Indirect effects of a predatory gastropod in a seagrass community

Abstract

While research and theory have largely focused on the effects of direct interspecific interactions such as predation and competition, there is growing evidence for the importance of indirect interactions in regulating populations and community composition. In St. Joseph Bay, FL, USA, pen shells [ Atrina rigida (Sol.)] are preyed on by horse conchs [ Pleuroploca gigantea (Kiener)]. The shells of dead Atrina are subsequently used as shelters and/or nest sites by a suite of benthic species, including crabs, octopi, and fish. By influencing the rate of new shelter availability, horse conchs may indirectly affect the distribution or reproduction of shell-occupant species. Feeding observations and an exclosure experiment indicate that Atrina are the primary prey of large horse conchs. The abundance of occupant species is strongly correlated to Atrina shell abundance, suggesting that shelter availability may influence population distributions. Experimental additions of new shelters mimicking freshly-killed pen shells caused an increase in the abundance of occupant species. Among these species, the blennies Hypsoblennius hentzi (Leseuer) and Chasmodes saburrae J. & G. and the clingfish Gobiesox strumosus Cope showed an increase in both abundance and frequency of egg-guarding, suggesting an important indirect effect of horse conchs on the distribution and reproductive output of these fish. Because the addition of new shells affected blennies and clingfish more strongly than other occupant species, community composition is influenced by shelter availability. In habitats where biogenic structures are prominent, such as mollusc shells in soft-sediment marine communities, this type of nontrophic indirect effect may be an important component of population and community regulation.