Costs of dispersal and offspring provisioning: insights from a marine bryozoan

Background/Question/Methods

Most studies addressing the optimal balance between the size and number of offspring a mother should produce typically focus on the effects of habitat quality, such as food availability, competitors, or predators. In many species, offspring disperse from their parents before the effects of habitat quality can be seen. Such dispersal in animals is often energetically or physiologically demanding. Longer dispersal durations can increase mortality after settlement when dispersal experiences ‘carry-over’ to influence survival in adult habitat. We predicted that smaller offspring with fewer resources might not have the stamina to survive after finally reaching distant habitat compared to larger offspring with more resources. To test this prediction, we experimentally delayed the settlement of larvae of marine bryozoan (Bugula neritina) in the laboratory and recorded subsequent growth and survival in the field. We then developed a theoretical model, based on a well-known optimality model, of how selection on offspring size is expected to change with dispersal distance.

Results/Conclusions

We found that smaller larvae had lowered post-settlement survival compared to larger larvae. This was probably because the energy a mother gives to smaller offspring is used to prolong the larval stage at the expense of being used by offspring for growth and development after colonization. Larger offspring on the other hand, may have more ‘fuel in the tank’ to cope with the energetic demands of dispersal. Our model predicted that, even when habitat quality does not vary, more isolated habitats favor the production of fewer, larger offspring when smaller offspring incur greater deferred costs of dispersal. Taken together, our results indicate that the spatial pattern of suitable habitat could impose very different selective regimes on offspring size compared to the effects of habitat quality. Furthermore, our predictions contrast to those predicted for seed size and dispersal in plants, where the production of smaller, more numerous seeds is often a more efficient way for mothers to access distant, suitable habitat.