Neuroendocrine regulation of osmoregulation and the evolution of air-breathing in decapod crustaceans

Gills are the primary organ for salt transport, but in land crabs they are removed from water and thus ion exchanges, as well as CO2 and ammonia excre tion, are compromised. Urinary salt loss is minimised in land crabs by redi recting the urine across the gills where salt reabsorption occurs, Euryhali ne marine crabs utilise apical membrane branchial Na+/H+ and Cl-/HCO3- exch ange powered by a basal membrane Na+/K+-ATPase, but in freshwater crustacea ns an apical V-ATPase provides for electrogenic uptake of Cl- in exchange f or HCO3-. The HCO3- is provided by carbonic anhydrase facilitating CO2 excr etion while NH4+ can substitute for K+ in the basal ATPase and for H+ in th e apical exchange. Gecarcinid land crabs and the terrestrial anomuran Birgu s latro can lower the NaCl concentration of the urine to 5% of that of the haemolymph as it passes across the gills. This provides a filtration-reabso rption system analogous to the vertebrate kidney. Crabs exercise hormonal control over branchial transport processes. Aquatic hyper-regulators release neuroamines from the pericardial organs, includin g dopamine and 5-hydroxytryptamine (5-HT), which via a cAMP-mediated phosph orylation stimulate Na+/K+-ATPase activity and NaCl uptake, Freshwater spec ies utilise a V-ATPase, and additional mechanisms of control have been sugg ested. Crustacean hyperglycaemic hormone (CHH) has now also been confirmed to have effects on hydromineral regulation, and a putative role for neurope ptides in salt and water balance suggests that current models for salt regu lation are probably incomplete. In a terrestrial crabs there may be controls on both active uptake and diff usive loss. The land crab Gecarcoidea natalis drinking saline water for 3 w eeks reduced net branchial Na+ uptake but not Na+/K+-ATPase activity, thus implying a reduction in diffusive Na+ loss. Further, in G. natalis Na+ upta ke and Na+/K+-ATPase were stimulated by 5-HT independently of cAMP, Convers ely, in the anomuran B. latro, branchial Na+ and Cl- uptake and Na+/K+-ATPa se are inhibited by dopamine, mediated by cAMP, There has been a multiple e volution of a kidney-type system in terrestrial crabs capable of managing s alt, CO2 and NH3 movements.