Variations in the density of two ectoparasitic gastropods (Coralliophila spp.) on scleractinian corals on a coast-reef scale

Ectoparasitic gastropods living and feeding on coral hosts are now being reported as ubiquitous elements of coral reef environments. While there have been several reports of the damage being caused by these gastropods in the Asia-Pacific region, detailed information on ectoparasitic gastropods from the Western Indian Ocean region is scarce. Small islands such as Mauritius are largely dependent on coral reefs for tourism and protection of the coastal zone against natural disasters and high wave action, thereby highlighting the need to conserve and manage coral reefs sustainably. This requires an assessment of potential threats, including ectoparasitic gastropods, to coral reefs. In an effort to enhance our knowledge on ectoparasitic gastropods from the Western Indian Ocean region, the present study investigates variations in the density of ectoparasitic gastropod species feeding on coral hosts at three zones, namely the coast, lagoon and reef flat during summer and winter in 2016 and 2017 at Flic en Flac, west coast of Mauritius. Fifteen random quadrats were surveyed at each zone and ectoparasitic gastropod density was expressed as the number of individuals m −2 . Three line intercept transects of 30 m each were surveyed at each zone to determine the percentage coral cover in October 2016. Two ectoparasitic gastropod species, Coralliophila erosa and C. radula , identified both morphologically and genetically, were found to be associated only with Acropora muricata and Porites lutea , respectively. Distribution of the coral host along the coast-reef scale seemed to affect the densities of the gastropods. No C. erosa was recorded on the reef flat where the percentage cover of branching coral was lowest (54.67 ± 2.40%). Similarly, no C. radula was observed at the coast where the percentage cover of massive coral was as low as 5.00 ± 2.08%. The density of both ectoparasitic gastropods differed significantly among the zones and between the years (Kruskal-Wallis, p 0.05). This study constitutes the first report on the variations in the density of ectoparasitic gastropods on a coast-reef scale at a tropical site in Mauritius and may have implications for coral reefs conservation and management.