Reproductive Strategies of Marine Benthic Invertebrates : A Settlement - Timing Hypothesis

Evidence for nudibranch molluscs is presented which shows a clear relationship between egg-size and developmental mode (planktotrophic, lecithotrophic and direct development), egg-size and development time, and thereby larval strategy and development time. Egg-to-juvenile periods differ significantly according to strategy, with lecithotrophic pelagic development being the briefest, and planktotrophic development the longest. In addition, support of the lecithotrophic strategy necessitates a greater absolute calorific threshold on the part of the adult than does the planktotrophic strategy. It is proposed that for a given species there is an optlmal time to spawn (when standing crop IS at a peak) and an optimal time for the larva to settle. While accepting that in selecting for a particular strategy a compromise must be struck between such mediating factors as total and relative energy demand on the adult, egg numbers, individual probabilities of larval survival, and especially dispersal requirements, the settlement-timing hypothesis may be instrumental in resolving some paradoxical situations. Thus, for example, a species in which two or more alternative strategies appear supportable, in energetic terms, may select that strategy which appropriately 'bridges' the period between the optimal time to spawn and to settle.