Pre-settlement behavior in larval bryozoans: the roles of larval age and size

Larval behaviors prior to settlement are important for both dispersal and the likelihood that larvae will encounter settlement habitat. The role of endogenous factors such as larval age and size are likely to be important in determining pre-settlement behavior but are less well understood than exogenous factors. In a simple experiment we explored the role of larval age and size on pre-settlement behavior in two species of bryozoan. We then used the results of this experiment to develop a theoretical model, which explored potential fitness benefits associated with phenotype-dependent changes in larval behavior (i.e., behaviors that changed depending on larval age or larval size) in a heterogeneous environment. In the experiment we delayed the metamorphosis of larvae of Bugula neritina andWatersipora arcuata and assessed the changes in the behavior of individual larvae (exploring the substratum vs. swimming away from it) as a function of larval age and size. In B. neritina, larval size had no effect on larval swimming behavior, but the youngest and oldest larvae spent more time exploring the substrate than did larvae of intermediate age. In W. arcuata, larval size and age had interactive effects on larval behavior. Our theoretical model predicted that phenotype-dependent behaviors carried a fitness benefit relative to phenotype-independent behaviors, but this depended strongly on the availability and quality of habitat elsewhere. We suggest that, taken together, larval age and size are important endogenous factors that act to affect pre-settlement larval behavior and that changes in behavior may act to increase fitness.