Impacts of low salinity on settlement by competent larvae of Pycnopodia helianthoides

Abstract

Freshwater input from terrestrial environments likely impact much of the biogeographic range of the sunflower sea star Pycnopodia helianthoides, particularly its planktonic larvae. Larvae that encounter these water masses need to be able to tolerate freshened seawater. In general, echinoderm larvae have poor tolerance to low salinity, but how changes in salinity affect the settlement and metamorphic success of P. helianthoides is not known. Larvae were exposed to an algal settlement cue within four salinity treatments ranging from 20–30‰– control for a duration of 8 days. No difference of overall survival occurred between treatments. By 6 h, settlement occurred in salinity ≥ 24‰. Mean settlement (%) increases steeply from 20‰ to ~24–27‰ indicating this decreased salinity range is likely where larvae delay their settlement. With time, salinity performance curves show the probability of all salinity/settlement cue groups to undergo a metamorphic response approaches 1, indicating settlement rate rather than absolute ability to settle under low salinity conditions may influence patterns of new recruits. Juveniles were first noticed at 177 h exposure and were only found in control and 27‰ treatments. These results suggest that varying salinity levels, commensurate with levels likely experienced by larvae in nature, impact absolute settlement and settlement rates in P. helianthoides. We provide important insight into how salinity conditions in the plankton may drive recruitment patterns in the endangered sunflower sea stars benthic habitat.