Trophic diversification in the evolution of predatory marine gastropods of the family Terebridae as inferred from stable isotope data

The family Terebridae includes approximately 400 species of predatory marine gastropod mollusks, commonly found in sandy bottom communities in the tropical Indo-Pacific. Like other Conoidea, Terebridae are typified by the presence of a venom gland and a highly specialized radula, which they use for hunting. Remarkably, some lineages of the family exhibit a tendency to reduction and complete loss of the radula, venom gland and proboscis. Recent studies on the mollusc community of Murray Beach (Nha Trang Bay, southern Vietnam) revealed an unusually rich and diverse fauna of Terebridae in a limited area of monotonous sandy sediments at depths of 6 to 18 m. In the present paper, mechanisms of resource partitioning between 16 syntopic Terebridae species from Murray Beach are studied using stable isotope analysis (SIA). The terebrid species studied exhibit considerable variation in isotopic signatures: mean delta C-13 and delta N-15 values of different species ranged between -16.9 and -9.3%, and between 5.8 and 9.5%, respectively. Although the isotopic niches of the species overlap considerably, the 5 most abundant species, contributing similar to 80% to the total abundance of the terebrid community, show little or no overlap. The studied Terebridae belong to 5 major phylogenetic lineages. By integrating phylogenetic data with the SIA results, we infer possible mechanisms of adaptive radiation in the family. Closely-related species within most phylogenetic clades, especially Clades E1 and E5, are well separated in isotopic niche space. In contrast, members of different clades overlap considerably in isotopic niche space, suggesting that trophic specialization has occurred independently in the evolution of different Terebridae lineages.