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

Abstract

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 (genusCupolaconcha), 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 cladeCupolaconcha, is described and named asVermetus biperforatusBieler, Collins, Golding & Rawlings n. sp.

Replacing mechanical protection with colorful faces–twice: parallel evolution of the non-operculate marine worm-snail genera Thylacodes (Guettard, 1770) and Cayo n. gen. (Gastropoda: Vermetidae)

“…Our phylogenetic analysis also included the newly described Vermetus biperforatus from Belize ( Bieler et al., 2019 ), an operculate species with ability of modifying its shell aperture by covering it with a shell dome with two equal shell openings. Its authors had placed it in Vermetus sensu lato for lack of another suitable position.…”

“…Our ongoing study of vermetids in the western Atlantic continues to uncover new morphological forms which molecular, anatomical, behavioral, and sperm characters are helping to confirm as genetically discrete lineages ( Golding et al., 2014 ; Bieler et al., 2017a ; Bieler et al., 2017b ; Bieler et al., 2019 ). The present study is particularly significant in revealing an entirely new clade of non-operculate vermetids which, upon first appearance, could easily be confused with members of the genus, Thylacodes .…”

“…A complete list of non-operculate taxa sampled, including locality details and collector information, is provided in Table 1 . We also extracted DNA from foot tissue from the recently described operculate vermetid, Vermetid biperforatus (Belize, Bieler et al., 2019 ). In addition, we supplemented our dataset with gene sequences available on GenBank for other operculate vermetid genera ( Ceraesignum, Cupolaconcha, Dendropoma, Eualetes, Novastoa, Petaloconchus, Thylaeodus, Vermetus ) to determine their relationship to focal non-operculate taxa.…”

“…As part of a long-term biodiversity study of Vermetidae, with special focus on the western Atlantic ( e.g. , Bieler, 1996 ; Bieler & Petit, 2011 ; Golding et al., 2014 ; Bieler et al., 2017a ; Bieler et al., 2019 ), this publication focuses on non-operculate taxa. Upon closer examination of previously undescribed non-operculate species in the western Atlantic that traditionally would have been classified in Thylacodes , we noticed fundamental morphological differences in soft-body anatomy, shell entrenching behavior, larval shell morphology, and sperm ultrastructure, which together indicated the presence of a previously unrecognized clade of vermetids.…”

Systematic revision of the snorkel snail genus Rhiostoma Benson, 1860 (Gastropoda, Caenogastropoda, Cyclophoridae) with descriptions of new species

“…Although the COI barcoding does not reflect the actual evolutionary relationship, it seemed to provide valuable traits for species-level classification; thus, those two species are synonymised with Rhiostoma hainesi . The complex gas-exchange structures have been detailed in studies of some alycaeid and vermitid caenogastropods, and the authors suggest that they can provide useful characteristics for higher-level systematics ( Páll-Gergely et al 2016b , 2017 , 2020b ; Bieler et al 2019 ; Jirapatrasilp et al 2022 ).…”