Larval period and its influence on post-larval life history in the nudibranch Phestilla sibogae

Abstract

Ontogenetic changes in larval phototaxis and metamorphic competence as well as the effects of an extended larval period on post-larval life history were examined in the opisthobranch gastropod Phestilla sibogae. Data presented in Chapter One showed that daily changes in larval phototaxis and the onset of metamorphic competence were not correlated. Considered together, these ontogenetic changes present a picture of how development affects dispersal and settlement of these larvae in their complex reef habitat. Data on the interactions between larval and post-larval life-history stages are presented in Chapters Two and Three. Weekly, unfed (i.e., lecithotrophic) larvae up to 21 days old and fed (i.e., facultatively planktotrophic) larvae up to 28 days old were induced to metamorphose by exposure to their natural prey, the coral Porites compressa. Pairs of juveniles and adults were raised through senescence and death on this coral. Egg-mass production, larval, juvenile, and adult weights, metamorphic success, and stage-specific survival and longevity were recorded. Extending the fed larval period significantly lowered larval survivorship and slightly lowered metamorphic success. Extending the unfed larval period significantly decreased larval and juvenile survival, metamorphic success, and larval and juvenile weights. Comparisons between the effects of a fed larval period and those of an unfed larval period showed significant differences for all the measured variables except adult longevity. Thus, long periods in the plankton can result in decreased individual fitness, and this effect on fitness is greatly exaggerated in species with lecithotrophic development. In Chapter Four the effects of an extended larval period on fitness are examined and the evolution of lecithotrophy from an ancestral planktotrophic condition is discussed. Three fitness parameters (population growth rate, generation time, and net reproductive rate) were calculated from demographic life tables. These tables were constructed using survivorship and fecundity data on laboratory-reared individuals plus field survivorship data on related invertebrate species. Extending the larval period resulted in significant decreases in the fitness parameters. These results are used to argue that reducing the competent larval period can facilitate an evolutionary shift from planktotrophy to lecithotrophy.