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Little is known about the functions of the crustacean gut microbiome, but environmental parameters and habitat are known to affect the composition of the intestinal microbiome, which may in turn affect the physiological status of the host. The mud crab Scylla serrata is an economically important species, and is wild-caught, and farmed across the Indo-Pacific region. In this study, we compared the composition of the gut microbiome (in terms of gut microbial species richness and abundance) of S. serrata collected from wild sites, and farms, from the east and west coast of India, and also tested the effects of the environment on the composition. The water temperature had a statistically significant effect on gut microbiome composition, with microbial biodiversity decreasing with increasing water temperature. This could have negative effects on both wild and farmed mud crabs under future climate change conditions, although further research into the effects of temperature on gut microbiomes is required. By comparison, salinity, crab mass and carapace width, geographical location as well as whether they were farmed or wild-caught crabs did not have a significant impact on gut microbiome composition. The results indicate that farming does not significantly alter the composition of the gut microbiome when compared to wild-caught crabs., (© 2021 The Authors. MicrobiologyOpen published by John Wiley & Sons Ltd.)

There is a growing body of evidence implicating the involvement of crustacean hyperglycaemic hormone (CHH) in ionic homeostasis in decapod crustaceans. However, little is known regarding hormonally influenced osmoregulatory processes in terrestrial decapods. As many terrestrial decapods experience opposing seasonal demands upon ionoregulatory physiologies, we reasoned that these would make interesting models in which to study the effect of CHH upon these phenomena. In particular, those (tropical) species that also undergo seasonal migrations might be especially informative, as we know relatively little regarding the nature of CHHs in terrestrial decapods, and hormonally mediated responses to seasonal changes in metabolic demands might also be superimposed or otherwise integrated with those associated with ionic homeostasis. Using Discoplax celeste as a model crab that experiences seasonal extremes in water availability, and exhibits diurnal and migratory activity patterns, we identified two CHHs in the sinus gland. We biochemically characterised (cDNA cloning) one CHH and functionally characterised (in terms of dose-dependent hyperglycaemic responses and glucose-dependent negative feedback loops) both CHHs. Whole-animal in situ branchial chamber (22)NaCl perfusion experiments showed that injection of both CHHs increased gill Na(+) uptake in a seasonally dependent manner, and (51)Cr-EDTA clearance experiments demonstrated that CHH increased urine production by the antennal gland. Seasonal and salinity-dependent differences in haemolymph CHH titre further implicated CHH in osmoregulatory processes. Intriguingly, CHH appeared to have no effect on gill Na(+)/K(+)-ATPase or V-ATPase activity, suggesting unknown mechanisms of this hormone's action on Na(+) transport across gill epithelia.

The Christmas Island red crab Gecarcoidea natalis undergoes extreme changes in metabolic status, ranging from inactivity during the dry season, to a spectacular annual breeding migration at the start of the wet season. The dramatic change in metabolic physiology that this polarisation entails should be reflected in changes in endocrine physiology, particularly that of the crustacean hyperglycaemic hormone (CHH), of which we know relatively little. CHH levels were measured using a novel ultrasensitive time-resolved fluoroimmunoassay (TR-FIA), together with metabolites (glucose, lactate), in the field at several scales of temporal resolution, during migratory activities (wet season) and during the inactive fossorial phase (dry season). Release patterns of CHH were measured during extreme (forced) exercise, showing for the first time an unexpectedly rapid pulsatile release of this hormone. A seasonally dependent glucose-sensitive negative-feedback loop was identified that might be important in energy mobilisation during migration. Haemolymph lactate levels were strongly correlated with CHH levels in both field and experimental animals. During migration, CHH levels were lower than during the dry season and, during migration, daytime CHH levels (when most locomotor activity occurred) increased. However, the intense dawn activity in both dry and wet seasons was not always associated with repeatable hyperglycaemia or CHH release. The results obtained are discussed in relation to the life history and behaviour of G. natalis.