Scaphopoda (; from Ancient Greek σκᾰ́φης (skáphēs) 'boat' and πούς (poús) 'foot') is a class of shelled marine molluscs (invertebrates within the phylum Mollusca), whose members are known as scaphopods () and commonly called tusk shells or tooth shells. They have a worldwide distribution and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range in length 0.5–18 cm (0.20–7.09 in), with Fissidentalium metivieri being the longest. Members of the order Dentaliida tend to be larger than those of the order Gadilida. These molluscs live in soft substrates offshore (usually not intertidally). Because of this subtidal habitat and the small size of most species, many beachcombers are unfamiliar with them; their shells are not as common or as easily visible in the beach drift as the shells of sea snails and clams. Molecular data suggest that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains unresolved.
Classification The group is composed of two subtaxa, the Dentaliida (which may be paraphyletic) and the monophyletic Gadilida. The differences between the two orders is subtle and hinges on size and on details of the radula, shell, and foot. Specifically, the Dentaliids are the physically larger of the two families, and possess a shell that tapers uniformly from anterior (widest) to posterior (narrowest); they also have a foot which consists of one central and two lateral lobes and which bends into the shell when retracted. The Gadilids, on the other hand, are much smaller, have a shell whose widest portion is slightly posterior to its aperture, and have a foot which is disk-like and fringed with tentacles which inverts into itself when retracted (in this state resembling a pucker rather than a disk).
According to the World Register of Marine Species:
Dentaliida da Costa, 1776 family Anulidentaliidae Chistikov, 1975 – 3 genera family Calliodentaliidae – 1 genus family Dentaliidae Children, 1834 – 14 genera family Fustiariidae Steiner, 1991 – 1 genus family Gadilinidae Chistikov, 1975 – 3 genera family Laevidentaliidae Palmer, 1974 – 1 genus family Omniglyptidae Chistikov, 1975 – 1 genus family Rhabdidae Chistikov, 1975 – 1 genus Gadilida Starobogatov, 1974 sub-order Entalimorpha Steiner, 1992 family Entalinidae Chistikov, 1979 – 9 genera sub-order Gadilimorpha Steiner, 1992 family Gadilidae Stoliczka, 1868 – 8 genera family Pulsellidae Scarabino in Boss, 1982 – 3 genera family Wemersoniellidae Scarabino, 1986 – 2 genera
Evolution
Fossil record
There is a good fossil record of scaphopods from the Mississippian onwards, making them the youngest molluscan class. The Ordovician Rhytiodentalium kentuckyensis has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from ribeirioid rostroconch molluscs such as Pinnocaris. However, a competing hypothesis suggests a Devonian/Carboniferous origin from a non-mineralized ancestor, or from a more derived, Devonian, conocardioid rostroconch.
Phylogeny The scaphopods are largely agreed to be members of the Conchifera, however their phylogenetic relationship with the other members of this subphylum remains contentious. The Diasoma concept proposes a clade of scaphopods and bivalves based on their shared infaunal lifestyle, burrowing foot, and possession of a mantle and shell. Pojeta and Runnegar proposed the extinct Rostroconchia as the stem group of the Diasoma. An alternative hypothesis proposes the cephalopods and gastropods as sister to the scaphopods with helcionellids as the stem group. A review of deep molluscan phylogeny in 2014 found more support for the scaphopods, gastropods, or cephalopods than for scaphopods and bivalves, thus the shared body features of scaphopods and bivalves may be convergent adaptations due to similar lifestyles. Analysis of the scaphopod nervous system demonstrated that both scaphopods and cephalopods share a similar nervous system structure, with ventrally shifted pedal nerves and lateral nerves that extend dorsally. These similarities led to the conclusion that scaphopods are sister to the cephalopods with gastropods as sister to them both. More recent studies, including the sequenced genome of tusk shells and a concordant genome-based phylogeny with fossils and morphology, support the Diasoma model with bivalves as the sister group.
Orientation The morphological shape of the scaphopod body makes it difficult to orient it satisfactorily. As a result, researchers have often disagreed as to which direction is anterior/ posterior and which is ventral/ dorsal. According to Shimek and Steiner, "[t]he apex of the shell and mantle are anatomically dorsal, and the large aperture is ventral and anterior. Consequently, the concave side of the shell and viscera are anatomically dorsal. The convex side has to be divided into anteriorly ventral and dorsally posterior portions, with the anus as the demarcation. Functionally, as in cephalopods, the large aperture with the foot is anterior, the apical area posterior, the concave side dorsal and the convex side ventral."
Anatomy
Shells
The shells of the members of the Gadilida are usually glassy-smooth and narrow, with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators. The Dentalids, by contrast, tend to have strongly ribbed and rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze. This makes them harder to detect by animals such as ratfish, which can sense the electrical signals given off by the most minute muscle movement.
… excerpt ends here. Continue reading the full article.
