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Diacavolinia angulosa (ms. Eydoux and Souleyet) (Gray, 1850)

Overview

This is a small uncoiled thecosomatous pteropod, 0.5 cm long. It has a flat dorsal side with moderately developed ribs. The ventral side is vaulted. The caudal spine is absent and the caudal spine mark is not left as the ventral and dorsal side grow together. The lateral spines are either small or well developed. Shell sculpture consists of faint growth lines and a faint transverse striation. The dorsal lip has a notch, no constriction and a gutter with an inner hump. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the Indian Ocean and Indo-Malayan waters at shallow depths (Diacavolinia angulosa 2).

Taxonomic Description

The transparent shell is triangular; it is brown, especially near the hump, the dorsal lip, the joint, and near the central part of dorsal and ventral side (Diacavolinia angulosa). The dorsal lip has a notch in the broad rostrum, but no constriction; characteristic is the nose on the middle of the rostrum. Owing to the strong inner hump, which is continuous with the underside of the lip shoulder, the central dorsal rib is not continuous on the rostrum. The ventral lip is moderately sized and shows a clear median depression. The lateral spines are perfectly straight and point laterally, they are hooked and have a sharp tip in the holotype; in some populations the lateral spines are reduced. The gutter corners are small (Diacavolinia angulosa drawing). The 18 ventral ribs are thin and linear; the anterior part of the shell has no ribs. The dorsal side is convex, with a well developed central rib and latero-dorsal ribs; the lock ribs are small though rather thick. The lock area is clearly visible with a small main tubercle and link. The lunar tubercle is well developed, the minor tubercle is absent or sometimes replaced by a hole. A second lock mechanism is absent although a slight irregularity in the latero-dorsal margin of the aperture may be a rudiment. In the NW-Pacific specimens have tubercles where the second lock system is found and in these the reduction of the minor tubercle does not occur. The growth lines are typical. A strong hump is present with a marked inner hump. On the dorsal side, especially near the hump, strong transverse ribs are found. These ribs, about 14 in number, are imbricate. (Diacavolinia angulosa ribs, Diacavolinia angulosa ventral surface, Diacavolinia angulosa lateral rib, Diacavolinia angulosa base ventral rib). Protoconch II area strongly projects with a clear caudal joint of about 1.04 mm; the caudal fold, about 0.66 mm long, is moon-shaped; the left and right fold make an angle in ventral view. There are no lip flaps; the lip bellies are moderate and the lip shoulders are small as they continue directly into the inner hump. The aperture is narrow; the lip angle is about 150°, the lock angle is 72°, and the side angle is 71°.
The shell length ranges from 4.80 to 3.00 mm and the width ranges from 3.72 to 2.32 mm.

Juveniles

In Diacavolinia angulosa protoconch I has fine transverse striations and blunt protoconch II has somewhat broader spaces between the striae as seen at the right in the next illustration (Diacavolinia angulosa protoc2). For a further description of juveniles and development see also Diacavolinia longirostris.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous.

Distribution

Diacavolinia angulosa is considered by most authors to be an Indo-Pacific taxon though Gray (1850) also described this species from the Atlantic Ocean. It is presently reported from the Banda Sea and the entire Indian Ocean and W-Pacific, see the Diacavolinia angulosa map. In our opinion the Atlantic Ocean specimens described by Gray (1850) do not belong to this species which is now considered endemic to the Indo-Pacific Ocean. It is a common epipelagic species exclusively found in shallow hauls, not collected by closing nets from layers below 200 m.

Types

Cavolina angulosa Gray, 1850: 8.
Lectotype and 3 paralectotypes are preserved in the MHNP.
Type locality is restricted to the Indian Ocean (originally Indian Ocean with 4 specimens, Atlantic Ocean with 6 specimens, and China Sea with 3 specimens).

Remarks

For the specimens that show reduced lateral spines, no special taxon was described as the spine length, the only difference, may be due to growth. The difference between Diacavolinia angulosa without spines and Diacavolinia bandaensis is the presence of a notch and the larger size of the former species.

Diacavolinia angulosa