Scaphopods are a small class of molluscs with shells shaped like elephant tusks, which gives them their common name of tusk shells. The name scaphopod means shovel-foot but it is actually their ‘head’ that they use for burrowing through the marine sediments where they live. This is an animal that spends its life upside down in the mud. The pointed back end of the tusk shell sticks out above the surface of the mud and is used for gas exchange and to get rid of wastes.
Internally tusk shells are simple animals. There is a mantle for secreting the shell but no gills in the mantle cavity. There is no heart and circulation is through open sinuses only. There are no eyes on the proboscis-shaped head. Instead there are a number of long and slender, almost squid-like, prehensile tentacles called captacula which are used for sensing the environment and capturing prey. These then retract to deliver food items such as diatoms and foraminifera to the mouth. There is a large buccal cavity with a radula used to break up and swallow the prey. This is followed by an esophagus and a small stomach with a caecum where digestive glands open. There is a coiled intestine and a ventral anus. The feces are carried out of the mantle cavity by the respiratory currents which are drawn in by cilia and pushed out by muscular contractions.
Sexes are separate. There is a single long gonad in the middle of the body and the gonoduct runs to the right kidney, similar to that of some of the gastropod molluscs. The females release the eggs one at a time into the water where they are fertilised and then develop through two planktonic larval stages just like other molluscs: a ciliated trochophore larva followed by a bilaterally symmetrical veliger stage. Eventually the larvae settle in the bottom sediments and take on the life of a burrowing adult. Scaphopods normally live in sediments that are a few meters deep but can occur as deep as 4500 meters.
Both the larvae and the adults are predators, eating such small planktonic and benthic organisms as diatoms and foraminifera. In turn, the larvae are eaten by bigger planktonic predators from crustaceans and small fish up to the baleen whales. Adults are preyed on by fish and crabs though they gain some protection from predation by living in sediments. Hermit crabs have been known to live in old scaphopod shells. Humans don’t have much to do with scaphopods but coastal American Indian tribes used to use the shells for decoration and a form of currency, trading them with other shells across the continent for goods they otherwise could not acquire.
Although they are single-shelled, scaphopods appear from their larval forms to be descended from the pelecypod or bivalve line of molluscs. There are fossil scaphopods from the Devonian period, some 400 million years ago. Currently there are about 900 known living species of this moderately successful class of molluscs.
References: http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/scaphopoda.php http://animaldiversity.ummz.umich.edu/site/accounts/information/Scaphopoda.html Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.
