Identification of potential markers and sensitive tissues for low or high salinity stress in an intertidal mud crab (Macrophthalmus japonicus)

Effect of endocrine-disrupting chemicals on the expression of a calcium ion channel receptor (ryanodine receptor) in the mud crab (Macrophthalmus japonicus)

Endocrine-disrupting chemicals (EDCs) are toxic pollutants generated by artificial activities. Moreover, their hormone-like structure induces disturbances, such as mimicking or blocking metabolic activity. Previous studies on EDCs have focused on the adverse effect of the endocrine system in vertebrates, with limited investigations conducted on ion channels in invertebrates. Thus, in this study, we investigated the potential adverse effects of exposure to bisphenol-A (BPA) and di-(2-ethylhexyl) phthalate (DEHP) at the molecular level on the ryanodine receptor (RyR), a calcium ion channel receptor in Macrophthalmus japonicus. In the phylogenetic analysis, the RyR amino acid sequences in M. japonicus clustered with those in the Crustacean and formed separated branches for RyR in insects and mammals. When exposed to 1 μg L-1 BPA, a significant increase in RyR mRNA expression was observed in the gills on day 1, although a similar level to the control group was observed from day 4 to day 7. However, the RyR expression due to DEHP exposure decreased on days 1 and 4, although it increased on day 7 following exposure to 10 μg L-1. The RyR expression pattern in the hepatopancreas increased for up to 4 days, depending on the BPA concentration. However, there was a tendency for the expression to decrease gradually after the statistical significance increased during the early stage of DEHP exposure (D1). Hence, the transcriptional alterations in the M. japonicus RyR gene observed in the study suggest that exposure toxicities to EDCs, such as BPA and DEHP, have the potential to disrupt calcium ion channel signaling in the gills and hepatopancreas of M. japonicus crabs.

Sediment temperature characteristics and their relation to distribution patterns of two sentinel crab species in intertidal flats in western Japan

In the context of global climate change, monitoring focused on temperature and benthic animals in intertidal flats and the development of metrics to assess climate change and ecosystem responses are essential for a sustainable society. However, few studies have assessed the relationship between intertidal sediment temperature and the distribution of benthic animals. To address this gap, in the present study, intertidal sediment temperatures were observed in 12 intertidal flats in 11 survey areas over 335 days, from October 2, 2019, to August 31, 2020, using water temperature data loggers. The characteristics of intertidal sediment temperatures were variable among the survey areas, and a correlation analysis suggested that such characteristics are possibly influenced by various spatial-scale factors, such as geographical, basin, and habitat scales. Furthermore, two sentinel crab species, Macrophthalmus japonicus and Macrophthalmus banzai were collected, and the number of wintering individuals of each species was estimated based on their carapace width to analyze the changes in abundances of the two species in each survey area. The results show that the number of days with daily minimum temperature ≥ 19 °C was the factor that influenced the abundance rate, suggesting that M. japonicus and M. banzai populations may decrease and increase, respectively, according to future climate change in Japan. Our findings emphasize the importance of long-term monitoring of sediment temperatures and benthic animals in intertidal flats to evaluate the influence of future climate change.

Growth retardation and suppression of ubiquitin-dependent catabolic processes in the brackish water clam Corbicula japonica in response to salinity changes and bioaccumulation of toxic heavy metals

The brackish water clam (Corbicula japonica) is constantly exposed to stressful salinity gradients and high levels of heavy metals in the freshwater-saltwater interface of estuary environments, which are introduced from upstream regions and land. To identify the key molecular pathways involved in the response to salinity changes and heavy metal bioaccumulation, we obtained the transcriptomes of C. japonica inhabiting different salinities and heavy metal distributions in Gwangyang Bay (Korea) using RNA sequencing. Among a total of 404,486 assembled unigenes, 5534 differentially expressed genes were identified in C. japonica inhabiting different conditions, 1549 of which were significantly upregulated and 1355 were significantly downregulated. Correlation analyses revealed distinct gene expression patterns between the low and high conditions of salinity and heavy metal bioaccumulation. Functional annotation revealed significant downregulation of genes involved in "ubiquitin-dependent protein catabolic process," "tricarboxylic acid cycle," and "intracellular protein transport" in C. japonica from the high condition compared to the low condition. Transcription and translation pathways were significantly enriched in the high condition. Additionally, upon comparison of the low and high conditions by qRT-PCR and proteasome enzyme activity analyses, our findings demonstrated that environmental stress could suppress the ubiquitin-proteasome complex (UPC). Additionally, transcriptomic changes under high salinity stress conditions may be related to an increase in cellular protection by defense enzymes, which leads to more energy being required and a disruption of energy homeostasis. Ultimately, this could cause growth retardation in the clam C. japonica. In summary, this study provides the first evidence of UPC suppression induced by a combination of high salinity and heavy metal bioaccumulation stress in C. japonica, which could compromise the survival and growth of estuarine bivalves.

Ammonia Stress Disturbs Moult Signaling in Juvenile Swimming Crab Portunus trituberculatus

Ammonia is a significant concern during hatchery culture in brachyuran species, and its accumulation may lead to abortive moulting and large-scale deaths of the early juveniles. To date, the underlying mechanism for ammonia-induced alteration of the moulting process is still unknown. In this study, we aimed to investigate the effects of ammonia on the moulting as well as the potential mechanisms in early juveniles of the swimming crab Portunus trituberculatus, an important aquaculture species in China. We evaluated the survival rate and moulting rate of the juvenile crabs (C2) and analyzed the expression pattern of the genes in key components of molt signaling during a complete moulting cycle under different concentrations of ammonia nitrogen (the control group: <0.1 mg/L; the LA group: 5 mg/L; and the HA group: 20 mg/L). The results showed that: (1) the survival rate in the LA and HA groups was lower than that in the control group at the end of the experiment, and moulting death syndrome (MDS) was only observed in the HA group; (2) the moulting rate was higher in the LA group and lower in the HA group compared to the control group; (3) consistent with the results of the moulting experiment, MIH showed decreased expression, and genes related to ecdysteroid synthesis, ecdysteroid receptors, and responsive effectors exhibited increased expression in the LA group compared to the control group; and (4) although MIH expression was upregulated, increased expression of the genes associated with ecdysteroid synthesis, ecdysteroid receptors and downstream effectors still observed in the HA group. Our results indicated that low levels of ammonia can promote moulting in juvenile swimming crabs by inhibiting the expression of MIH and activating moult signaling, whereas high levels of ammonia inhibit moulting and lead to MDS through impairing moult signaling.

RNA-seq Provides Novel Insights into Response to Acute Salinity Stress in Oriental River Prawn Macrobrachium nipponense

The oriental river prawn Macrobrachium nipponense is an important aquaculture species in China, Vietnam, and Japan. This species could survive in the salinity ranging from 7 to 20 ppt and accelerate growth in the salinity of 7 ppt. To identify the genes and pathways in response to acute high salinity stress, M. nipponense was exposed to the acute high salinity of 25 ppt. Total RNA from hepatopancreas, gills, and muscle tissues was isolated and then sequenced using high-throughput sequencing method. Differentially expressed genes (DGEs) were identified, and a total of 632, 836, and 1246 DEGs with a cutoff of significant twofold change were differentially expressed in the hepatopancreas, gills, and muscle tissues, respectively. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genome pathway enrichment analyses were conducted. These DEGs were involved in the GO terms of cellular process, metabolic process, membrane, organelle, binding, and catalytic activity. The DEGs of hepatopancreas and gill tissues were mainly enriched in PPAR signaling pathway, longevity regulating pathway, protein digestion and absorption, and the DEGs of muscle tissue in arginine biosynthesis, adrenergic signaling in cardiomyocytes, cardiac muscle contraction, and cGMP-PKG signaling pathway. Real-time PCR conducted with fifteen selected DEGs indicated high reliability of digital analysis using RNA-Seq. The results indicated that the M. nipponense may regulate essential mechanisms such as metabolism, oxidative stress, and ion exchange to adapt the alternation of environment, when exposed to acute high salinity stress. This work reveals the numbers of genes modified by salinity stress and some important pathways, which could provide a comprehensive insight into the molecular responses to high salinity stress in M. nipponense and further boost the understanding of the potential molecular mechanisms of adaptation to salinity stress for euryhaline crustaceans.

Integrated analysis of exoskeletal surface profile and chitin-related gene expression on Macrophthalmus japonicus mud crabs exposed to hexabromocyclododecane

Hexabromocyclododecanes (HBCDs), widely used brominated flame retardants, easily accumulate in aquatic organisms such as Macrophthalmus japonicus crabs, which inhabit tidal flat sediments. To analyze the effects of HBCD exposure in chitin-formed exoskeleton, we investigated molecular responses of chitin-related genes as well as physical changes of the exoskeletal surface form as a new biological end-point on M. japonicus. The expression patterns of chitin biosynthesis-, modification-, and degradation-related genes in the gills and hepatopancreases of M. japonicus were also analyzed. Additionally, the survivability and exoskeleton surface profiles of M. japonicus crabs were evaluated. M. japonicus chitin synthase expression was significantly downregulated, whereas that of the chitinase transcript was significantly upregulated upon exposure to all HBCD concentrations on day 7. Contrastingly, the gene expression of chitin deacetylase 1 significantly increased upon exposure to all HBCD concentrations on day 1, and this increase was significantly elevated on day 4. The expression of chitin deacetylase 1 was dose-dependent. Additionally, decreased survival and exoskeleton surface profile changes were observed in M. japonicus crabs exposed to all HBCD concentrations. These results suggest that exposure to HBCD induces changes in the synthesis, modification, and degradation of chitin, a pivotal component of the cuticular exoskeleton, and may disrupt the exoskeletal surface structure in M. japonicus crabs.

EDCs trigger immune-neurotransmitter related gene expression, and cause histological damage in sensitive mud crab Macrophthalmus japonicus gills and hepatopancreas

Endocrine-disrupting chemicals (EDCs), distributed at various concentrations in freshwater and marine ecosystems, affect the survival, reproduction, and behavior of wide ranges organisms. Most toxicology studies on EDCs have focused on the endocrine system of invertebrates, and research on invertebrate neurotransmitters is limited. In the present study, we investigated the expression of Macrophthalmus japonicus genes encoding γ-aminobutyric acid transporter subtype 2 (GAT-2) and glutamine synthetase (GS), which play important roles as neurotransmitters at synapses. We observed differences in the mRNA expression levels of GAT-2 and GS as well as histological changes in various tissues after exposure to bisphenol-A (BPA) and di-(2-ethylhexyl) phthalate (DEHP). The amino acid sequences of M. japonicus GAT-2 and GS formed separate branches in crustaceans, fish, insects, and mammals. M. japonicus GAT-2 and GS expression levels were highest in the gills, hepatopancreas, and stomach, and showed different between DEHP or BPA treatments. In particular, hepatopancreas GS expression on Day 1, the first step in the presynaptic process, was upregulated after BPA and DEHP exposure, while GAT-2, sequential step in the presynaptic process, was significantly elevated only in DEHP. After BPA treatments, gill GS expression was increased at all concentrations, whereas GAT-2 expression was overall down regulations. In contrast, in DEHP treatment groups hepatopancreatic GS and GAT-2 expression at Day 1 was only significantly higher and all groups including gill GS and GAT-2 expression were downregulation. Histological changes in the gills and hepatopancreas were observed in a concentration-dependent manner. Accordingly, BPA and DEHP exposure in crabs could be stimulate neurotransmitter gene expression and alter the morphological structure of gill and hepatopancreas.

Effects of di-(2-ethylhexyl) phthalate on Transcriptional Expression of Cellular Protection-Related HSP60 and HSP67B2 Genes in the Mud Crab Macrophthalmus japonicus

Di-2-ethylhexyl phthalate (DEHP) has attracted attention as an emerging dominant phthalate contaminant in marine sediments. Macrophthalmus japonicus, an intertidal mud crab, is capable of tolerating variations in water temperature and sudden exposure to toxic substances. To evaluate the potential effects of DEHP toxicity on cellular protection, we characterized the partial open reading frames of the stress-related heat shock protein 60 (HSP60) and small heat shock protein 67B2 (HSP67B2) genes of M. japonicus and further investigated the molecular effects on their expression levels after exposure to DEHP. Putative HSP60 and small HSP67B2 proteins had conserved HSP-family protein sequences with different C-terminus motifs. Phylogenetic analysis indicated that M. japonicus HSP60 (Mj-HSP60) and M. Japonicus HSP67B2 (Mj-HSP67B2) clustered closely with Eriocheir sinensis HSP60 and Penaeus vannamei HSP67B2, respectively. The tissue distribution of Heat shock proteins (HSPs) was the highest in the gonad for Mj-HSP60 and in the hepatopancreas for Mj-HSP67B2. The expression of Mj-HSP60 Messenger Ribonucleic Acid (mRNA) increased significantly at day 1 after exposure to all doses of DEHP, and then decreased in a dose-dependent and exposure time-dependent manner in the gills and hepatopancreas. Mj-HSP67B2 transcripts were significantly upregulated in both tissues at all doses of DEHP and at all exposure times. These results suggest that cellular immune protection could be disrupted by DEHP toxicity through transcriptional changes to HSPs in crustaceans. Small and large HSPs might be differentially involved in responses against environmental stressors and in detoxification in M. japonicus crabs.

Evaluation of immunological response in shrimp Penaeus vannamei submitted to low temperature and air exposure

The objective of this study is to explore the immune response of the shrimp Penaeus vannamei to low temperature and air exposure during the mimic waterless transportation. Shrimp were cold shocked at 13 °C for 3 min, then exposed to air at 15 °C for 12 h, and finally revived in water at 25 °C. Hemocyte structure remained intact with only slight distortions of some organelles and nuclear membrane under the stress. Phenoloxidase (PO), lysozyme (Lys) and gamma-glutamyl transferase (GGT) activities, glutamine (Gln) content and relative mRNA expressions of prophenoloxidase (proPO), β-1,3-glucan binding protein (LGBP), ferrin (Fer) and glucose regulated protein 78 (GRP 78) increased and reached peak levels after 3 h-9 h of air exposure, and then decreased to relatively stable levels in the prolonged period of air exposure. The total hemocyte count (THC) and gene expressions of proPO, Fer and LGBP at the end of waterless storage were significantly lower (p < 0.05) than those in fresh shrimp while no significant difference (p ≥ 0.05) was found between revived and fresh shrimp in PO, Lys, GGT activities, Gln content and gene expression level of GRP 78. Of all the hemocytes, the percentage of semi granular cells (SGC) and granular cells (GC) significantly decreased after 6-9 h of storage, while hyaline cells (HC) dramatically increased after 9 h of storage to compensate the loss of SGC and GC. Results suggested that the low temperature and air exposure caused significant immunological response to the shrimp, but the damages to the immune system were partly reparable.

Individual and combined effects of ammonia-N and sulfide on the immune function and intestinal microbiota of Pacific white shrimp Litopenaeus vannamei

In this study, we explored the individual and combined effects of ammonia-N and sulfide stress (1 mg/L sulfide and 15 mg/L ammonia-N) on the oxidation resistance, immune response and intestinal health of Litopenaeus vannamei during 72 h exposure. The total antioxidant capacity (T-AOC), malonaldehyde (MDA) and nitric oxide (NO) content, superoxide dismutase (SOD) and catalase activity (CAT), the immune-relative gene (caspase-3, hsp70 and IMD) expression in hepatopancreas and intestine of L.vannamei and the intestinal microbiota were measured. The result showed that MDA and NO contents in hepatopancreas of L. vannamei in all treatment groups increased and remain were at high levels at the end of the stress exposure. The L. vannamei employ antioxidant defense system by increasing the activities of T-AOC, SOD and CAT enzymes in hepatopancereas and intestine to reduce oxidant damage. More severe damages with combined ammonia-N and sulfide stress to antioxidant systems were observed. The gene expression results also demonstrated that antioxidant capacity of L. vannamei was severely impaired and the apoptosis cell was initiated under the ammonia-N and sulfide stress. In addition, the environmental stress also reshaped the intestinal microbial community structure of L. vannamei that a number of original genera decreased, such as Cellvibrio, Vibrio and Rheinheimera; some new genera increased or appeared, such as Photobacterium in all treatment groups, Arcobacter and Fusibacter in sulfide stress group. Therefore, the health of L. vannamei was severely impacted when exposed to the stress of ammonia nitrogen and sulfide and these two factors can have weak synergic effects.

Environmental Pollutants Impair Transcriptional Regulation of the Vitellogenin Gene in the Burrowing Mud Crab (Macrophthalmus Japonicus)

Vitellogenesis is a pivotal reproductive process of the yolk formation in crustaceans. Vitellogenin (VTG) is the precursor of main yolk proteins and synthesized by endogenous estrogens. The intertidal mud crab (Macrophthalmus japonicus) inhabits sediment and is a good indicator for assessing polluted benthic environments. The purpose of this study was to identify potential responses of M. japonicus VTG under environmental stresses caused by chemical pollutants, such as 1, 10, and 30 µg L−1 concentrations in di(2-ethylhexyl) phthalate (DEHP), bisphenol A (BPA) and irgarol. We characterized the M. japonicus VTG gene and analyzed the transcriptional expression of VTG mRNA in M. japonicus exposed to various chemicals and exposure periods. A phylogenetic analysis revealed that the M. japonicus VTG clustered closely with Eriocheir sinensis (Chinese mitten crab) VTG, in contrast with another clade that included the VTG ortholog of other crabs. The basal level of VTG expression was the highest in the hepatopancreas and ovaries, and tissues. VTG expression significantly increased in the ovaries and hepatopancreas after 24 h exposure to DEHP. Increased responses of VTG transcripts were found in M. japonicus exposed to DEHP and BPA for 96 h; however, VTG expression decreased in both tissues after irgarol exposure. After an exposure of 7 d, VTG expression significantly increased in the ovaries and hepatopancreas for all concentrations of all chemicals. These results suggest that the crustacean embryogenesis and endocrine processes are impaired by the environmental chemical pollutants DEHP, BPA, and irgarol.

Endocrine-disrupting chemicals impair the innate immune prophenoloxidase system in the intertidal mud crab, Macrophthalmus japonicus

Endocrine-disrupting chemicals (EDCs), xenobiotics that interfere with endogenous hormone function, have been studied for their impacts in aquatic environments. However, there is limited information about the potentially hazardous impact of bisphenol A (BPA) and di-(2-ethylhexyl) phthalate (DEHP) on the marine environment. The aim of this study was to investigate the effects of BPA and DEHP on the immune response of the intertidal mud crab, Macrophthalmus japonicus. In order to examine immunological responses involving the prophenoloxidase (proPO) system, mRNA transcript and activity levels of six immune-related genes, including lipopolysaccharide and β-1,3-glucan-binding protein (LGBP), proPO, phenoloxidase (PO), peroxinectin (PE), serine protease inhibitor (Serpin), and trypsin (Tryp), were assessed in M. japonicus hepatopancreas and gills exposed to BPA or DEHP. Expression of immune genes generally decreased in M. japonicus hepatopancreas and gills exposed to all concentrations of BPA by days 4 and 7. However, at day 1, expression of Serpin and Tryp genes was significantly increased in M. japonicus hepatopancreas and gills exposed to BPA. For DEHP exposure, all genes, with the exception of Serpin, were significantly downregulated in M. japonicus gills. In the hepatopancreas, gene expression of PO, proPO, and LGBP increased at day 1, and then decreased by day 7, while mRNA expression of Serpin and Tryp exhibited up-regulation over all exposure periods. In addition, PE gene expression was upregulated in hepatopancreas at day 7 in a dose-dependent manner. Taken together, these results indicated that the crab immune responses were perturbed by exposure to BPA, and, in particular, DEHP.

Changes in exoskeleton surface roughness and expression of chitinase genes in mud crab Macrophthalmus japonicus following heavy metal differences of estuary

Risk assessment of heavy metals is important for the health evaluation of inhabiting species in aquatic ecosystem. This study investigated whether chitin exoskeleton of mud crab Macrophthalmus japonicus is affected by heavy metals in estuary sediments in Korea. We compared heavy metal concentrations and analyzed the expression of M. japonicus chitinase genes, which play the crucial role in the formation of chitin exoskeleton. Concentrations of heavy metals were highly observed in crab body inhabiting Hampyeong among estuarine sites. High expressions of chitinase 1 were observed in crab gill and hepatopancreas from Myodo, which is the site with the lowest concentration of heavy metal in crab body. The surface roughness of the exoskeleton decreased with the increased concentration of heavy metals accumulated in the crab body. These results suggest that the total bioconcentration of heavy metals in crabs affected the expression of chitinase genes and changes in the exoskeleton surface roughness.

Changes of exoskeleton surface roughness and expression of crucial participation genes for chitin formation and digestion in the mud crab (Macrophthalmus japonicus) following the antifouling biocide irgarol

Irgarol is a common antifoulant present in coastal sediment. The mud crab Macrophthalmus japonicus is one of the most abundant of the macrobenthos in the costal environment, and its exoskeleton has a protective function against various environmental threats. We evaluated the effects of irgarol toxicity on the exoskeleton of M. japonicus, which is the outer layer facing the environment. We analyzed transcriptional expression of exoskeleton, molting, and proteolysis-related genes in the gill and hepatopancreas of these exposed M. japonicus. In addition, changes in survival and exoskeleton surface characteristics were investigated. In the hepatopancreas, mRNA expression of chitinase 1 (Mj-chi1), chitinase 4 (Mj-chi4), and chitinase 5 (Mj-chi5) increased in M. japonicus exposed to all concentrations of irgarol. Mj-chi1 and Mj-chi4 expressions from 1 to 10μgL(-1) were dose- and time-dependent. Ecdysteroid receptor (Mj-EcR), trypsin (Mj-Tryp), and serine proteinase (Mj-SP) in the hepatopancreas were upregulated in response to different exposure levels of irgarol at day 1, 4, or 7. In contrast, gill Mj-chi5, Mj-Tryp, and Mj-SP exhibited late upregulated responses to 10μgL(-1) irgarol compared to the control at day 7. Mj-chi1 showed early upregulation upon exposure to 10μgL(-1) irgarol and Mj-chi4 showed no changes in transcription in the gill. Gill Mj-EcR presented generally downregulated expression patterns. In addition, decreased survival and change of exoskeleton surface roughness were observed in M. japonicus exposed to the three concentrations of irgarol. These results suggest that exposure to irgarol induces changes in the exoskeleton, molting, and proteolysis metabolism of M. japonicus.

Ammonia and salinity tolerance of Penaeus monodon across eight breeding families

Ammonia nitrogen and salinity tolerance of Penaeus monodon from eight selected breeding families were evaluated at the concentration of 67.65 mg L⁻¹ ammonia-N and reducing salinity from 15 to 0 ‰. The final survival of family A (88.67 ± 9.81 %) was highest, and the final survival of family B was lowest (24.33 ± 14.01 %) after the ammonia tolerance test. Upon completing the sudden drop salinity test from 15 to 0 ‰, the highest survival was observed in family B (98.00 ± 1.73 %), and the lowest survival was found in family H (18.00 ± 1.73 %). Family A showed the strongest ability to tolerate ammonia stress, and family B showed the strongest tolerance to low salinity. This study suggests that the tolerance of salinity and ammonia nitrogen varied between breeding families. Results from the present study provide useful information towards selective breeding in shrimp in aquaculture for environmental tolerance.

Chitinase gene responses and tissue sensitivity in an intertidal mud crab (Macrophthalmus japonicus) following low or high salinity stress

Changes in salinity affect the physiological status of the marine habitat including that of the intertidal mud crab Macrophthalmus japonicus. Chitinases play significant biological roles in crustaceans such as morphogenesis, nutrient digestion, and pathogen defense. In this study, the osmoregulatory function of three chitinase gene transcripts was determined compared to seawater (SW, 31 ± 1 psu) in M. japonicus gills and hepatopancreas under different salinities (10, 25, and 40 psu) for 1, 4, and 7 days. In SW-exposed crab, quantitative real-time PCR analysis showed chitinase 1 (Mj-chi1) and chitinase 4 (Mj-chi4) transcripts constitutively expressed in all the tested tissues with strong expression in hepatopancreas, but chitinase 5 (Mj-chi5) showed highest expression in stomach. When exposed to different salinities, Mj-chi1 showed significant up-regulation at day 4 whereas Mj-chi4 showed late up-regulation (day 7) at all the salinities in hepatopancreas. In the gills, early up-regulation (day 1) in Mj-chi1 and time-dependent late up-regulation (day 7) in Mj-chi4 at high salinity were observed. These results indicate the possibility of using Mj-chi4 as a marker against salinity changes. Moreover, our results further suggest that Mj-chi1 and Mj-chi4 transcriptions were significantly affected by changes in salinity; however, Mj-chi5 in gills was less affected by salinity and showed no effect in hepatopancreas. Thus, chitinase transcription modulations in the gills are more sensitive than hepatopancreas to salinity stress. Further, present data indicate the possible existence of different physiological roles among chitinase gene families, which need to be clarified in more detail by future biochemical and physiological functional studies.