Settlement Preferences in Temperate Sea Stars

AbstractMany marine invertebrates possess biphasic life histories, during which larvae develop in the plankton and adults inhabit the benthos. The transition between phases entails the settlement of larvae onto substrata, completion of metamorphosis, and survival as vulnerable early juveniles. The perimetamorphic period, encompassing settlement and the interval immediately following settlement, is a key determinant of adult abundance and distribution. However, because settling larvae and early juveniles are difficult to observe in the field, the ecology of this period remains poorly understood. We performed experiments to elucidate the settlement preferences of Asterias forbesi and Asterias rubens, keystone predators on the east coast of North America, on substrata common to their intertidal habitats. Larval Asterias exhibit clear selectivity in settlement, with shells of the blue mussel, Mytilus edulis, most preferred. The algae Chondrus crispus and crustose coralline algae also induced high rates of settlement, while little settlement was observed on rocks with biofilm and no settlement occurred in controls. When inductive cues were subsequently added to controls, high frequencies of settlement occurred immediately, confirming the competency of larvae to settle and their ability to delay metamorphosis in the absence of appropriate cues. Our results demonstrate that Asterias larvae have specific settlement preferences and that settlement can be postponed in this species if no suitable substrate is available.