Feeding behaviour and the operculum in Olividae (Gastropoda): the case of Callianax biplicata (G. B. Sowerby I, 1825)

Olividae are marine gastropods living as predators or scavengers on soft sediments. The complex prey handling behaviour of large predatory species includes the storage of food in a pouch formed temporarily by bending and contraction of the posterior foot. Such metapodial pouches had been observed only in Olividae that lack an operculum, prompting the hypothesis that the folding of the metapodium into a pouch biomechanically required the absence of the operculum. Here we report metapodial pouch formation in an operculate olivid, Callianax biplicata (formerly Olivella biplicata). Since the operculum is too small to close the shell aperture in mature C. biplicata, a protective function seems unlikely. The operculum may rather serve as an exoskeletal point for muscle attachment, but may also represent a ‘vestigial organ’ in the process of evolutionary reduction, or an ontogenetic remnant functional at early life stages but not at maturity. Consequently, our observations refute the notion that only inoperculate olivids can form pouches, but not necessarily the idea that the ability to form a metapodial pouch evolved in parallel with operculum reduction