Tusk shell

Scaphopoda ( ; ) 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 , 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.

Scaphopoda ( ; ) 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 , 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

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."

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.

Reproduction and development

Scaphopods have separate sexes, and external fertilisation. They have a single gonad occupying much of the posterior part of the body, and shed their gametes into the water through the nephridium. Once fertilized, the eggs hatch into a free-living trochophore larva, which develops into a veliger larva that more closely resembles the adult, but lacks the extreme elongation of the adult body. The three-lobed foot originates prior to metamorphosis while the cephalic tentacles develop post metamorphosis. Scaphopods remain univalved throughout their morphogenesis contrary to bivalves. File:Necklace PRE.2009.0.238.1-a.jpg|Tusk shell necklace from Bronze Age (MHNT) File:Necklace PRE.2009.0.238.1-IMG 1851.jpg|Tusk shell necklace from Bronze Age (MHNT) File:Necklace PRE.2009.0.238.1-IMG 1992.jpg|Tusk shell necklace from Bronze Age (MHNT) File:Necklace PRE.2009.0.238.1-IMG 2001.jpg|Tusk shell necklace from Bronze Age (MHNT)

Text derived from https://en.wikipedia.org/w/index.php?oldid=1346974316 under CC BY-SA 4.0; modifications have been made.