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Diacavolinia bicornis Van der Spoel, Bleeker and Kobayasi, 1993

Overview

This is a medium sized, uncoiled thecosomatous pteropod, 0.8 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 when the ventral and dorsal sides grow together. The lateral spines are large. Shell sculpture consists of faint growth lines and faint transverse striation. The broad dorsal lip has a notch, a gutter but no constriction. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in warm water of the Atlantic and Indian Oceans at shallow depths (Diacavolinia bicornis).

Taxonomic Description

The hyaline shell is triangular; with a brown hue especially on lower ventral and dorsal side. The dorsal lip has a notch in the broad rostrum and no constriction, but a gutter is present. Both ventral and dorsal lips have a thicker rim. The ventral lip is moderately sized and shows a weak median depression. The lateral spines do not bent, they are sharp and lanceolate; the gutter corners are small. The thin ventral ribs are line-shaped, only 5 were distinct in the holotype. The lateral lines are slightly convex and the flanks are not very clear. The dorsal side is convex, with well-developed central and lateral ribs; the lock rib is moderately developed. The lock area is average sizes like the main tubercle and link; the lunar and minor tubercle are rather small and all tubercles are placed close together. A second lock mechanism is present. The growth lines are normal. A hump is absent. The protoconch II area does not projecting far. In the holotype it has a very small caudal joint of 1.20 mm (only 20% of shell width); the moon-shaped caudal fold is 2.00 mm long. There sometimes is an indication of lip flaps; well-developed lip bellies are present, and the lip shoulders are very small. The aperture is narrow. In the holotype the lip angle is 150°, the lock angle is 74°, and the side angle is 66°.
The shell length ranges from 7.84 to 4.60 mm and the width from 7.84 to 3.88 mm.

Juveniles

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

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous.

Distribution

Diacavolinia bicornis is found in the subtropical and tropical NW-Atlantic and W-Indian Oceans; it probably has a wide distribution, see the Diacavolinia bicornis map.

Types

Diacavolinia bicornis Van der Spoel, Bleeker and Kobayasi, 1993: 138, fig. 11.
Holotype (ZMUC. gas.159) in ZMUC, 2 paratypes (ZMA. moll. 3.92.011) in ZMAN.
Type locality: 3°45'S 56°33'E CDAE., Stat. 3922v, 0-50 m depth, 12 December 1929.
Etymology: The name of this species is given for the large notch, projecting like two horns in front of the shell.

Others

It resembles most Diacavolinia longirostris but that species has a smaller rostrum and the spines are bent dorsally.

Diacavolinia bicornis