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Creseis acicula (Rang, 1828b) forma clava (Rang, 1828b)

Overview

This is a shelled pelagic snail, up to 0.6 cm long, with a transparent uncoiled shell. The cross-section is round. The surface is smooth. The shell is short and narrow, tube shaped and not curved. The visceral mass is seen through the shell. It is a clumsy swimmer that feeds on phytoplankton and protozoa. It lives in the warmer waters of all oceans in the upper water layers. The adults never grow as long as in the forma acicula (Creseis a. clava 2).

Taxonomic Description

The shell is straight, up to 6 mm long, with a diameter at the aperture of about 1 mm. The animal is connected with the columellar muscle to the caudal tip of the shell. This muscle disappears quickly above its attachment to the shell in the visceral mass, only a small part of it runs free in the shell lumen. The embryonic shell is rounded at its end, the caudal part, in profile has two or more swellings caused by transverse rings below a concave portion ending in the perfectly round embryonic end. Sometimes transverse striations are found caudally, and near the rings the shell is thicker. The embryonic shell is never darker than the rest of the shell. The wings have a wing protrusion and opposite a wing gland. The wing glands overlap the base of the wing protrusion. The radula formula is 1-1-1.
The shell is up to 6 mm long, with a diameter at the aperture of about 1 mm.

Juveniles

Protoconch I is long conical with blunt tip and a clear incision between protoconch I and II. Protoconch II is not well separated and continues without interruption into the slender conical teleoconch (Creseis a. clava 1, Creseis a. clava).

Reproduction

The form is a protandric hermaphrodite.

Ecology

This form is phytophagous and epipelagic. Descending rate is controlled by angle-pitch of wings and their horizontal breadth. During upward locomotion the wings are active. When descending the animal may be in a horizontal or vertical position, which causes a slow and a quicker sinking respectively. When attacked by small predators the animal does not retract completely in its shell which results in a slow descent in horizontal position controlled by the wing position described. When attacked by larger animals, the animal retracts completely which results in a quick sinking in a vertical position.

Distribution

This form is found in warm waters of the Atlantic, Indian, and Pacific Oceans, for more information see also the form acicula, see the Creseis a. clava map.

Geological Record

This species is known from the Late Quaternary of the Red Sea.

Types

Cleodora (Creseis) clava Rang, 1828: 317, pl. 17, fig. 5.
Lectotype: MHNP (dry collection) Paralectotypes: MHNP, 4 fragments (dry collection).
Type locality: Banc des Aiguilles. Coll.: Rang [1837].

Creseis acicula clava