Ecology of a corallivorous gastropod, Coralliophila abbreviata, on two scleractinian hosts, I: Population structure of snails and corals

Despite their potential importance in structuring reef communities, invertebrate corallivores and their population structures are poorly understood. We found distinct differences in the population structures (length-frequency distribution and sex ratio) of the corallivorous gastropod Coralliophila abbreviata residing on two coral-host taxa, Montastraea spp. and Acropora palmata, in the Florida Keys. In each of two survey years, around 50% of the Montastraea spp. colonies were infested, with a mean snail density of eight snails per infested colony (range 1-45), while around 20% of A. palmata colonies harbored three snails per infested colony (range 1-23). Variation in patterns of snail occurrence was also observed within a host taxon. A. palmata occurred in low- and high-density stands (0.4 and 1 colony m^-2, respectively, at the initial survey) at different sites. Hurricane Georges struck the area in September 1998. When resurveyed in 1999, density of colonies in low-density stands had decreased by 75% to 0.1 colonies m^-2. This decrease was accompanied by a doubling in the proportion of colonies infested with snails (from 19% to 46%) and an increase in snail density per infested colony (from 3.7 ± 3.3 SD to 5.4 ± 4.6 SD) as snails apparently concentrated on surviving A. palmata. In contrast, sites with high density A. palmata stands (thickets) retained colony densities of about ∼1 colony m^-2 among years, while snail infestation increased only from 9% to 14% of colonies surveyed and snail density essentially remained unchanged (from 2.7 ± 1.8 to 2.9 ± 1.9 snails per infested colony). Snails collected from Montastraea spp. were shorter than those from A. palmata in low-density stands and were longest on A. palmata in thickets. On both host taxa, female snails were longer than males. The sex ratio of snails on Montastraea spp. hosts was even (1:1), while that of snails on A. palmata was skewed (70% males). Factors that could explain observed differences in size structure and sex ratio between Coralliophila populations on the two coral host taxa include: differential susceptibility to predators, influence of host tissue nutritional quality and/or secondary metabolite content, and genetic differences (cryptic species). The host-specific characteristics of C. abbreviata populations imply that the impact of gastropods on reef communities will vary with the coral species composition.