Adaptive evolution of osmoregulatory-related genes provides insight into salinity adaptation in Chinese mitten crab, Eriocheir sinensis

Effects of low salinity stress on osmoregulation and gill transcriptome in different populations of mud crab Scylla paramamosain

Animals living in estuaries suffer from rapid and continuous salinity fluctuations, while the global warming and extreme precipitation aggravate this situation. Osmoregulation is important for estuarine animals adapt to salinity fluctuations. The present study investigated the effects of low salinity stress on osmoregulation and gill transcriptome in two populations of mud crab from Hangzhou Bay and Zhangzhou Bay of China, respectively. Crabs were transferred from salinity 25 ppt to 5 ppt for 96 h. Edematous swelling in gill filaments was caused by low salinity stress and was more serious in Zhangzhou Bay population. Gill Na⁺/K⁺-ATPase activity increased (p < 0.01) in both populations under the low salinity stress and was significantly higher (p < 0.01) in Hangzhou Bay population than in Zhangzhou Bay population. According to transcriptome analysis, there were 191 genes differentially expressed under the low salinity stress in gill tissue of both populations. Several ion transport and energy metabolism related pathways, as well as the arginine and proline metabolism pathway, were enriched by these genes. On the other hand, 272 genes were identified to differentially express between two populations under the low salinity stress, but not under the control salinity. The enrichment analysis showed that these genes were mainly related to ion transport, energy metabolism, osmolytes metabolism and methyltransferase activity. In conclusion, the present study suggested that mud crab exploited a combination of extracellular anisosmotic regulation and intracellular isosmotic regulation for osmoregulation under the low salinity stress. Hangzhou Bay population showed a greater osmoregulatory capacity, which is probably due to the enhanced ion transport, energy supply, and osmolytes regulation. Meanwhile, epigenetic modification might also contribute to an inherent osmoregulation ability for Hangzhou Bay population to response to salinity fluctuation rapidly.

Evolutionary trade-offs in osmotic and ionic regulation and expression of gill ion transporter genes in high latitude, cold clime Neotropical crabs from the 'end of the world'

Osmoregulatory findings on crabs from high Neotropical latitudes are entirely lacking. Seeking to identify the consequences of evolution at low temperature, we examined hyperosmotic/hypo-osmotic and ionic regulation and gill ion transporter gene expression in two sub-Antarctic Eubrachyura from the Beagle Channel, Tierra del Fuego. Despite sharing the same osmotic niche, Acanthocyclus albatrossis tolerates a wider salinity range (2-65‰ S) than Halicarcinus planatus (5-60‰ S); their respective lower and upper critical salinities are 4‰ and 12‰ S, and 63‰ and 50‰ S. Acanthocyclus albatrossis is a weak hyperosmotic regulator, while H. planatus hyperosmoconforms; isosmotic points are 1380 and ∼1340 mOsm kg-1 H2O, respectively. Both crabs hyper/hypo-regulate [Cl-] well with iso-chloride points at 452 and 316 mmol l-1 Cl-, respectively. [Na+] is hyper-regulated at all salinities. mRNA expression of gill Na+/K+-ATPase is salinity sensitive in A. albatrossis, increasing ∼1.9-fold at 5‰ compared with 30‰ S, decreasing at 40-60‰ S. Expression in H. planatus is very low salinity sensitive, increasing ∼4.7-fold over 30‰ S, but decreasing at 50‰ S. V-ATPase expression decreases in A. albatrossis at low and high salinities as in H. planatus. Na+/K+/2Cl- symporter expression in A. albatrossis increases 2.6-fold at 5‰ S, but decreases at 60‰ S versus 30‰ S. Chloride uptake may be mediated by increased Na+/K+/2Cl- expression but Cl- secretion is independent of symporter expression. These unrelated eubrachyurans exhibit similar systemic osmoregulatory characteristics and are better adapted to dilute media; however, the expression of genes underlying ion uptake and secretion shows marked interspecific divergence. Cold clime crabs may limit osmoregulatory energy expenditure by hyper/hypo-regulating hemolymph [Cl-] alone, apportioning resources for other energy-demanding processes.

Correlation analysis of migration in Eriocheir sinensis of the Yangtze River estuary based on salinity preference

Reproduction and development are the most important stages in the life of aquatic animals, which are also the most sensitive stages to environmental stress. As a migratory crab, parent Eriocheir sinensis need to migrate to estuary (brackish water) for reproduction, and megalopas need to migrate to freshwater for development. Yangtze River estuary is located at the junction of the Yangtze River and the East China Sea, and the salinity of different regions varies greatly (salinity 0-30‰). Therefore, salinity is one of the key environmental factors affecting E. sinensis. In this study, the salinity preference of parent and megalopas E. sinensis was observed and analyzed by the six-chambered and seven-chambered devices of laboratory experiments. Two groups of alternative salinity were set for parent crabs (7, 14, 21, 28, and 35 ‰) and megalopas (0, 5, 10, 15, 20, 25, and 30‰). The results indicated that males showed no obvious preference for salinity, while females had obvious preference for different salinity and stronger dependence and tolerance to saltwater. Megalopas showed a significant preference for low salinity which was beneficial to migrating to freshwater quickly. It was speculated that the behavior of megalopas tending to freshwater was an inherited behavior in the evolutionary process rather than influenced by environment. The salinity preference of E. sinensis might change the position of spawning grounds and migratory routes, and then the annual fluctuation of salinity in Yangtze River estuary will affect the change of E. sinensis resources.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Label-free quantification proteomics analysis reveals acute hyper-osmotic responsive proteins in the gills of Chinese mitten crab (Eriocheir sinensis)

Chinese mitten crab (Eriocheir sinensis) is a typical euryhaline crustacean to study osmotic regulation of crustaceans. Osmotic-regulation of Chinese mitten crab is a complex process. In order to study the osmotic-regulation related proteins of Chinese mitten crab, we domesticated Chinese mitten crab for 144 h with 25 salinity sea water (SW) and 0 salinity fresh water (FW) respectively, and then analyzed the proteome of its posterior gills. A total of 1453 proteins were identified by label free proteomics. Under the threshold of 2 fold change (FC), 242 differentially expressed proteins (DEPs) were screened, including 122 up-regulated DEPs and 120 down-regulated DEPs. GO database and KEGG database were used to annotate and enrich DEPs. It was found that DEPs were significantly enriched in energy metabolism, signal transduction, ion transport, actin cytoskeleton, immunity, lipid metabolism, amino acid metabolism and other biological functions. After 144 h of high salinity stress, the energy metabolism of Chinese mitten crab decreased and the expression of actin and cytoskeleton protein increased. In order to cope with oxidative damage caused by high salinity, Chinese mitten crab improved its immunity and antioxidant capacity.

Toxic effects of metal copper stress on immunity, metabolism and pathologic changes in Chinese mitten crab (Eriocheir japonica sinensis)

Copper (Cu²⁺), which represents a major physiological challenge for crab culture, is ubiquitous in the aquatic culture environment, and gills are the first organs that come into direct contact with the environment. However, the molecular basis of the response of crabs to Cu²⁺ stress remains unclear. Here, we conducted a transcriptome and differential expression analysis on the gills from Chinese mitten crab unexposed and exposed to Cu²⁺ for 24 h. The comparative transcriptome analysis identified 2486 differentially expressed genes (DEGs). GO functional analysis and KEGG pathway analysis revealed some DEGs, which were mostly related to immunity, metabolism, osmotic regulation, Cu²⁺ homeostasis regulation, antioxidant activity, and detoxification process. Some pathways related to humoral and cellular immunity, such as phagosome, peroxisome, lysosome, mTOR signaling pathway, PI3K-Akt signaling pathway, Toll-like receptor signaling pathway, and T cell receptor signaling pathway were enhanced under Cu²⁺ stress. In addition, Cu²⁺ stress altered the expression patterns of key phagocytosis and apoptosis genes (lectin, cathepsin L, Rab7, and HSP70), confirming that Cu²⁺ can induce oxidative stress and eventually even apoptosis. Histological analysis revealed that the copper can induce damage at the cellular level. This comparative transcriptome analysis provides valuable molecular information to aid future study of the immune mechanism of Chinese mitten crab in response to Cu²⁺ stress and provides a foundation for further understanding of the effects of metal toxicity.

Comparative transcriptome analysis of the gills of Cardisoma armatum provides novel insights into the terrestrial adaptive related mechanism of air exposure stress

Cardisoma armatum is a typical member of the Gecarcinidae which show significant behavioral, morphological, physiological, and/or biochemical adaptations permitting extended activities on the land. The special gills (branchiostegal lung) of C. armatum play an important role in maintaining osmotic pressure balance and obtaining oxygen to adapt to the terrestrial environment. However, adaptive molecular mechanisms responding to air exposure in C. armatum are still poorly understood. In this study, transcriptomic analysis and histological analysis were conducted on the gills to test adaptive capabilities over 8 h between the aerial exposure (AE) and the water immersion (WI) group. Differentially expressed genes (DEGs) related to terrestrial adaptation were categorized into four broad categories: ion transport, acid-base balance, energy metabolism and immune response. This is the first research to reveal the molecular mechanism of terrestrial adaptation in C. armatum, and will provide new insight into the molecular genetic basis of terrestrial adaptation in crabs.

Evolution of digestive enzyme genes associated with dietary diversity of crabs

Crabs feed on a wide range of items and display diverse feeding strategies. The primary objective of this study was to investigate 10 digestive enzyme genes in representative crabs to provide insights into the genetic basis of feeding habits among crab functional groups. Crabs were classified into three groups based on their feeding habits: herbivores (HV), omnivores (OV), and carnivores (CV). To test whether crabs' feeding adaptations matched adaptive evolution of digestive enzyme genes, we examined the 10 digestive enzyme genes of 12 crab species based on hepatopancreas transcriptome data. Each of the digestive enzyme genes was compared to orthologous sequences using both nucleotide- (i.e., PAML and Datamonkey) and protein-level (i.e., TreeSAAP) approaches. Positive selection genes were detected in HV crabs (AMYA, APN, and MGAM) and CV crabs (APN, CPB, PNLIP, RISC, TRY, and XPD). Additionally, a series of positive selection sites were localized in important functional regions of these digestive enzyme genes. This is the first study to characterize the molecular basis of crabs' digestive enzyme genes based on functional feeding group. Our data suggest that HV crabs have evolved an enhanced digestion capacity for carbohydrates, and CV crabs have acquired digestion capacity for proteins and lipids.

Comparative transcriptome analysis of Eriocheir japonica sinensis response to environmental salinity

Chinese mitten crabs (Eriocheir japonica sinensis) are catadromous, spending most of their lives in fresh water, but moving to a mixed salt-fresh water environment for reproduction. The characteristics of this life history might imply a rapidly evolutionary transition model for adaptation to marine from freshwater habitats. In this study, transcriptome-wide identification and differential expression on Chinese mitten crab groups were analysed. Results showed: clean reads that were obtained totalled 93,833,096 (47,440,998 in Group EF, the reference, and 46,392,098 in Group ES, the experimental) and 14.08G (7.12G in Group EF 6.96G in Group ES); there were 11,667 unigenes (15.29%) annotated, and they were located to 230 known KEGG pathways in five major categories; in differential expression analysis, most of the top 20 up-regulated pathways were connected to the immune system, disease, and signal transduction, while most of the top 20 down-regulated pathways were related to the metabolism system; meanwhile, 8 representative osmoregulation-related genes (14-3-3 epsilon, Cu2+ transport ATPase, Na+/K+ ATPase, Ca2+ transporting ATPase, V-ATPase subunit A, Putative arsenite-translocating ATPase, and Cation transport ATPase, Na+/K+ symporter) showed up-regulation, and 1 osmoregulation-related gene (V-ATPase subunit H) showed down-regulation. V-ATPase subunit H was very sensitive to the transition of habitats. These results were consistent with the tests of qRT-PCR. The present study has provided a foundation to further understand the molecular mechanism in response to salinity changing in water.