Desperate larvae: influence of deferred costs and habitat requirements on habitat selection

As marine invertebrate larvae age, some accept a wider variety of settlement cues. A conceptual argument, the desperate larva hypothesis, has been proposed to explain this change in behaviour, and focuses on the idea that larvae should accept less preferred habitats as time goes by because the deferred costs of continued searching are too great. Whilst this model has explained why some species change their preferences as they age, it struggles to account for other species that do not. General theoretical considerations of the issue have tended to focus on a parameter space outside that which is likely to be typical of marine larvae. We adapted a more general dispersal/search model specifically for marine larvae and examined the influence of larval energy intake, planktonic mortality and habitat quality and abundance on the benefits of decreased selectivity at settlement. We found that decreased selectivity carries an adaptive benefit across the majority of our parameter space. Whenever planktonic mortality is high, larvae deplete their resources quickly (as is the case for most lecithotrophs) or there is little difference in the quality of different habitats (as for generalists); therefore, larvae should become less choosy with regards to settlement. However, our model suggests that decreasing selectivity will not be adaptive when larvae can feed or when there are large differences in the quality of potential habitats. Initial indications from the literature generally conform to the predictions of our model and the occurrence of decreasing selectivity can be predicted based on an organism's habitat specificity and ability to feed during the facultative planktonic stage. Our model predicts that habitat selection behaviour should also vary within species. For non-feeding larvae, larger larvae (i.e. those with more resources) should remain selective for longer than smaller larvae. For feeding larvae, local food availability in the plankton should strongly affect the benefits of delaying metamorphosis in the absence of settlement cues.