Competing for Space: Factors That Lead to Sponge Overgrowth When Interacting with Octocoral

Abstract

The overgrowth of octocorals by sponges is generally disadvantageous for the octocorals. When the growth of octocoral populations increases the reefs’ structural complexity, more substrate space becomes available for epibionts to grow. One of these epibionts is the sponge Desmapsamma anchorata, known to grow exponentially faster than other sponges, and for its ability to overwhelm the substrate over which it grows. Our objective in this study is to determine if the overgrowth of branching octocorals by varying sponges is defined only by species-specific interactions, or if some physical attributes make this octocoral more susceptible to being overgrown than others. To determine this, on a reef south of Carrie Bow Cay, Belize, we monitored the attachment of various demosponges to branching octocorals in a series of experiments to assess the response effect of the interactions and properties leading to attachment and subsequent sponge overgrowth on branching octocorals. Substrate effects, species-specific reactions, and time of exposure were considered. Additionally, following successful-attachment, preliminary growth rates of two sponge species epizoic on two octocoral species were monitored. Differences in the observed interaction between species and treatments were analyzed with a multinomial response model to isolate a species-specific effect from other interacting effects of the properties tested. Results indicate that time of exposure of a sponge in direct contact with the octocoral fragment significantly affects the outcome of the interaction; secondly, physical properties and species-specific interactions also influence the attachment. Following the successful attachment to an octocoral, the growth of the sponge varies as a function of the sponge’s differential growth rate and the substrate species over which it grows. In the context of species successions, understanding interactions between coralreef organisms is important as these interactions explain species composition and diversity, and provide a basis for the prediction of future changes in complex reef communities.