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Diacria quadridentata (De Blainville, 1821a)

Overview

This is a small, uncoiled, shelled thecosomatous pteropod, 0.2 cm long. It is globular with rounded ventral and dorsal sides and moderately developed ribs. The lateral spines are small. The caudal spine is absent, as in juveniles, the protoconch is broken off, leaving only a mark. Shell sculpture consists of faint growth lines and 3 dorsal striae. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the Indo-Pacific Ocean (Diacria quadridentata lateral).

Taxonomic Description

The shell globular, hyaline, with only a brown-red along the thickened border of the upper shell lip, in the centre of the ventral lip and the adjacent ventral side, along the lateral ribs between the lateral spines and the corners of the caudal spine mark. (Diacria quadridentata drawing, Diacria quadridentata shell details, Diacria quadridentata). Dorsal ribs are less developed, transverse striation on dorsal and ventral surfaces is faint, posterior portion of the ventral surface is concave. The lateral spines are well developed and curved caudad. A small part of the ventral side is visible in dorsal view. The embryonic parts are usually absent. The place of rupture of the caudal spine is oval; there are two small spines at its lateral corners. The small lateral sides, and lateral spines, form an arch (Diacria quadridentata lateral spine). There are three dorsal striae. The dorsal aperture lip is thickened near its ends, coming close to the ventral lip and forms a kind of closing apparatus. This mechanism is not as developed as in Cavolinia, but it does split off the two posterior slits of the aperture. The aperture ends near the lateral spines. The upper aperture lip is curved ventrally and forms a vault above the aperture. The radula (formula 1-1-1) is composed of about 9 transverse rows of teeth.
Shell length 2.04-3.0 mm, shell width 1.84-2.48, diameter caudal spine mark 0.76-1.08 mm.

Morphology and Structure

This species is a protandric hermaphrodite. The digestive gland is bilobate, the two lobes are equal in size, with the left one in direct contact with the intestine. Tesch (1946) did not find gills but Vayssiére (1915a) found them as horseshoe-shaped organs. In Tesch's opinion the function of the gills is taken over by the mantle appendages near the lateral parts of the shell aperture, the place where the mantle is in direct contact with the outside. These two structures are in fact present, being strongly folded and provided with many septa through which the blood can circulate as it passes the sinuses. Together they form a horseshoe-shaped structure, but as they are embedded in the latero-dorsal mantle tissue they are not homologous with the horseshoe-shaped gills found in Cavolinia.

Juveniles

The juvenile has a long, dorso-lateral flattened needle-like shell with lateral sides diverging anteriorly. The protoconch I is rounded, slightly egg-shaped. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous.

Distribution

This species occurs between 30°N-30°S in the Indo-Pacific Ocean, see the Diacria quadridentata map.

Other

The protoconch described for Diacria quadridentata by Grecchi and Bertolotii (1988) does not belong to this species.

Types

Hyalaea quadridentata De Blainville, 1821: 81.
Types were not found in the MHNP.

Diacria quadridentata