Prior exposure to low salinity affects the vertical distribution of Pisaster ochraceus (Echinodermata: Asteroidea) larvae in haloclines

Increased freshwater discharge during spring/summer from the Fraser River into Puget Sound makes this region ideal to study the effects of low salinity (similar to 20%) on the vertical distribution of marine invertebrate larvae. We investigated whether exposing early developmental stages of the sea star Pisaster ochraceus to low salinity for 3, 7, and > 25 d affects larval morphology and the ability of later developmental stages (brachiolariae) to swim to the surface in the presence or absence of haloclines. We also determined the effect of food patches at the halocline on swimming behavior. Larvae reared in low salinity throughout development had 55-95% shorter posterolateral arms than those in other treatments. In the absence of a halocline, exposure to low salinity for 7 or > 25 d reduced the percentage of brachiolariae that swam to the surface (3 and 30%, respectively) compared to brachiolariae held for 0 or 3 d at low salinity (64 and 84%, respectively). Brachiolariae with no exposure to low salinity remained around the halocline in the presence of food, while those reared in continuous low salinity swam directly to the surface. Brachiolariae acclimated to low salinities and then transferred to a stratified water column avoided the halocline even in the presence of food. We conclude that exposure of early developmental stages to low salinity affects the swimming ability of brachiolariae. Our results suggest that sea star populations in the Pacific Northwest could be threatened if the influence of the Fraser River during the spring/summer continues to intensify in the southern Puget Sound region.