A novel and enigmatic two-holed shell aperture in a new species of suspension-feeding worm-snail (Vermetidae)

• Main Authors: Rüdiger Bieler, Timothy M. Collins, Rosemary Golding, Timothy A. Rawlings
• Format: Article
• Language: English
• Published: PeerJ Inc. 2019-02-01
• Series: PeerJ
• Subjects: Mollusca; Morphology; Barrier reef; Diversity; Anatomy; Western Atlantic
• Online Access: https://peerj.com/articles/6569.pdf

Shell aperture modifications are well known in terrestrial and aquatic gastropods, with apertural lip thickening and tooth development common in species with terminal (determinate) shell growth. In contrast, secondary shell openings are rare in snails and are largely limited to slit shells, keyhole limpets, and abalone of the Vetigastropoda. When such features occur in other groups, they are noteworthy and raise interesting questions concerning the functional/adaptive significance of these shell modifications. Here we report on one such modification in a newly described species of vermetid snail. Members of the worm-snail family Vermetidae are sessile, suspension-feeding caenogastropods found in warm temperate to tropical marine environments worldwide. As juveniles, vermetids permanently cement their shells to hard substrata and subsequently produce irregularly coiled polychaete-like shell tubes with indeterminate growth and typically a simple circular shell aperture. In one previously studied group (genus Cupolaconcha), the aperture can be covered by a shell dome with a central slit that retains its widest opening in the center of the aperture. Vermetid specimens collected in the barrier reefs of Belize and the Florida Keys show an extreme aperture modification previously unknown in Gastropoda, in which the shell opening is covered by an apertural dome that leaves two equal-sized circular holes, each corresponding to the inflow and outflow water exchange currents of the animal’s mantle cavity. The function of this perforated apertural dome is unknown, and it is in some ways antithetical to the suspension feeding habit of these snails. Further field and laboratory-based studies will be needed to clarify the functional significance and trade-offs of this unique morphology. The new taxon, which is not closely related to the previously described dome-building clade Cupolaconcha, is described and named as Vermetus biperforatus Bieler, Collins, Golding & Rawlings n. sp.