Impact of hypoxia stress on the physiological responses of sea cucumber Apostichopus japonicus: respiration, digestion, immunity and oxidative damage

Evidence of size-dependent toxicity of polystyrene nano- and microplastics in sea cucumber Apostichopus japonicus (Selenka, 1867) during the intestinal regeneration

Microplastics are ubiquitous pollutants in the global marine environment. However, few studies have adequately explored the different toxic mechanisms of microplastics (MPs) and nanoplastics (NPs) in aquatic organisms. The sea cucumber, Apostichopus japonicus, is a key organism in the marine benthic ecosystem due to its crucial roles in biogeochemical cycles and food web. This study investigated the bioaccumulation and adverse effects of polystyrene micro- and nanoplastics (PS-M/NPs) of different sizes (20 μm, 1 μm and 80 nm) in the regenerated intestine of A. japonicus using multi-omics analysis. The results showed that after 30-day exposure at the concentration of 0.1 mg L-1, PS-MPs and PS-NPs accumulated to 155.41-175.04 μg g-1 and 337.95 μg g-1, respectively. This excessive accumulation led to increased levels of antioxidases (SOD, CAT, GPx and T-AOC) and reduced activities of immune enzymes (AKP, ACP and T-NOS), indicating oxidative damage and compromised immunity in the regenerated intestine. PS-NPs had more profound negative impacts on cell proliferation and differentiation compared to PS-MPs. Transcriptomic analysis revealed that PS-NPs primarily affected pathways related to cellular components, e.g., ribosome, and oxidative phosphorylation. In comparison, PS-MPs had greater influences on actin-related organization and organic compound metabolism. In the PS-M/NPs-treated groups, differentially expressed metabolites were mainly amino acids, fatty acids, glycerol phospholipid, and purine nucleosides. Additionally, microbial community reconstruction in the regenerated intestine was severely disrupted by the presence of PS-M/NPs. In the PS-NPs group, Burkholderiaceae abundance significantly increased while Rhodobacteraceae abundance decreased. Correlation analyses demonstrated that intestinal regeneration of A. japonicus was closely linked to its enteric microorganisms. These microbiota-host interactions were notably affected by different PS-M/NPs, with PS-NPs exposure causing the most remarkable disruption of mutual symbiosis. The multi-omic approaches used here provide novel insights into the size-dependent toxicity of PS-M/NPs and highlight their detrimental effects on invertebrates in M/NPs-polluted marine benthic ecosystems.

Sea cucumber physiological response to abiotic stress: Emergent contaminants and climate change

The ocean is facing a multitude of abiotic stresses due to factors such as climate change and pollution. Understanding how organisms in the ocean respond to these global changes is vital to better predicting consequences. Sea cucumbers are popular echinoderms with multiple ecological, nutritional, and pharmaceutical benefits. Here, we reviewed the effects of environmental change on an ecologically important echinoderm of the ocean, aiming to understand their response better, which could facilitate healthy culture programs under environmental changes and draw attention to knowledge gaps. After screening articles from the databases, 142 studies were included on the influence of emergent contaminants and climate variation on the early developmental stages and adults of sea cucumbers. We outlined the potential mechanism underlying the physiological response of sea cucumbers to emerging contaminants and climate change. It can be concluded that the physiological response of sea cucumbers to emergent contaminants differs from their response to climate change. Sea cucumbers could accumulate pollutants in their organs but are aestivated when exposed to extreme climate change. Research showed that the physiological response of sea cucumbers to pollutants indicates that these pollutants impair critical physiological processes, particularly during the more susceptible early phases of development compared to adults, and the accumulation of these pollutants in adults is often observed. For climate change, sea cucumbers showed gradual adaptation to the slight variation. However, sea cucumbers undergo aestivation under extreme conditions. Based on this review, critical suggestions for future research are presented, and we call for more efforts focusing on the co-occurrence of different stressors to extend the knowledge regarding the effects of environmental changes on these economically and ecologically important species.

Investigating Immunotoxicity in Black Carp (Mylopharyngodon piceus) Fingerlings Exposed to Niclosamide

Niclosamide (NIC) is a potent salicylanilide molluscicide/helminthicide commonly utilized for parasite and mollusc control in aquatic environments. Due to its persistent presence in water bodies, there is growing concern regarding its impact on aquatic organisms, yet this remains inadequately elucidated. Consequently, this study aims to assess the hepatotoxic effects and detoxification capacity of black carp (Mylopharyngodon piceus) in a semi-static system, employing various parameters for analysis. NIC was applied to juvenile black carp at three different concentrations (0, 10 and 50 μg/L) for 28 days in an environmentally realistic manner. Exposure to 50 μg/L NIC resulted in an increase in hepatic lysozyme (LYZ), alkaline phosphatase (ALP), and complement 4 (C4) levels while simultaneously causing a decrease in peroxidase (POD) activity. Additionally, NIC exposure exhibited a dose-dependent effect on elevating serum levels of LYZ, ALP, complement 3 (C3), C4, and immunoglobulin T (IgT). Notably, the mRNA levels of immune-related genes tnfα, il8, and il6, as well as nramp and leap2, were upregulated in fish exposed to NIC. RNA-Seq analysis identified 219 differentially expressed genes (DEGs) in M. piceus after NIC exposure, with 94 upregulated and 125 downregulated genes. KEGG and GO analyses showed enrichment in drug metabolism pathways and activities related to oxidoreductase, lip oprotein particles, and cholesterol transport at 50 μg/L NIC. Additionally, numerous genes associated with lipid metabolism, oxidative stress, and innate immunity were upregulated in NIC-exposed M. piceus. Taken together, these findings indicate that NIC has the potential to cause hepatotoxicity and immunotoxicity in M. piceus. This research offers important insights for further understanding the impact of molluscicide/helminthicide aquatic toxicity in ecosystems.

Importance of the ECM-receptor interaction for adaptive response to hypoxia based on integrated transcription and translation analysis

Low dissolved oxygen (LO) conditions represent a major environmental challenge to marine life, especially benthic animals. For these organisms, drastic declines in oxygen availability (hypoxic events) can trigger mass mortality events and thus, act as agents of selection influencing the evolution of adaptations. In sea cucumbers, one of the most successful groups of benthic invertebrates, the exposure to hypoxic conditions triggers adaptive adjustments in metabolic rates and behaviour. It is unclear, however, how these adaptive responses are regulated and the genetic mechanisms underpinning them. Here, we addressed this knowledge gap by assessing the genetic regulation (transcription and translation) of hypoxia exposure in the sea cucumber Apostichopus japonicus. Transcriptional and translational gene expression profiles under short- and long-term exposure to low oxygen conditions are tightly associated with extracellular matrix (ECM)-receptor interaction in which laminin and collagen likely have important functions. Finding revealed that genes with a high translational efficiency (TE) had a relatively short upstream open reading frame (uORF) and a high uORF normalized minimal free energy, suggesting that sea cucumbers may respond to hypoxic stress via altered TE. These results provide valuable insights into the regulatory mechanisms that confer adaptive capacity to holothurians to survive oxygen deficiency conditions and may also be used to inform the development of strategies for mitigating the harmful effects of hypoxia on other marine invertebrates facing similar challenges.

Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.

Living in a hypoxic world: A review of the impacts of hypoxia on aquaculture

Hypoxia is a harmful result of anthropogenic climate change. With the expansion of global low-oxygen zones (LOZs), many organisms have faced unprecedented challenges affecting their survival and reproduction. Extensive research has indicated that oxygen limitation has drastic effects on aquatic animals, including on their development, morphology, behavior, reproduction, and physiological metabolism. In this review, the global distribution and formation of LOZs were analyzed, and the impacts of hypoxia on aquatic animals and the molecular responses of aquatic animals to hypoxia were then summarized. The commonalities and specificities of the response to hypoxia in aquatic animals in different LOZs were discussed lastly. In general, this review will deepen the knowledge of the impacts of hypoxia on aquaculture and provide more information and research directions for the development of fishery resource protection strategies.

Copper exposure does not alter the ability of intertidal sea cucumber Cucumaria miniata to tolerate emersion during low tide

Intertidal animals experience cycles of tidal emersion from water and are vulnerable to copper (Cu) exposure due to anthropogenic toxicant input into marine waters. Both emersion and Cu toxicity can cause damage to physiological processes like aerobic metabolism, ammonia excretion, and osmoregulation, but the interactions of the combination of these two stressors on marine invertebrates are understudied. Mixed effects of 96 h of low and high Cu exposure (20 and 200 μg/L) followed by 6 h of tidal emersion were evaluated on the intertidal sea cucumber Cucumaria miniata. The respiratory tree accumulated the highest concentrations of Cu, followed by the introvert retractor muscle, body wall, and coelomic fluid. Emersion affected accumulation of Cu, perhaps by inhibiting excretion. 200 μg/L of Cu increased lactate production in the respiratory tree, indicative of damaged aerobic metabolism. Cu diminished ammonia excretion, but emersion increased oxygen uptake and ammonia excretion upon re-immersion. The combination of the two stressors did not have any interactive effects on metabolism or ammonia excretion. Neither Cu exposure nor emersion altered ion (sodium, potassium, calcium, magnesium) content of the coelomic fluid. Overall, results of this study suggest that Cu exposure does not alter C. miniata's high tolerance to emersion, and some potential strategies that this species uses to overcome environmental stress are illuminated.

Adaptation to hypoxic stress involves amino acid metabolism: A case in sea cucumber

Low dissolved oxygen (LO) in seawater negatively affects aquatic animals and has received considerable attention. However, there is still much to learn about how echinoderms, which are keystone species in benthic ecosystems, respond to hypoxic stress. Here, we detected differentially expressed metabolites (DEMs) in sea cucumber (Apositchopus japonicus) between normoxic conditions (NC group) and hypoxic conditions (2 mg L^-1) for 3 and 7 days (i.e., LO3 and LO7 groups). A total of 156, 180, and 95 DEMs were found in the NC versus LO3, NC vs. LO7, and LO3 vs. LO7 comparisons, respectively. Amino acids were the most abundant class of DEMs, and "biosynthesis of amino acids" was an enriched pathway in all three comparisons. Most of the enriched metabolite sets under hypoxic stress were related to metabolism. As the duration of the hypoxia treatment extended, the metabolism-related process maintained an upward trend, and signaling pathways maintained a downward trend. Thus, metabolism-related processes are affected in hypoxia-stressed sea cucumber, and amino acid metabolism is the most important process for adaption to hypoxic conditions, potentially function in osmotic regulation and energy regulation. Our results shed light on the adaptative strategies of sea cucumber to challenging environmental conditions.

Oxidative stress responses in the respiratory tree and the body wall of sea cucumber Apostichopus japonicus (Selenka) to high temperature

Sea cucumber Apostichopus japonicas (Selenka) is one of the important aquaculture species distributed in northern China. In recent years, global warming caused frequent high temperature weather in summer in northern China, resulting in dramatic losses of the sea cucumber aquaculture industry. In the present study, we focused on the effect of oxidative stress in Apostichopus japonicus (Selenka) subjected to high temperature stress. Sea cumbers were exposed to the control (16 °C), and high temperature treatments (20 °C, 24 °C, and 28 °C) for 7 days. Then, reactive oxygen species (ROS) level, superoxide dismutase (SOD) activity, catalase (CAT) activity, peroxidase (POD) activity, reduced glutathione (GSH) content, malondialdehyde (MDA) content and 8-hydroxy-2'-deoxyguanosine (8-OHdG) level in the respiratory tree and body wall were detected, respectively. Results showed that 24 °C and 28 °C acute exposure induced the elevation of ROS level, SOD, CAT, POD activities, GSH content, MDA content and 8-OHdG level in the respiratory tree of sea cucumber. In contrast, no significant changes were observed for ROS and 8-OHdG levels in the body wall of sea cucumber, while the antioxidants including SOD, CAT, POD, and GSH decreased to some extent. Moreover, MDA content exhibited a noticeable increase in the body wall, similarly to that in the respiratory tree, indicating that high temperature could induce severe lipid peroxidation in two tissues. Considering the differences in various biomarkers measured in two tissues, respiratory tree might be more susceptible to the high temperature changes compared to the body wall. Our findings may help understand the oxidative stress response to high temperature in the respiratory tree and the body wall in A. japonicus.

Marine heatwaves and upwelling shape stress responses in a keystone predator

Climate change increases the frequency and intensifies the magnitude and duration of extreme events in the sea, particularly so in coastal habitats. However, the interplay of multiple extremes and the consequences for species and ecosystems remain unknown. We experimentally tested the impacts of summer heatwaves of differing intensities and durations, and a subsequent upwelling event on a temperate keystone predator, the starfish Asterias rubens. We recorded mussel consumption throughout the experiment and assessed activity and growth at strategically chosen time points. The upwelling event overall impaired starfish feeding and activity, likely driven by the acidification and low oxygen concentrations in the upwelled seawater. Prior exposure to a present-day heatwave (+5°C above climatology) alleviated upwelling-induced stress, indicating cross-stress tolerance. Heatwaves of present-day intensity decreased starfish feeding and growth. While the imposed heatwaves of limited duration (9 days) caused slight impacts but allowed for recovery, the prolonged (13 days) heatwave impaired overall growth. Projected future heatwaves (+8°C above climatology) caused 100% mortality of starfish. Our findings indicate a positive ecological memory imposed by successive stress events. Yet, starfish populations may still suffer extensive mortality during intensified end-of-century heatwave conditions.

Exosomal microRNAs regulate the heat stress response in sea cucumber Apostichopus japonicus

Exosomes are small extracellular vesicles that contain nucleic acids such as microRNAs and may participate in important biological processes. We made the initial report of exosomes from sea cucumber Apostichopus japonicus, that were classically cup-shaped and had an average size of 74.65 nm, and identified specific exosome biomarkers (HSP70, TSG101, and CD9). We explored changes in the global expression of microRNAs in exosomes from the commercially important A. japonicus under normal conditions and heat-stressed conditions for 3 and 7 d. We found that heat stress increased exosome production and modified the expression profiles of the microRNAs that they contained. Novel_mir31, novel_mir132, novel_mir26, miR-92_1, and novel_mir27 were commonly found to be differentially expressed in three comparison groups, indicating their importance in the heat stress response. The microRNA expression levels were validated by qPCR. Function analysis of the target genes of these microRNAs indicated they were involved mainly in replication and repair in the initial response of A. japonicus to heat stress exposure. Conversely, during acclimation to the high temperature conditions, the target genes of the differentially expressed microRNAs were primarily involved in metabolism adjustments. Our results will contribute to a better understanding of the regulatory roles of exosomes in sea cucumber, and provide insights into the functions of sea cucumber exosome-shuttled microRNAs against environmental stresses exacerbated by global warming.

Comparative transcriptome analysis reveals physiological responses in liver tissues of Epinephelus coioides under acute hypoxia stress

Hypoxia is a common stressor for aquatic animals, including Epinephelus coioides, with a considerable impact on sustainable aquaculture. E. coioides is a widely consumed fish in China owing to its high nutritious value and taste. However, water hypoxia caused by high density culture process has become a great threat to E. coioides culture, and its response to hypoxia stress has not been discussed before. Therefore, the aim of this study was to examine the response of E. coioides to acute hypoxia using transcriptomic techniques. To this end, RNA sequencing was performed on the liver tissues of fish exposed to normoxic and hypoxic conditions for 1 h. The results presented 503 differentially expressed genes (DEGs) in the liver tissue of fish exposed to hypoxic condition compared with those in the normoxic group. Enrichment analysis using the Gene Ontology database showed that the DEGs were mainly enriched for functions related to cell apoptosis signaling pathways, insulin resistance, antioxidant enzymes, and glycolysis/gluconeogenesis signaling pathways. KEGG enrichment analysis showed that HIF-1, PI3K-AKT, IL-17, NF-kappa B, and MAPK signaling pathways were significantly enriched by the DEGs. The DEGs were mainly involved in immune response, inflammatory response, cell apoptosis regulation, energy metabolism, and substance metabolism. Additionally, the hypoxia response in E. coioides was mainly regulated via the PI3K-AKT-HIF-1 signaling axis. Overall, the findings of this study contribute to the understanding of hypoxia stress response in E. coioides, and provides target genes for breeding hypoxia-tolerant Epinephelus spp.

Metabolomic Profiling Analysis of Physiological Responses to Acute Hypoxia and Reoxygenation in Juvenile Qingtian Paddy Field Carp Cyprinus Carpio Var Qingtianensis

The Qingtian paddy field carp (Cyprinus carpio var qingtianensis) is a local carp cultivated in the rice field of Qingtian county, Zhejiang province, China. The paddy field environment is distinct from the pond environment. Due to the inability to artificially increase oxygen, the dissolved oxygen greatly changes during the day. Therefore, investigating the physiological regulation to the changes of acute dissolved oxygen in Qingtian paddy field carp (PF-carp) will dramatically clarify how it adapts to the paddy breeding environment. The high tolerance of Qingtian paddy field carp to hypoxia makes it an ideal organism for studying molecular regulatory mechanisms during hypoxia process and reoxygenation following hypoxia in fish. In this study, we compared the changes of metabolites in the hepatopancreas during hypoxia stress and the following reoxygenation through comparative metabolomics. The results showed 131 differentially expressed metabolites between the hypoxic groups and control groups. Among them, 95 were up-regulated, and 36 were down-regulated. KEGG Pathway enrichment analysis showed that these differential metabolites were mainly involved in regulating lipid, protein, and purine metabolism PF-carps could require energy during hypoxia by enhancing the gluconeogenesis pathway with core glutamic acid and glutamine metabolism. A total of 63 differentially expressed metabolites were screened by a comparison between the reoxygenated groups and the hypoxic groups. Specifically, 15 were up-regulated, and 48 were down-regulated. The KEGG Pathway enrichment analysis supported that PF-carp could continue to gain energy by consuming glutamic acid and the glutamine accumulated during hypoxia and simultaneously weaken the ammonia-transferring effect of amino acids and the toxicity of ammonia. By consuming glycerophospholipids and maintaining the Prostaglandin E content, cell damage was improved, sphingosinol synthesis was reduced, and apoptosis was inhibited. Additionally, it could enhance the salvage synthesis and de novo synthesis of purine, reduce purine accumulation, promote the synthesis of nucleotide and energy carriers, and assist in recovering physiological metabolism. Overall, results explained the physiological regulation mechanism of PF-carp adapting to the acute changes of dissolved oxygen at the metabolic level and also provided novel evidence for physiological regulation of other fish in an environment with acute changes in dissolved oxygen levels.

Comparative transcriptome analysis provides novel insights into the molecular mechanism of the silver carp (Hypophthalmichthys molitrix) brain in response to hypoxia stress

The brain of fish plays an important role in regulating growth and adapting to environmental changes. However, few studies have been performed to address the changes in gene expression profiles in fish brains under hypoxic stress. In the present study, silver carp (Hypophthalmichthys molitrix) were kept under hypoxic experimental conditions by using the method of natural oxygen consumption, which resulted in a significant decrease in malondialdehyde (MDA) and glutathione (GSH) content and superoxide dismutase (SOD) activity in the brain. In addition, RNA sequencing (RNA-Seq) was performed to analyze transcriptional regulation in the brains of silver carp under normoxia (control group), hypoxia, semi-asphyxia, and asphyxia conditions. The results of KEGG enrichment pathway analysis showed that the immune system, such as antigen processing and presentation, natural killer cell-mediated cytotoxicity, was enriched in the hypoxia group; the nervous system (e.g., "glutamatergic synapse"), signal transduction (e.g., "calcium signaling pathway"; "foxo signaling pathway"), and signaling molecules and interactions (e.g., "neuroactive ligand-receptor interaction") were enriched in the semi-asphyxia group; and signaling molecules and interactions (e.g., "neuroactive ligand-receptor interaction") were enriched in the asphyxia group. These results provide novel insights into the molecular regulatory mechanism of the fish brain coping with hypoxia stress.

Transcriptomic responses to air exposure stress in coelomocytes of the sea cucumber, Apostichopus japonicus

During rearing in hatcheries and transportation to restocking sites, sea cucumbers are often exposed to air for several hours, which may depress their non-specific immunity and lead to mass mortality. We performed transcriptome analysis of Apostichopus japonicus coelomocytes after air exposure to identify stress-related genes and pathways. After exposure to air for 1 h, individuals were re-submerged in aerated seawater and coelomocytes were collected at 0, 1, 4, and 16 h (B, H1, H4, and H16, respectively). We identified 6148 differentially expressed genes, of which 3216 were upregulated and 2932 were downregulated. Many genes involved in the immune response, antioxidant defense, and apoptosis were highly induced in response to air exposure. Enrichment analysis of Gene Ontology terms showed that the most abundant terms in the biological process category were oxidation-reduction process, protein folding and phosphorylation, and receptor-mediated endocytosis for the comparison of H1 vs. B, H4 vs. H1, and H16 vs. H4, respectively. Kyoto Eecyclopedia of Genes and Genomes enrichment analysis showed that six pathways related to the metabolism of proteins, fats, and carbohydrates were shared among the three comparisons. These results indicated that sea cucumbers regulate the expression of genes related to the antioxidant system and energy metabolism to resist the negative effects of air exposure stress. These findings may be applied to optimize juvenile sea cucumber production, and facilitate molecular marker-assisted selective breeding of an anoxia-resistant strain.

Proteomics reveals gender differences in physiological characteristics of the gonads and tube feet of the sea cucumber, Apostichopus japonicus

The sea cucumber Apostichopus japonicus is an important aquaculture species in China because of its high nutritional and medicinal values. Gender, as a factor affecting the physiology of organisms, is always considered when improving the breeding efficiency of economically important animals. In the present study, protein expression profiles of the gonads and tube feet of male and female A. japonicus were investigated using a comparative proteomics approach. A set of 7499 proteins were identified, which covered a broad range of functions based on function annotations. A significant difference in protein expression profiles was observed between the gonads and tube feet of A. japonicus; gonads showed more apparent gender differences than tube feet. Moreover, the findings revealed that male A. japonicus had more specific functions and most of these functions were associated with energy consumption. Further analyses suggested that the regulation of ERK activity and the capacity of tyrosine production and virus immunity might be more powerful in male and female A. japonicus, respectively. Some candidate proteins were also recognized as potential targets for gender identification of A. japonicus. Overall, our study provides new insights into the understanding of molecular mechanisms underlying gender-based physiological differences in A. japonicus. SIGNIFICANCE: The current study aimed to reveal gender differences in the physiological characteristics of gonads and tube feet of the sea cucumber A. japonicus. To the best of our knowledge, this is the first proteomics study to analyze the differences in the protein expression profiles of external organs between male and female A. japonicus. The analysis revealed gender differences in the protein expression profiles of both gonads and tube feet of A. japonicus, and the gender differences in gonads were quite apparent. Moreover, according to the recognition of differentially expressed proteins and the enrichment analyses based on Kyoto Encyclopedia of Genes and Genomes, a draft view of how the physiological functions of A. japonicus were affected by gender was obtained. Male A. japonicus could have more specific functions related to energy consumption than females. The regulation of ERK activity and virus immunity might be more robust in male and female A. japonicus, respectively. Some candidate proteins were also recognized as potential targets for gender identification of A. japonicus. The findings presented here will improve the understanding of researchers about the molecular mechanisms underlying gender-based differences in A. japonicus and contribute to the meticulous breeding of A. japonicus.

Emerging roles of circRNAs in regulating thermal and hypoxic stresses in Apostichopus japonicus (Echinodermata: Holothuroidea)

Some sea cucumbers are economically and ecologically important, but they are threatened by thermal and hypoxic stress in changing oceanographic conditions. We construct circRNAs profiles, reveal circRNAs characters, and illustrate the potential regulatory roles of circRNAs in one commercially important species of sea cucumber, Apostichopus japonicus. Reads are distributed in intergenic (44.14%), exonic (48.26%) and intronic (7.60%) regions of the genome. A total of 1684 circRNAs were identified, and the most common spliced length is 269 nt in the present study. In three treatments (HT [thermal stress], LO [hypoxic stress], and HL [combined thermal and hypoxic stress]), 24, 27 and 27 differentially expressed (DE) circRNAs were identified, respectively. Five novel DE-circRNAs commonly occur in these treatments (novel_circ_0003311, novel_circ_0000229, novel_circ_0003944, novel_circ_0001458 and novel_circ_0000707), and based on them, potential circRNA-miRNA binding pairs were predicted. Sanger sequencing, RNase R treatment experiment and qPCR validation identified the accuracy of the circRNAs. Key circRNAs identified in the present study were covalently closed and were more stable under RNase R treatment than linear RNAs. Based on function analysis, circRNAs could regulate metabolic process, signal transduction, and ion responses in A. japonicus when exposed to thermal and hypoxic stress, and 'regulation of response to stimulus' is a common gene ontology (GO) term that is significantly enriched in each treatment; GO terms for 'DNA' and 'stress' are commonly enriched in heat-related treatments (HT and HL); and GO terms for 'protein' are commonly enriched in hypoxia-related treatments (LO and HL). When environmentally stressed, 'metabolism,' 'transport and catabolism,' 'membrane transport,' and 'signal transduction' were significantly responded in sea cucumber based on KEGG analysis. We provide insights into circRNA functions in stress regulation and lay a foundation for invertebrate circRNA research.

Effect of chronic exposure to microplastic fibre ingestion in the sea cucumber Apostichopus japonicus

Microplastics (MPs) in the form of microfibres (MFs) are of great concern because of their size and increasing abundance, which increase their potential to interact with or be ingested by aquatic organisms. Although MFs are the dominant shape of MPs ingested by sea cucumbers in habitats, their effect on sea cucumbers remains unclear. This study examined the effect of dietary exposure to MFs on the growth and physiological status of both juvenile and adult Apostichopus japonicus sea cucumbers. MFs were mixed into the diet of sea cucumbers for 60 d at environmentally relevant concentrations of 0.6 MFs g^-1, 1.2 MFs g^-1 and 10 MFs g^-1. Dietary exposure to MFs, with concentrations at or above those commonly found in the habitats, did not significantly affect the growth and faecal production rate of either juvenile or adult sea cucumbers. However, a disruption in immunity indices (acid phosphatase and alkaline phosphatase activity) and oxidative stress indices (total antioxidant capacity and malondialdehyde content) was observed in juvenile and adult sea cucumbers, indicating that these indices might be useful as potential biomarkers of the exposure to MF ingestion in sea cucumbers. This study provides insights into the toxicity mechanism of MF ingestion in a commercially and ecologically important species.

Ocean acidification, hypoxia and warming impair digestive parameters of marine mussels

Global change and anthropogenic activities have driven marine environment changes dramatically during the past century, and hypoxia, acidification and warming have received much attention recently. Yet, the interactive effects among these stressors on marine organisms are extremely complex and not accurately clarified. Here, we evaluated the combined effects of low dissolved oxygen (DO), low pH and warming on the digestive enzyme activities of the mussel Mytilus coruscus. In this experiment, mussels were exposed to eight treatments, including two degrees of pH (8.1, 7.7), DO (6, 2 mg/l) and temperature (30 °C and 20 °C) for 30 days. Amylase (AMS), lipase (LPS), trypsin (TRY), trehalase (TREH) and lysozyme (LZM) activities were measured in the digestive glands of mussels. All the tested stress conditions showed significant effects on the enzymatic activities. AMS, LPS, TRY, TREH showed throughout decreased trend in their activities due to low pH, low DO, increased temperature and different combinations of these three stressors with time but LZM showed increased and then decreased trend in their activities. Hypoxia and warming showed almost similar effects on the enzymatic activities. PCA showed a positive correlation among all measured biochemical parameters. Therefore, the fitness of mussel is likely impaired by such marine environmental changes and their population may be affected under the global change scenarios.

Plasticity of Respiratory Function Accommodates High Oxygen Demand in Breeding Sea Cucumbers

Physiological plasticity allows animals to adjust their physiological function to abiotic and biotic variations. It has been mostly studied in the context of response to external factors and not much is known on how animals adjust their physiology to cope with variations in internal conditions. The process of reproduction implies gonadal maturation and other internal changes, bringing various challenges to the animal such as an increased demand for energy and oxygen. Here, the capacity of the sea cucumber, Apostichopus japonicus to adjust its respiratory function and physiological mechanisms during reproduction was studied using a time-lapse videography and metabolomics approach. The results showed that reproduction caused a significant increase in oxygen consumption in A. japonicus. Interestingly, breeding sea cucumbers can accommodate the high oxygen demand by accelerating respiratory rate. However, to maintain a necessary high level of respiratory activity during reproduction, sea cucumbers need consume large amounts of adenosine triphosphate (ATP). In addition, the metabolomic data suggests that oxidative stress and hormone regulation are the physiological mechanisms linking reproduction and respiratory function. Altogether, these findings suggest that plasticity of respiratory function is an effective tactic to cope with high oxygen demand during reproduction.

The regulation mechanism of lncRNAs and mRNAs in sea cucumbers under global climate changes: Defense against thermal and hypoxic stresses

The aquatic environment can be greatly impacted by thermal and hypoxic stresses, particularly caused by intensified global warming. Hence, there is an urgency to understand the response mechanisms of marine organisms to adverse environment. Although long non-coding RNAs (lncRNAs) are involved in many biological processes, their roles in stress responses still remain unclear. Here, differentially expressed (DE) lncRNAs and mRNAs were identified as responses to environmental stresses in the economically important sea cucumber, Apostichopus japonicus, and their potential roles were explored. Based on a total of 159, 355 and 495 significantly upregulated genes and 230, 518 and 647 significantly downregulated genes identified in the thermal, hypoxic and combination thermal + hypoxic stress treatments, respectively, we constructed DE-lncRNA-mRNA coexpression networks. Among the networks, eight shared pairs were identified from the three treatments, and based on the connectivity degree, MSTRG.27265, MSTRG.19729 and MSTRG.95524 were shown to be crucial lncRNAs. Among all the significantly changed lncRNAs identified by RT-qPCR and sequencing data, binding sites were found in four other lncRNAs (MSTRG.34610, MSTRG.10941, MSTRG.81281 and MSTRG.93731) with Aja-miR-2013-3p, a key miRNA that responds to hypoxia in sea cucumbers. The hypoxia-inducible factor (HIF-1α) was also shown as the possible targeted mRNA of Aja-miR-2013-3p. As indicated by a dual-luciferase reporter assay system, "HIF-1α gene/Aja-miR-2013-3p/MSTRG.34610" network and the "HIF-1α gene/Aja-miR-2013-3p/MSTRG.10941" network may play important roles in sea cucumbers under environmental stresses. Moreover, environmental stress altered the expression of multiple lncRNAs and mRNAs, thus affecting various biological processes in A. japonicus, including immunity, energy metabolism and the cell cycle. At the molecular level, more comprehensive responses were elicited by the combined thermal/hypoxic stress treatment than by individual stresses alone in sea cucumbers. This study lays the groundwork for future research on molecular mechanisms of echinoderm responses to thermal and hypoxic stress in the context of global climate changes.

Air Exposure Affects Physiological Responses, Innate Immunity, Apoptosis and DNA Methylation of Kuruma Shrimp, Marsupenaeus japonicus

Air exposure stress is a common phenomenon for commercial crustacean species in aquaculture and during waterless transportation. However, the antioxidant responses to air exposure discussed in previous studies may be insufficient to present the complexities involved in this process. The comprehensive immune responses, especially considering the immune genes, cell apoptosis, and epigenetic changes, are still unknown. Accordingly, we investigated the multifaceted responses of Marsupenaeus japonicus to air exposure. The results showed that the expression profiles of the apoptosis genes (e.g., IAP, TXNIP, caspase, and caspase-3) and the hypoxia-related genes (e.g., hsp70, hif-1α, and HcY) were all dramatically induced in the hepatopancreas and gills of M. japonicus. Heart rates, T-AOC (total antioxidant capacity) and lactate contents showed time-dependent changes upon air exposure. Air exposure significantly induced apoptosis in hepatopancreas and gills. Compared with the control group, the apoptosis index (AI) of the 12.5 h experimental group increased significantly (p < 0.05) in the hepatopancreas and gills. Most individuals in the experimental group (EG, 12.5 h) had lower methylation ratios than the control group (CG). Air exposure markedly reduced the full-methylation and total-methylation ratios (31.39% for the CG and 26.46% for the EG). This study provided a comprehensive understanding of the antioxidant responses of M. japonicus considering its physiology, innate immunity, apoptosis, and DNA methylation levels, and provided theoretical guidance for waterless transportation.

Microplastic fibers transfer from the water to the internal fluid of the sea cucumber Apostichopus japonicus

Microplastics (MPs) are small plastic particles less than 5 mm in diameter. MPs in the form of microfibers (MFs) are widely detected in aquatic habitats and are of high environmental concern. Despite many reports on the effects of MFs on marine animals, their effect on sea cucumbers is still unclear. In addition, our previous filed study has shown that MFs may transfer to the coelomic fluid of the sea cucumber Apostichopus japonicus (A. japonicus). Here, we show how MFs transfer to the coelomic fluid of the sea cucumber. We captured the MFs during their transfer from the water to the coelomic fluid through the respiratory tree. A. japonicus ingested in the MFs along with the water during respiration; the MFs got stuck in the respiratory tree or transferred to the coelomic fluid. The transferred MFs increased during 72 h of exposure and persisted for 72 h after the transfer to clean water. Among the immunity indices, lysozyme (LZM) levels increased in response to the transferred MFs, which confirms the defensive role of LZMs against strange substances. Additionally, non-significantly decreased levels of total antioxidant capacity (T-AOC), malondialdehyde (MDA), peroxidase (POD) and phenol oxidase (PPO) were observed at 24 h and 48 h post-exposure, suggesting minimal oxidative imbalance. Furthermore, there were no significant changes in the speed and the total distance moved by A. japonicus post MFs transfer. This study revealed that MFs transfer and accumulate in the coelomic fluid of A. japonicus.

Time course analysis of immunity-related gene expression in the sea cucumber Apostichopus japonicus during exposure to thermal and hypoxic stress

Temperature and dissolved oxygen concentration are important abiotic factors that can limit the growth and survival of sea cucumbers by affecting their immune systems. As global warming intensifies, sea cucumbers are increasingly exposed to adverse environmental conditions, which can cause severe economic losses and limit the sustainable development of sea cucumber aquaculture. It is therefore important to better understand how sea cucumbers respond to environmental stress, especially with regard to its effects on immunity. In the present study, the time series of immunity-related gene expression in sea cucumbers under thermal and hypoxic stresses were analyzed separately. The expression trends of 17 genes related to the nuclear factor κB (NF-κB) pathway, the protease family, the complement system, heat shock proteins (HSPs) and the transferrin family during exposure to two stresses at eight time points were concluded. These genes have interconnected roles in stress defense. The expression levels of genes relating to the NF-κB pathways and HSPs were strongly affected in the sea cucumber thermal stress response, while melanotransferrin (Mtf), ferritin (Ft) and mannan-binding C-type lectin (MBCL) were affected by hypoxia. In contrast, complement factor B (Bf), myosin V (Mys) and serine protease inhibitor (SPI) were not that sensitive during the initial period of environmental stress. Similar expression patterns under both thermal and hypoxic stress for certain genes, including an increase in Hsp90 and decreases in lysozyme (Lys), major yolk protein (MYP) and cathepsin C (CTLC) were observed in sea cucumbers. Conversely, NF-κB and Hsp70 were differentially affected by the two stress treatments. Lysozyme-induced immune defense was inconstant in sea cucumbers coping with stress. A gene ontology (GO) analysis of the selected genes revealed that the most co-involved terms related to immunity and iron ion. Our analysis suggests that sea cucumbers demonstrate complex and varied immune responses to different types of stresses. This dynamic image of the immune responses and stress tolerance of sea cucumbers provides new insights into the adaptive strategies of holothurians in adverse environments.

Transcriptome analysis and the effects of polyunsaturated fatty acids on the immune responses of the critically endangered angtze sturgeon (Acipenser dabryanus)

The poor understanding of nutrition needed has become a significant obstruction to artificial conservation of Yangtze sturgeon (Acipenser dabryanus) and the relationship between ployunsaturated fatty acid nutrition and the immune response of Yangtze sturgeon is remains unclear. To explore this relationship, the immune response was determined by the activities of serum immune-related enzymes and the transcriptome pattern in the spleen after feeding different fat source diets for 7 weeks. In addition, the gene expression pattern after a lipopolysaccharide (LPS) challenge was investigated in the presence of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Long-term feeding of the fish oil diets increased the serum immune-related enzyme activities, including lysozyme, acid phosphatase, and alkaline phosphatase of Yangtze sturgeon. More than 653,999 transcripts with an N50 length of 1047 bp were obtained and a final set of 280,408 unigenes was generated. After annotating the unigenes, 3549 genes were assigned to the immune system and 2839 were identified to participate in the response to the different fat sources. A transcriptome assay showed the fish oil diets moderately upregulated immune-related signaling pathways in the spleen of Yangtze sturgeon, including NLR signaling, platelet activation, Fc gamma R-mediated phagocytosis, Th17 cell differentiation, and Th1 and Th2 cell differentiation. The quantitative polymerase chain reaction (qPCR) results of candidate genes for these pathways showed similar results. The LPS challenge study revealed that DHA and EPA moderately upregulated the candidate immune-related genes and modulated excessive activation of the immune pathway by the pathogen. This study confirmed the immunomodulatory function of unsaturated fatty acids in Yangtze sturgeon. This research will provide a reference for the preparation of artificial diets for Yangtze sturgeon.

Physiological and immunological responses of sea cucumber Apostichopus japonicus during desiccation and subsequent resubmersion

Desiccation is one of the extremely stressful situations experienced by aquatic animals, and sea cucumber usually suffers from desiccation stress during transportation without water. The present study was conducted to evaluate the effect of desiccation and subsequent resubmersion on physiological stress, oxidative damage, antioxidant status and non-specific immune response of Apostichopus japonicus, providing valuable information on the health management of sea cucumber culturing. Control and desiccation groups were set up, and each group has three replicates. After 1, 3 and 6 h of desiccation, individuals were resubmersed in aerated seawater for a 24 h recovery in three batches, which were represented as D1, D3 and D6, respectively. The results showed that glucose level in coelomic fluid of sea cucumber significantly decreased after desiccation, whereas lactate, cortisol and osmolality showed remarkable ascending trends. Thereafter, all stress parameters gently recovered towards normal levels as control group during 24 h resubmersion. The prolonged desiccation at D6 treatment induced the significant increases of malondialdehyde (MDA) and reactive oxygen species (ROS) contents, as well as relatively lower superoxide dismutase (SOD) and catalase (CAT) activities. During the period of desiccation and subsequent resubmersion, sea cucumber adjusted antioxidant defense to reduce the concentrations of MDA and ROS as a strategy for protecting against oxidative damage. Desiccation also had significant effects on non-specific immune parameters (total coelomocytes counts, TCC; complement C3; total nitric oxide synthase, T-NOS; lysozyme, LSZ; alkaline phosphatase, AKP) of A. japonicus, which could be recovered to some extent during resubmersion. In conclusion, less than 6 h of desiccation did not induce irreparable damage to sea cucumber, and was recommended for handling and shipping live sea cucumbers.

Metabolome responses of the sea cucumber Apostichopus japonicus to multiple environmental stresses: Heat and hypoxia

Economically important marine organisms face severe environmental challenges, such as high temperature and low dissolved oxygen, from global climate change. Adverse environmental factors impact the survival and growth of economically important marine organisms, thereby negatively influencing the aquaculture industry. However, little is known about the responses of sea cucumbers to combined environmental co-stressors till now. In this study, ultra-performance liquid chromatography (UPLC) was utilized to obtain metabolic profiles of sea cucumbers. Changes in the concentrations of 84, 68, and 417 metabolites related to the responses of sea cucumbers to heat (26 °C), hypoxia (2 mg/L) and the combined stress, respectively, were observed and analyzed. Representative biomarkers were discussed in detail, including deltaline, fusarin C, halichondrin B and rapanone. The concentration of metabolites involved in the regulation of energy metabolism, including amino acid, carbohydrate and lipid metabolism were significantly changed, and the tricarboxylic acid (TCA)-cycle was significantly altered under heat plus hypoxia. We interpreted these changes partly as an adaptation mechanism in response to environmental stress. Based on the decreased accumulation of glutamine, we hypothesized that heat stress is the main factor that interferes with the process of glutamic acid-glutamine metabolism. The present study showed that combined environmental stressors have a more extensive impact on the metabolites of the respiratory tree in sea cucumbers than single stress. These results would facilitate further development of the sea cucumber as an echinoderm model to study mechanisms of response to adverse environments, as well as to help advance knowledge of the adaptation of marine organisms to global climate change.