Three functionally distinct kinds of pelagic development

Interest in pelagic embryos and larvae of benthic marine animals has centered on dispersal and its consequences, but longer pelagic larval durations appear to have evolved and to persist for other reasons than dispersal advantages. Here are explanatory hypotheses for three kinds of pelagic development. (1) Pelagic development prior to ability to feed or settle avoids constraints of oxygen supply to aggregated embryos and scarcity of deposition sites. Also, costs of protection may be greater and survival benefits less than indicated by present evidence. (2) Obligately feeding pelagic larvae migrate to the plankton for greater growth or lower mortality. Under this hypothesis, the observed large scale dispersal is an incidental byproduct of a migration from benthic to pelagic habitat for greater growth or survival. Given pelagic development, unevenly favorable benthic environments and varying currents might result in selection for spread of sibling larvae as a form of bet hedging, but selection for dispersal as a result of pelagic development does not explain evolution of pelagic development. (3) For larvae released near competence for settlement and metamorphosis, the function of the pelagic stage is dispersal and habitat selection. For these larvae, the observed precompetent pelagic periods of seconds to a day may indicate pelagic durations adapted for dispersal because offspring could be protected until release at competence.