The challenge of small and rare species in marine biodiversity surveys: microgastropod diversity in a complex tropical coastal environment

Although molluscs feature prominently in the semi-popular and academic literature on marine biodiversity, field surveys largely ignore the small and rare species that form the majority of marine molluscan diversity. As a result of a massive effort to sample the benthic molluscs of a complex tropical coastal environment, 23,238 gastropod specimens representing 259 species of Triphoridae—a family with most adult species ranging from 2 to 10 mm—were obtained from a 45,000 hectares study area off the island of Espiritu Santo, Vanuatu. Most species are represented by fewer than 20 specimens and, despite the intensity of the sampling effort, 13% of the species are unique singletons. Spatial heterogeneity was high: out of 416 sampling events, 187 contained triphorids, and 42% of the species occurred at fewer than 5 stations. Most species were small (68% below 5 mm) or very small (22% below 3 mm). A faunal turnover was documented at around 10 m, and another at around 60 m, at the onset of the “twilight zone” that is particularly difficult to sample. On the order of 70% of the species are probably new to science. When dealing with taxonomically difficult groups a morphospecies segregation approach is operationally appropriate to detect patterns of richness, rarity and spatial turn-over. Very few, if any, conservation surveys have the human and funding resources to carry out baseline surveys of the intensity that generated the results presented here. However, as species numbers are often used to promote the conservation interest of a reef, a bay or a stretch of coast, it is essential to know how the numbers were generated: absolute numbers of species are meaningless unless sampling effort and techniques, area surveyed, and size classes targeted are described. This is very rarely the case, even in the academic literature.