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Larval tolerances and developmental rates of four species and one hybrid cross of echinoderms were determined as a function of salinity. Fertilization and development of Strongylocentrotus droebachiensis (O.F. Muller, 1776), Strongylocentrotus purpuratus (Stimpson, 1857), Strongylocentrotus pallidus (G.O. Sars, 1871), and Pisaster ochraceus (Brandt, 1835) larvae were observed. A hybrid was created by fertilizing eggs of S. droebachiensis with sperm from S. pallidus. Fertilized eggs were placed in filtered, natural seawater (30‰ S) diluted to 30, 27.5, 25, 22.5, and 20‰ S and cleavage and larval development were observed over 32 days. Developmental rates differed among salinity treatments within each species. For each species, embryos and larvae at lower salinities (20, 22.5, and 25‰ S) tended to develop more slowly than those at higher salinities (27.5 and 30‰ S). Pisaster ochraceus and S. droebachiensis larvae survived all salinity treatments throughout the experimental period. The hybrid larvae exhibited salinity tolerances and developmental rates intermediate between values reported for the more stenohaline larvae of S. pallidus and those for the more euryhaline larvae of S. droebachiensis. The interaction of a secondary stressor (food availability) with salinity was examined; algal concentration did not influence the survival, developmental rate, or length of P. ochraceus or S. droebachiensis larvae.

Larval survival and development rates of Strongylocentrotus droebachiensis and Strongylocentrotus pallidus were determined as a function of salinity in two experiments by (i) directly transferring fertilized eggs obtained from adults acclimated to sea water at a salinity of 30‰ to cultures containing seawater at salinities of 30, 27.5, 25, 22.5, 20, 17.5, 15, 12.5, and 10‰ at 10 °C; and (ii) acclimation of adult sea urchins to the salinity–temperature conditions described above for 2, 3, and 4 weeks prior to spawning. Subsequent development occurred under these acclimation conditions. Development rates and percent survival of larvae prior to metamorphosis varied directly with salinity. Survival of S. pallidus plutei to metamorphosis decreased at salinities below 30‰. Strongylocentrotus droebachiensis plutei survived to metamorphosis at 20‰ and above. Lactic acid concentrations in the coelomic cavity fluid of adult S. droebachiensis and S. pallidus acclimated to low salinity were significantly higher than initial controls at 30‰. Cell volumes of fertilized eggs of both species exhibited osmotic swelling when exposed to lowered salinity. LC50 values (‰), development rates, and percent survival to metamorphosis indicate that acclimation of adult urchins to lower salinity prior to spawning and fertilization does not enhance development or survival of embryos of these two species exposed to low salinity. Furthermore, our results show that S. pallidus larvae are stenohaline when compared with larvae of other echinoderm species.