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Diacria schmidti Van der Spoel, 1971a subsp. occidentalis Leyen and Van der Spoel, 1982

Overview

This is a small, uncoiled, shelled thecosomatous pteropod, 0.5 cm long, it is a relatively large form of this group. It is globular with rounded ventral and dorsal sides and strongly 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 6 to 14 dorsal striae. Micro- zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the W-Pacific Ocean (Diacria s. occidentalis).

Taxonomic Description

The rather globular shell is red-brown in colour and semi-transparent. The dorsal lip is arch-shaped and borders a very narrow shell aperture. The ventral and dorsal sides are both heavily vaulted, and show striae near the aperture lips. The caudal spine mark is oval. Between the small lateral spines and the corners of the spine mark the curved lateral sides are thickened. Usually a faint median depression is found on the ventral side and lip. The aperture lips are thickened. A pair of closing mechanisms, less developed than in Cavolinia, are found in the aperture. The radula formula is 1-1-1. The shell is rounded with dorsally five well-developed longitudinal ribs continuing to the caudal spine mark. In dorsal view the ventral 'valve' of the shell is visible. The lateral spines are slightly curved dorsally and caudally. The distance between the lateral spines is smaller than the shell width. Dorsally six up to fourteen clear imbricate transverse striae are present; ventrally thin but clear transverse striae are present which are more pronounced near the aperture. The sediment material did not show a colour pattern. In older specimens the dorsal lip extends parallel to the dorsal surface, as found in Diacria s. schmidti. In lateral view the posterior portion of the ventral and dorsal sides are straight or slightly convex.
Shell length 4.2-5 mm width 3.2-4.2 mm width caudal spine mark 1.04-1.64 mm.

Juveniles

The juvenile has a long, dorso-lateral flattened needle-like shell, with lateral sides diverging only 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 form is protandric hermaphrodite.

Ecology

This form is phytophagous.

Distribution

This form is restricted to the tropical W-Pacific, see the Diacria s. occidentalis map.

Types

Diacria schmidti subsp. occidentalis Leyen and Van der Spoel, 1982 : 144, fig. 6.
Holotype and 19 paratypes preserved in USNM 289901.
Type locality: USBF stat 5586, 337 fms gy. M. 44° Borneo Sibuko Bay off Sipadan Island.

Diacria schmidti occidentalis