Selection of predator-free settlement sites by larval ascidians

Abstract

In rocky subtidal habitats of the San Juan Islands, Washington, the solitary ascidian Boltenia villosa occurs primarily as an epizooite on two other ascidians, Pyura haustor and Halocynthia igaboja, where it obtains refuge from the abundant predatory gastropod Fusitriton oregonensis. To investigate the prediction that larvae of this species should prefer refuge habitats as settlement sites, I offered choices among various “safe” and “unsafe” substrata to the tadpole larvae of B. villosain laboratory experiments. Tadpoles selected tunic of the refuge ascidians significantly more often than subtidal rocks or other substrata, but did not distinguish between refuge ascidians and conspecific adults. Larvae delayed metamorphosis, often into their post-competent period, when refuge substrata were not available to them. Four ascidian species that do not occur as epizooites in the field were offered a series of choices that included the refuge substratum Pyura haustor. None of the four favored tunic of P. haustor over natural rock or conspecific adults. Larval behavior is important in controlling small-scale distributional patterns within this assemblage, and predation appears to be a selective pressure that maintains habitat selection in the behavioral repertoire of Boltenia villosa tadpoles.