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Coral reefs are under pressure from numerous anthropogenic and natural threats. A species of vermetid worm, Dendropoma maxima, has a negative interaction with the corals it lives on. Vermetid worm interactions result in structural homogeneity of reef habitats, reduced coral growth, and increased coral death. Dendropoma maxima and corals have a naturally evolved competitive relationship. Therefore, it is inherent that removal of D. maxima from the environment would alleviate stress to coral associated with vermetid worm activity and lead to increased coral growth and survival. A 2015 die-off of D. maxima in French Polynesia is likely to have had direct positive effects to coral and cascading effects on reef community structure. The aim of this investigation was to contribute to the understanding of vermetid worm and coral interactions by quantifying the extent of stress on corals under reduced vermetid worm densities. This study measured D. maxima size and abundance on several coral types and assessed Porites coral health by the presence of pink lesions and growth along edges of coral. Surveys of D. maxima size and Porites coral health occurred at Temae Reef, Moorea in October and November 2016. Live D. maxima were less than half the size of dead D. maxima. Average size of live D. maxima was different between Porites coral and dead coral substrates and between Top or Side positions. There was a strong positive correlation between number of live D. maxima and number of lesions on Porites coral and positive correlation between live D. maxima and percent of coral death. A positive but insignificant correlation between number of live D. maxima and length of new growth was observed. The die-off of D. maxima in 2015 appears to have positively impacted coral health and the results are important to understanding shifts in community structure and ecological health, especially as environmental changes become more frequent in the future.
Hi Liam - We read your paper with interest as we and our colleagues (Tom Frazer and Jeff Shima) have been following the die-off of Dendropoma (now Ceraesignum) maximum for the past several years. Our extensive surveys in 2015, 2016, and 2017 suggest that C. maximum was completely killed in Mo'orea by August 2015. We have yet to see any recovery -- our most recent surveys (in June 2017 on the north shore of Mo'orea found 0 live C.maximum; our surveys in Temae in 2015 found 0 live C. maximum). We are therefore guessing that what you have documented are C. maximum (the dead snails) vs. D. platypus (the live "D. maxima"). Do you have any photographs of the live snails that you sampled? If so, we could quickly determine if they are C.maximum or D.platypus. D.platypus tends to have an aperture diameter of 4-8mm, whereas C.maximum is more typically 9-25mm.
Even if the live vermetids turn out to be D. platypus, it obviously doesn't change the importance of the other effects you documented on the corals.