Echinoderm responses to variation in salinity

Although Echinodermata is one of the only stenohaline phyla in the animal kingdom, several species show remarkable abilities to acclimate and survive in euryhaline habitats. The last comprehensive review of this topic was over 25 years ago and much work has been published since. These recent studies expand the field reports of species living in hyposaline environments and detail experimental research on the responses, physiological range, and limits of echinoderms to salinity challenges. I provide a brief review of the historical concepts and measures of salinity and relate this overview to the physiological and ecological studies on echinoderms. Many marine biologists are not aware that chemical oceanographers advocate abandoning today's commonly used measure of salinity, 'PSU', in favour of absolute salinity (SA)-a return to the ppt (‰) metric. The literature survey reveals only one euryhaline-tolerant species in the Southern Hemisphere (there are 42 in the North) and more euryhaline species in the geologically older, brackish seas. The green sea urchin, Strongylocentrotus droebachiensis, is one of the most tolerant echinoids to hyposalinity. Different source populations have varying levels of acclimation and tolerance to hyposalinity. Experiments show that green urchins previously unexposed to hyposalinity experience a clear decrease in growth rates; however, this adverse effect is short lived. Green urchins already acclimated to hyposalinity can endure intense and repeated bouts and grow at the same rate of urchins not exposed. Promising future work on the physiological and cellular mechanisms of hyposalinity acclimation includes comparative studies of the role of heat shock proteins in the response to changing salinities.