Response of the sea urchin, Strongylocentrotus droebachiensis, to reduced salinities: importance of size, acclimation, and interpopulation differences

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Abstract
Weight change, mortality, and neuromuscular coordination, as measured by righting time, were used to evaluate the resistance of Strongylocentrotus droebachiensis to hypoosmotic stress. Acclimation to a salinity of 24 or 25‰ decreased the righting time of urchins after 96 h of hypoosmotic stress. The effect of acclimation was the same for four size groups of urchins (10–15, 20–25, 34–37, and 50–60 mm in test diameter). A comparison of urchins from the St. Lawrence Estuary and from Nova Scotia, both acclimated to 24‰, showed that for the three larger size classes, the salinity at which there was a significant increase in righting time was lower for urchins from an estuarine environment than for urchins from the oceanic environment. The percentage weight gain following hypoosmotic stress increased with decreasing urchin size. For small urchins the increase was less marked after acclimation to 24‰. The mortality rates following these experiments showed that acclimation to a reduced salinity increased the tolerance to hypoosmotic stress, that urchins from the St. Lawrence Estuary were better able to withstand low salinities than urchins from Nova Scotia, and that small urchins were more sensitive to osmotic stress than large urchins. The latter was confirmed in a field experiment in which four size groups of urchins were kept in cages at a location exposed to periodic decreases in surface salinities.