Marine Biology (2+1)

Species: Dentalium octangulatum

Phylum : Mollusca

Class : Scaphopoda

Family : Dentalidae

Common Name : Ribbed elephant tusk shell

Habitat : Sandy bootom of near shore waters

Description: The shell is large, moderately stout and about ten times as long as the diameter of the front end. It is well curved. The sculpture normally consists of eight very strong, raised, rounded, longitudinal ribs which are particularly sharp and prominent towards the narrow posterior end; they often become slightly less strongly raised towards the anterior aperture. They are separated by wide, fairly deep more or less flattened or only slightly concave interstices which are traversed by numerous longitudinal striae, particularly towards the wider end; growth lines are inconspicuous. The aperture is slightly oblique, a little longer than broad and sharply octagonal in outline. The anal aperture is minute and slightly indented on the convex side. The shell is white, or slightly bluish white, and displays a fine gloss. In young specimens the shell is more strongly curved than in the adult and the interstices between the ribs are smooth. There are two abnormal specimens form Chennai in the Museum collection, one of which has nine longitudinal ribs, and the other only seven.

Similar to all scaphapods, ribbed elephant tusk shells live below the surface of the sand where they reside head down at an oblique angle with the posterior end projecting from the sand.

They have no eyes, no heart and no gills but are good detectors of vibration. Long, thin tentacles around the mouth capture the shelled protozoan on which they feed. In common with all tusk shells, it is carnivorous, feeding on foraminiferans and other microorganisms. Tusk shells, which look like miniature elephant tusks, live buried more or less vertically in sand. The shells are open at both ends. The tapered end projects just above the sand surface where water is circulated into a long mantle cavity for breathing and waste removal. After several minutes of very slow water inhalation, muscular contraction of the foot expels the water from the same opening. The foot protrudes from the opposite end.

Once the tusk shell has burrowed into the sand, contractions and extensions of the foot pack the sand, creating a small cavity in which the tusk shell can feed. Long tentacles extend from the ‘head’ and explore the cavity for organic debris, protozoan and bivalve larvae, which stick to the adhesive ends of the tentacles and are passed back to the mouth. Living tusk shells are rarely seen, but many are washed up dead on beaches, indicating that they are numerous. D. octangulatum, is the commonest species of the four species of Dentalium reported from East coast. These shells are abundant between Kanyakumari and Tuticorin.

Size : 60 mm