Vermetid snails reduce growth and survival of corals and shift coral species composition

The dynamics and restoration of corals are of critical concern given their ecological importance and the deleterious impacts caused by disease and anthropogenic activities. Previous research has focused on the possible effects of nutrients, algae and disturbance. Vermetid gastropods, which are sessile and feed via an extensive mucus net, are poorly studied members of the coral reef community, yet also may have important effects on corals. We examined the possible effects of vermetids on coral growth and survival in a field study in a lagoon of Moorea, French Polynesia. Vermetids were either left at ambient densities or removed from small (~3m²) patch reefs. Coral nubbins were collected, weighed (using the buoyant mass technique), and outplanted to these patch reefs. Growth was assessed after ~7 weeks and survival after ~8 months. Eight experiments were conducted using four different species of coral (Pocillopora, Montipora, Porites lobata, and Porites rus). Growth and survival were both greater offshore than inshore, although effects of vermetids were similar in both regions. Effects of vermetids were strongly deleterious but varied among coral taxa. Vermetids reduced the growth of coral by as much as 90% (Pocillopora) and by as little as 30% (Montipora). Survival was reduced from >98% in the “removals” to 79% (P. rus), 70% (P. lobata), and 45% (Pocillopora). These dramatic short-term effects can have dramatic consequences for coral regeneration and dynamics. We projected changes in coral community composition resulting from observed effects of vermetids and found that the effects rival those documented from other factors.