High densities, rapid infestation and high feeding rates of corallivore gastropods on corals in Bora-Bora Island, French Polynesia

Abstract

The global decline of coral reefs has prompted research into understanding the impacts of anthropogenic stressors, as well as the interactions between corals and their natural predators, especially those that proliferate into outbreaks. However, studies on gastropod corallivores are lagging behind. The present study, conducted on Bora-Bora (French Polynesia), focused on two genera of corallivore muricid gastropods: Drupella spp. living on Acropora colonies and Coralliophila violacea on Porites rus colonies. An observational experiment of Drupella spp. and C. violacea showed strong spatial variation in densities across different habitats (fringing reef, channel, barrier reef) in Bora-Bora lagoon. The highest densities found were 21 ± 6 ind.m−3 of coral for Drupella spp. on the fringing reef, and 342 ± 51 ind.m−3 for C. violacea on the barrier reef. A manipulation experiment monitored the infestation dynamics of these gastropods. For both genera, colonization of vacant coral colonies was rapid during the first 14 days and then stabilized over the next 42 days. The manipulation experiment showed that the feeding rate on live and healthy coral tissue did not change significantly with Drupella spp. density (0.6 ± 0.2 cm 2/day/ind for 1 Drupella spp., and 0.5 ± 0.1 cm 2/day/ind for 5 Drupella spp.). Overall, our study highlights the high densities of two gastropod species, their rapid movement and infestation of vacant colonies and their role in the necrosis of corals during feeding. In view of the possible outbreaks involving high densities of such predatory gastropods in a major tourist destination in the Pacific Ocean, human resources could be put in place to manually remove these gastropods in a sustainable manner, but this would require a lot of effort in terms of manpower and time. Nature-based solutions such as the conservation of natural predators should be considered to prevent large-scale destructions by these gastropod aggregations.