| Tusk shells Temporal range: Mississippian–Recent[1][2] |
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| A shell of the scaphopod Antalis vulgaris from France | |||||
| Scientific classification | |||||
| Kingdom: | Animalia | ||||
| Phylum: | Mollusca | ||||
| Class: | Scaphopoda Bronn, 1862 |
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The tusk shells or scaphopods are a class of shelled marine mollusks. The scientific name of this class is Scaphopoda, meaning "shovel-footed". 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 on 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.
Molecular data suggests that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains somewhat unresolved.[3]
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Scaphopods feed on microscopic organisms; some supplement their diet of zooplankton with vegetable matter.[4]
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The aragonitic[5] shells of scaphopods are conical and curved in a planispiral way, and they are usually whitish in color. Because of these characteristics, the shell somewhat resembles a miniature elephant's tusk, hence the common name tusk shell. However, unlike an elephant's tusk, the shells of these molluscs are hollow and open at both ends; the opening at the larger end is the main or anterior aperture of the shell. The smaller opening is known as the apical aperture. Some tusk shells are minute, and most are 4 to 6 centimetres (1.6 to 2.4 in) long; however, a few species reach 15 centimetres (5.9 in) in length.
The scaphopod shell bears a prominent cross-lamellar layer,[6] and has a bioeroded tip.[6]
Tusk shells live in seafloor sediment where they feed on detritus, foraminiferans and microscopic animals.
The group is composed of a two subtaxa, the Dentaliida (which may be paraphyletic) and the monophyletic Gadilida.[1] The differences between the two orders is subtle and hinges on details of the radula and shell.
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 remainder of the digestive system consists of an oesophagus, 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.[7]
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 fertilised, 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.[7]
There is a good fossil record of scaphopods from the Mississippian onwards,[6] 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.[8]
As such they were the most recent of all molluscan classes to evolve. They are most closely related to the extinct molluscan class Rostroconchia.[1]
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.[9]
Shells of the species Dentalium pretiosum were used as money.
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. .