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Diacavolinia limbata (d'Orbigny, 1836) forma africana Van der Spoel, Bleeker and Kobayasi, 1993

Overview

This is a large uncoiled thecosomatous pteropod, 1 cm long. It has a flat dorsal side with moderately developed ribs. The ventral side is vaulted. The caudal spine is absent and a caudal spine mark is not left as the ventral and dorsal sides grow together. The lateral spines are well developed. Shell sculpture consists of faint growth lines and slight faint transverse striation. The broad dorsal lip has a small notch and gutter but no constriction. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in Atlantic Ocean at shallow depths (Diacavolinia l. africana).

Taxonomic Description

The shell is less triangular than in the preceding forma. It has a brown-purple hue and the wings are intensively purple (Diacavolinia l. africana balancing machamism, Diacavolinia l. africana 2). The dorsal lip, with notch in the broad rostrum, has a small gutter and a constriction; there is also a small nose at the extreme tip of the lip. The ventral lip is normal sized and shows a median depression. The lateral spines are well developed and slightly bent dorsally, the gutter corners are well developed. The dorsal side is convex, the central rib and latero-dorsal ribs are well developed, the lock rib is strong. The ventral ribs, 27 in number, are strongly developed, comb-shaped. The flanks are moderately developed and the lateral lines are slightly convex. The upper half of the ventral side is flattened in profile. The lock area is visible but small and the lock mechanism is relatively large. A second lock mechanism is found at the upper level of the attachment of the spines to the lateral line. The growth lines are well developed. There is no hump. The protoconch II area is not far projecting. In the holotype the caudal joint is 1.92 mm, the caudal fold is 1.92 mm long and nearly straight. There are no lip flaps; the lip belly is well developed and low, the lip shoulders are small. The aperture is well open. In the holotype the lip angle is 146°, the side angle is 75° and the lock angle is 68°. The dorsal aperture rim is sharp; the ventral one is slightly thickened (Diacavolinia l. africana).
The length ranges from 10.40 to 7.84 mm and for width it is 8.64 to 7.04 mm.

Juveniles

A special description for this forma is not available, see Diacavolinia longirostris.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous.

Distribution

Diacavolinia limbata forma africana has the same distributional pattern as Diacavolinia atlantica and appears to be endemic to the NE-Atlantic, see the Diacavolinia l. africana map.

Others

This forma varies in the development of the constriction in the rostrum. There are specimens with a constriction, and specimens without a constriction but instead these have a long upper lip. As there evidently are intermediates between the two forms of this species they have not been separated as species but as formae. The shell length is on an average larger in the forma africana than in the forma limbata. Though the differences in shell shape between this species and Diacavolinia atlantica are small, the striking difference of the colour of the soft part clearly separates the two species clearly.

Types

Diacavolinia limbata (d'Orbigny, 1836) forma africana Van der Spoel, Bleeker and Kobayasi, 1993: 133, fig. 5.
Holotype (ZMA. moll3.92.004) and 6 paratypes (ZMA. moll3.92.005) in ZMAN.
Type locality: 29°48.1'N 29°57.5'W, 110-205 m depth, AMNAPE Stat. 24, trawl 2, 29 April 1980, RMT 8.
Etymology: The forma is common off the Atlantic coasts of Africa, hence the name africana.

Diacavolinia limbata africana