37% associated with an 85% increase in ventilation rate. Octopine levels were increased to a small but significant level in mantle, whereas there was no change in gill or heart. There were no changes in mantle free glucose or glycogen levels. Similarly, the hypoxic period did not result in changes in HSP70 or polyubiquinated protein levels in mantle, gill, or heart. As such, it appears that although there was a decrease in metabolic rate there was only a minor increase in anaerobic metabolism as evidenced by octopine accumulation and no biochemical changes that are hallmarks of alterations in protein trafficking. Experiments with isolated preparations of mantle, gill, and heart revealed that pharmacological inhibition of protein synthesis could decrease oxygen consumption by 32 to 42% or Na⁺/K⁺ ATPase activity by 24 to 54% dependent upon tissue type. We propose that the decrease in whole animal oxygen consumption was potentially the result of controlled decreases in the energy demanding processes of both protein synthesis and Na+/K+ ATPase activity.