Temporal range: Mississippian–Recent
|A shell of the scaphopod Antalis vulgaris from France|
The tusk shells or scaphopods // (Greek, "boat-footed") are a class of shelled marine mollusc with worldwide distribution, and are the only exclusively class of infaunal marine molluscs. Shells of species within this class range from about 0.5 to 15 cm in length. Members of the Order Dentaliida are generally significantly 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 usually not nearly as common or as easily visible in the beach drift as the shells of sea snails and clams.
The mantle of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow through the substrate. The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting up into the water.
Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by cilia. There are no gills; the entire surface of the mantle cavity absorbs oxygen from the water. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.
A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food, which they then convey to the mouth. The mouth has a grinding radula that breaks the bit into smaller pieces for digestion. The massive radula of the scaphapoda is the largest such organ relative to body size of any of the molluscs. The remainder of the digestive system consists of a digestive diverticulum, esophagus, stomach, and intestine. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The anus opens on the underside of the animal, roughly in the middle of the mantle cavity.
The scaphopod vascular system is rudimentary, lacking both heart and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. Metabolic waste is excreted through a pair of nephridia close to the anus.
The nervous system is generally similar to that of gastropods. One pair each of cerebral and pleural ganglia lie close to the oesophagus, and effectively form the animal's brain. A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body. Scaphopods have no eyes, or other distinct sensory organs.
Reproduction and development
The group is composed of a 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).
There is a good fossil record of scaphopods from the Mississippian onwards, making them the youngest molluscan class; but the origin of the group remains contentious.
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.
The shells of Dentalium hexagonum, a scaphopod mollusc, were strung on thread and used by the natives of the Pacific Northwest as shell money. Dentalium shells were also used to make belts and headdresses by the Natufian culture of the Middle East, and are a possible indicator of early social stratification.
Shells of the species Dentalium pretiosum were used as money.
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For a comprehensive overview, see Reynolds, P. D. (2002). "The scaphopoda". Molluscan Radiation - Lesser-known Branches. Advances in Marine Biology 42. pp. 137–236. doi:10.1016/S0065-2881(02)42014-7. ISBN 9780120261420. PMID 12094723. .