Salinity-induced morphological changes in Pisaster ochraceus (Echinodermata: Asteroidea) larvae

Abstract

A regular occurring event in the coastal waters of the Pacific Northwest is a decrease in salinity (to <15‰ in some areas) as a result of increased precipitation and ice melt. Low salinities can suppress feeding activity and act as a barrier to the vertical movement of adults of the echinoderm Pisaster ochraceus. How larvae of this keystone species would respond to a decrease in salinity in the Puget Sound region is unknown. The purpose of the present study was to determine how larvae of P. ochraceus respond to low salinity in the laboratory. Sea star larvae at different stages of development were kept at 32‰ for 51 days (controls) or exposed to fluctuating salinity (that is transferring gastrulae, bipinnaria or brachiolaria larvae from 32 to 20‰) for 3, 7 or 14 days and then back to 32‰. Total larval length and width, the posterolateral arm length and stomach length and width were measured during development. Gastrulae exposed to 20‰ for 3 days and bipinnaria larvae exposed to 20‰ for 14 days developed into wider and shorter brachiolariae while larvae from the controls and brachiolariae subjected to low salinity for 3 days developed into longer and slender brachiolariae. This is the first study to report salinity-induced morphological changes during development and metamorphosis at 20‰. These morphological changes might allow brachiolaria larvae close to settlement to move from an osmotically stressful environment at the surface of the water column to higher salinity waters in the benthic habitat. © Crown copyright 2012.