Temperature tolerance, salinity tolerance, osmoregulation, and water permeability of arctic marine isopods of the Mesidotea (=Saduria) complex

Abstract

Temperature tolerance, salinity tolerance, osmoregulation, and integumental water permeability have been studied in three arctic marine isopods of the Mesidotea complex, as well as in a freshwater variant of Mesidotea entomon. Temperature has little influence on their distribution in the southern Beaufort Sea. Habitat temperatures are far below the 96 h tolerance limits of the species which range from 21.5 to 26.3 °C. Salinity is an important factor in their distribution. The relative salinity tolerances and osmoregulatory capabilities of the isopods correlate well with their distribution in coastal waters. The marine M. entomon could not be adapted to fresh water even by a 7-week, stepwise transfer. The freshwater form, however, survives indefinitely in undiluted seawater. In all three species, the haemolymph is hyperosmotic in dilute seawater and isosmotic at higher salinities. Mesidotea entomon is the most effective osmoregulator and Mesidotea sabini is the least effective. All three species reduce their integumental permeability as the salinity falls. The capacity to alter the permeability to water increases in proportion to the euryhalinity of the species. The permeability varies directly with temperature, with Q10 coefficients ranging from 1.75 to 2.68.