Aggregation patterns of two corallivorous snails and consequences for coral dynamics

Abstract

Spatial heterogeneity plays an important role in consumer–resource interactions. It arises from variability in the underlying distribution of the resource and/or the consumer, as well as the habitat in which the consumer–resource interaction occurs. In some cases, the resource is the habitat, especially when the habitat is biogenic (e.g., kelp, corals, seagrasses). In these systems, the resulting dynamics can be particularly rich because the consumer–resource interactions are coupled with changes in the habitat (i.e., resource) that are due to the consumer–resource interaction. In Moorea, French Polynesia, two corallivorous snails, Coralliophila violacea and Drupella cornus, feed and live on massive Porites corals. Here, I (1) document the spatial patterns of the snails among sites, within sites, and on corals; (2) examine the drivers of small-scale aggregations by testing the attraction of the snails to chemical cues coming from conspecifics and corals; (3) test the effects of aggregations of snails on coral growth by manipulating snail density. The distributions of both snails were highly heterogeneous among sites across the island, and both species were spatially aggregated both among and within corals. The source of chemical attraction that caused the small-scale clustering differed between the two snails. D. cornus was attracted to conspecifics and to corals damaged by conspecifics, whereas C. violacea was attracted to damaged corals (regardless of the cause). Increasing snail density caused a linear decline in coral growth that was similar for the two snail species. The combination of the clustered spatial pattern of both snail species and their negative effects on coral growth could lead to important feedbacks in which high densities of snails reduce coral cover in localized areas and create spatial dynamics that affect the spatial distributions of both corals and snails across the reef.