An inordinate fondness for turrids

“Turrids” are a group of small predatory gastropods that are highly diversified in shallow as well as deep water. They sing a hymn to specialisation and rarity. A dataset of 34,810 specimens collected through a quarter-century of qualitative exploration off New Caledonia reveals 1409 species of turrids at depths deeper than 100 m. Much of this diversity is constituent of the “rare biosphere”: as many as 41% are singletons, and 73% of the species are represented by empty shells only. Species numbers vary with depth, reflecting both differences in sampling intensity and in actual species richness. The richest depth interval is 301–600 m, with 831 species, and a moderately good saturation. By contrast, the 601–1000 m interval, with 389 species, is still far from saturated. The overlap in species composition with other South Pacific tropical island groups (Vanuatu, Fiji, Tonga and the Marquesas) is only 17%, but we lack data to back up extrapolations at broader geographical scales (West Pacific, Indo-Pacific, World Ocean). These results champion the slopes of tropical islands, consisting mostly of highly heterogeneous hard bottoms, as a vast reservoir of biodiversity. However, the technological challenges associated with sampling hard bottoms and the geographical remoteness of these target areas combine in making these benthic communities among the least sampled and least studied in the world.