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Cavolinia tridentata (Niebuhr, 1775) forma australis (Péron, 1807)

Overview

This is a large shelled thecosomatous pteropod, 1.6 cm long, living at moderate depths. It has a flat dorsal side with moderately developed ribs. There are keels along the lateral sides. The caudal spine is straight. The ventral side moderately vaulted. Shell sculpture consists of faint growth lines and a very faint transverse striation. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the Southern Ocean (Cavolinia t. australis dorsal).

Taxonomic Description

The hyaline or opaque shell shows a slightly vaulted dorsal surface with three well developed longitudinal ribs and two less prominent ribs along the lateral sides. The dorsal aperture lip is separated from the dorsal side by a prominent arch-shaped rib between the closing mechanisms, The aperture lip is strongly truncated, usually showing two anterior projections(Cavolinia t. australis outline, Cavolinia t. australis, Cavolinia t. australis lateral). The lateral sides are provided with strong side keels composed by thin "wings" of the ventral and the dorsal shell surface. The lateral spine points are directed caudad. The caudal spine is straight. The shell margin between the caudal and lateral spines is irregularly concave. The ventral side is strongly vaulted. The shell aperture is trapezoid in shape. The shell structure consists of growth lines and transverse ribs on the ventral side which are quote variable, sometimes they are coarse, sometimes faint. The shell colour consists of a brown to dark brown or purple pattern over the ventral and dorsal sides. The dorsal aperture lip is always lighter in colour and the lateral keels are whitish.
The shell length varies between 10.5 and 16.0 mm, the width varies between 8.0 and 10.5 mm, with averages of 13.3 and 10.2 mm respectively.

Juveniles

A special description is not available.

Reproduction

This forma is a protandric hermaphrodite.

Ecology

This forma is phytophagous and epipelagic.

Distribution

This forma is found only in subantarctic waters of the Atlantic, Indian and Pacific Oceans, see the Cavolinia t. australis map.

Types

Hyalaea australis Péron, 1807: pl. 31, fig. 5.
The type material, formerly kept in the Staatliches Museum fur Naturkunde at Stuttgart, has been lost (Dr. Janus in lit.).
Type locality: Vals Bay (South Africa).

Cavolinia tridentata australis