The ups and downs of life in a halocline: The response of Pisaster ochraceus larvae to food patches and prior exposure to low salinity

Abstract

Salinity fluctuations are expected to become increasingly frequent in the Salish Sea due to glacial melting caused by global warming. Pisaster ochraceus, a keystone predator of the intertidal zone will be especially affected by these fluctuations since they lack the ability to ion- or osmo-regulate. We examined the impact of prior exposure to low salinity and the presence of a food patch on the behavior of P. ochraceus larvae in a halocline. In the gastrula stage, P. ochraceus larvae were split into a low salinity treatment (reared in 20-22‰ filtered sea water) and a control treatment (reared in 30- 32‰ filtered sea water). In the first experiment, bipinnariae were introduced into the top or bottom of 20/30 haloclines. In the second experiment brachiolariae were introduced into control columns with and without the algae Isochrysis galbana, and into 20/30 haloclines with and without I. galbana patches at the halocline. The halocline posed a major barrier to larval vertical migration, especially for the younger bipinnaria stage. Low salinity larvae changed their distribution in the water column at a much slower rate than control larvae, indicating that they may have impaired swimming abilities. Brachiolariae from the controls aggregated in the halocline when a food patch was present but those from low salinity swam right through the food patch to the top of the column. Lowered salinity in the Salish Sea could result in larvae arriving in unsuitable habitats since their different vertical distribution in the water column may result in currents carrying them to unsuitable habitats. Smaller adult P. ochraceus populations in the rocky intertidal could result in drastic changes to the ecosystem including a reduction in species diversity.