The tusk shells or scaphopods are a class of marine mollusks which vary in size from very small to medium sized. The scientific name of this class is Scaphopoda, meaning "shovel-footed".
These mollusks 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. The class Bivalvia represents their nearest living relatives.
Description of the shell
The shells of scaphopods are conical and curved in a planispiral way, and they are usually whitish in color. Because of these characters, 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 mollusks 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, most are small, however, a few species reach 15 centimetres
Habitat and distribution
Tusk shells live in the bottom sediment where they feed
Anatomy of the soft parts
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. 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 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.
The fossil record
Tusk shells are well-known in the fossil record
, having first appeared in the Ordovician
. As such they were the most recent of all molluscan classes to evolve. They are most closely related to the extinct molluscan class Rostroconcha
. Among living creatures, their closest relatives are the Pelecypoda
The shells of Dentalium hexagonum
, a scaphopod mollusk, were strung on thread and used by the natives of the Pacific Northwest
, shell money
Shells of the species Dentalium pretiosum were used as money
- G. Steiner, "Phylogeny and classification of Scaphopoda", Journal of Molluscan Studies 58:385-400 (1992)