Tiny Snails Are Secretly Devastating Coral Reefs Across the Pacific

We studied coral reefs in Moorea's lagoons and documented a pattern: some coral colonies studying coral reefs in Moorea's lagoons documented a pattern: some coral colonies appeared healthy and three-dimensional, while others were stunted and flattened. This morphological difference correlated with the presence of small, tube-building snails called vermetids that most reef scientists rarely study.

We investigated whether these gastropods, specifically Dendropoma maximum, harm reef-building corals. The snails are sessile filter-feeders that cast out mucus nets to capture food, and those nets often contact neighboring corals. We surveyed 90 patch reefs to document patterns, then conducted controlled field experiments by transplanting coral fragments to small patch reefs, randomly removing vermetids from half the reefs while leaving them at natural densities on the others.

Vermetids reduced skeletal growth rates of Pocillopora by 68-81%, P. rus by 62%, P. lobata by 40-62%, and Montipora by 24%. They also reduced coral survival by up to 52% for three of the four species tested. Averaged across species, vermetids reduced coral skeletal growth by 56% and colony survival by 40%. Field surveys supported the experimental results: vermetid presence was strongly associated with flattened coral morphology, and higher vermetid densities correlated with more dead coral substrate.

The effects varied considerably among coral species. Montipora maintained 100% survival even with vermetids present, while Pocillopora suffered the most severe growth reductions. This variation suggests that vermetids might reshape reef communities by affecting coral species differently.

These findings are significant because coral reefs face mounting pressure from climate change, pollution, and other human impacts. The discovery that an abundant but overlooked species causes substantial additional damage highlights the potential importance of poorly studied species to coral dynamics. The differential effects on coral species suggest vermetids could alter coral community composition, potentially affecting reef diversity and resilience.

Citation

Shima, Jeffrey S.; Osenberg, Craig W.; Stier, Adrian C. (2010). The vermetid gastropod Dendropoma maximum reduces coral growth and survival. Biology Letters.

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