Life Histories in an Epifaunal Community: Coupling of Adult and Larval Processes

Marine invertebrates growing epifaunally on red mangrove (Rhizophora mangle) prop roots in the Indian River, Florida, USA, were studied in a small mangrove island (Jim Island) through which a number of channels had been cut. Roots hanging down into the water supported diverse epifaunal communities including sponges, oysters, barnacles, bryozoans, and acidians. To determine what factors control species' population dynamics and contribute to the high degree of spatial heterogeneity characteristic of communities in the unique habitat, two hypotheses were tested: (1) Distributions of species on the roots are controlled by differential growth and mortality due to physical features; and (2) Recruitment, as influenced by larval supply, structures the community. Four channels of the island were chosen for comparison and experimentation. Distributions of abundances of epifauna in the channels were determined and physical parameters (i.e., temperature, pH, salinity, flow, turbidity) were measured over a 13—mo period. Adult and early juvenile organisms were transplanted among channels and growth and survival were monitored. Patterns of water flow in the island were studied, and plankton samples were taken to determine how larval supply varied among channels in different parts of the island. General patterns or recruitment were measured for 9 mo and patterns were compared to adult distributions. Epifaunal cover differed among the study channels, with dramatic differences in abundance and species diversity. Except for flow, physical factors did not differ significantly among channels. Flow rate, per se, was not responsible for disjunct distributions since neither adult nor juvenile survival (processes expected to be most affected by flow) differed among channels. Plankton samples and recruitment measurements revealed that the importance of larval supply depended on the life history of the individual species. Those producing short—lived lecithotrophic larvae showed patchy distributions that were strongly affected by the location of source populations and prevaling patterns of water flow. Species with long—lived planktotrophic larvae were more evenly distributed and post—settlement processes played a more important role in their populations dynamics. On large temporal or spatial scales, the effects of physical factors on juvenile and adult organisms are probably quite important in controlling epifaunal distributions. However, in this study, the distributions of organisms on Rhizophora mangle root within Jim Island were best explained by differential larval input, with larval life history determining the strength of coupling between adult populations, larval supply, and patterns of recruitment.