Settlement of Planktonic Larvae: A Theory of Habitat Selection in Varying Environments

Abstract

A theory of habitat selection is developed which, although fairly general in form, is intended to apply with some realism to the settlement behavior of planktonic larvae of marine organisms. It is assumed that different substrates represent different habitats and that the environment can be heterogenous on two scales of distance: (1) a "fine grained" heterogeneity in which larvae are exposed to a homogeneous mixture of substrate types during the settlement period, and (2) a "course grained" heterogeneity in which the composition of the mixture varies from place to place. The objective is to predict whether settlement will occur on both substrates in the fine grained mixtures or only on one--in other words, whether larvae will exhibit habitat preference. Settlement behavior is treated as an absorbing Markov chain in which the transition probabilities are the probability on encountering a substrate, the probability of metamorphosing on it, the probability of adult survival and the probability of death in the plankton. Observations on the serpulid tube-worm Spirorbis borealis are introduced to illustrate the model. The relative attractiveness of species of fucoid algae as substrates for Spirorbis larvae is shown to differ between populations of Spirorbis. Both the field occurrence of Spirorbis on these substrates and its relative settlement on them in experimental tests are correlated with the availability and suitability of the substrates.