Chapter 8 - Osmoregulation

Abstract

Carcinus maenas is a successful globally invasive species in part due to its ability to survive in waters of low and fluctuating salinity. An osmotic/ionic conformer in full-strength seawater, this species makes the physiological transition to osmoregulation in salinities below 27ppt. C. maenas is classified as a moderately strong regulator, maintaining its haemolymph osmotic concentrations up to 350mOsmkg−1 above ambient at the lower end of its salinity tolerance. The mechanism of osmoregulation lies in the active transport of ions by the posterior gills, with some conservation of ions through reabsorption via the antennal gland. Na+ and Cl− are actively taken up from the ambient medium through a transport mechanism that involves multiple transport proteins localized to the apical and basolateral membranes of the gill and in the cytoplasm. The expression and activity of these proteins are salinity sensitive, being upregulated in low salinity when the crab makes the transition from osmoconformity to osmoregulation. The regulatory process is believed to be under inhibitory control by a putative repressor found in the major endocrine complex of the crab’s eyestalk, the X organ/sinus gland complex. The regulatory process appears to break down at the level of protein synthesis near the lower limit of salinity tolerance of the species, providing an insight into the nature of physiological stress.