Elephant Tusk Shells (Scaphopoda)

 
Shells of Dentalium rubescens, a Medi-
terranean scaphopod. (Source)
Class Species No.
Snails (Gastropoda) 43.000
Mussels (Bivalvia) 10.000
Squids (Cephalopoda) 650
Elephant Tusks (Scaphopoda)        600
Neopilina (Tryblidia) 20
Chitons (Placophora) 750
Solenogastres 230
Caudofoveata 120
Molluscs (Mollusca) 55.400
Species number of molluscs. Diagram.
 

On the coast shells can often be found that belong to an animal that looks roughly like an elephant's tusk, which it is therefore colloquially called. In contrary to a real elephant tusk this shell, however, is open on both ends. Most often, besides, it is much smaller in size, ranging between 2,5 and 12 cm (1 to 5 in.).

The shell's former inhabitant, Dentalium entalis, is a member of a further mollusc class, the Scaphopoda. Similar to mussels, scaphopods are generally sessile animals living on or better in the ocean floor. Biology calls this a benthic form of life, in contrary, for example, to the pelagic life of swimming animals. Scaphopods live from the shoreline until a depth of 7,000 m (21,000 ft.).

Watching a living scaphopod you will usually see nothing more than the rear third of the shell. The scientific name literally means boat-foot animals, referring to the foot's shape resembling a boat. A scaphopod's foot is made for digging the animal into the ocean floor. To do so, the animal pushes its foot into the ground, anchors it by pressing blood into it and then pulls in the rest of the shell.


Foraminifers, a scaphopod's main food.
 

In the ocean floor scaphopods feed on smallest creatures, like the one-celled Foraminifers. A scaphopod catches its food with the captacula, thin tentacles with sticky ends. After catching a foraminifer, the captacula then transport it into the scaphopod's mouth. Foraminifers have a calcareous shell, which the scaphopods cracks with its radula, before digesting the contents in the stomach using digestion fluids from the finger-shaped protrusions of the main digestion gland (hepato-pancreas).

After depleting a feeding ground the scaphopod moves to another part of the ocean floor.

Looking at the composition of a scaphopod's body, one can see, that the mantle covers all of the shell's inner surface. The mantle cavity is shaped like a tube from the front to the rear end of the animal. It is open in front as well as at the back, as is the shell. Scaphopods do not possess gills. By movement of cilia water is sucked into the rear end of the mantle cavity. Depleted from oxygen and loaded with carbon dioxide it then leaves the mantle cavity by the same way.

 
Schematic illustration of a scaphopod's body.
Source: Biodidac, further editing: R. Nordsieck.

A scaphopods' heart is reduced to such an extent, that it is not even shown in the schematic illustration on the right side. The blood circulation is completely free, blood not being pumped by heart action, but by foot contractions. Kidney and gonads have a common exit, opening into the mantle cavity as well as the intestine. Scaphopods have separate sexes. After an external fertilisation their development passes a planktontic larval stage of the trochophora type, of which evolve, after a metamorphosis, young scaphopods.


Two sides of a scaphopod (Dentalium vulgare).
Above: Rear end, through which water in driven
into the pallial cavity by ciliar action.

Burrowing foot pulling the animal into the ground.
Pictures: Marc Cochu, Nature 22, Dentalium.
 

Systematically the scaphopods (phylogenetically the youngest mollusc group, known as fossils only since the Ordovician Geological Timeline) resemble in more than one way the bivalves (mussels, clams and cockles). Both groups are bilaterally symmetric animals (not all molluscs are) living on or in the ground with a strong digging foot. Both groups of animals possess a very large mantle, the two halves of which have partly grown together at the seams.

In contrary to the mussel group, scaphopods do possess a radula and them having like a frontal proboscis also the head is not completely reduced, as it is among mussels. Also unlike mussels, scaphopods to not feed by filtering the water, they catch their prey with the captacula.

Today it is assumed both groups have evolved from a common ancestor among primordial benthic molluscs, but also separated in their subsequent evolutionary history.

Two orders of scaphopods are known: Dentaliida and Gadilida, different mainly in shell form and radula type. The Dentaliida are thought to be the phylogenetically older group of the about 600 known scaphopod species.

Not only snails and mussels have been used by prehistoric tribes to craft jewellery. The shells of scaphopods were crafted into bracelets and necklaces, as can be seen in an exhibition of the Asparn (Zaya) prehistoric museum.

Prehistoric museum in Asparn (Zaya), Lower Austria.

Especially in Northern America, scaphopod shells have been used as decoration, jewellery and money by many peoples.

 
Native Indian necklace from Dentalium and Haliotis shells. The abalone (Haliotis) is a sea shell, 
which unlike other gastropods also produces pearls. Source: "Art by Vivian Hailstone".

Quite well known are the chest armours of Sioux and Kiowa made from scaphopod shells. The elongate shells are even today used for ornamental chains and jewellery. Their importance as money, however, has been lost with the settlement of white nations in North America and the introduction of money from metals.

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