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Oxygyrus keraudreni (Lesueur, 1817)

Overview

This is a large, dextrally shelled, pelagic snail, 1 cm in diameter with large eyes and a single swimming fin. The keel and teleoconch are not calcareous but formed of cartilage-like substance. There is only one whorls visible, the others are visible in the umbilicus. The brown shell is strongly flattened and keeled. The spire of the operculum is strongly excentrical. It is a carnivorous species predating on relatively large zooplankton. This species occurs in the warm waters of all oceans (Oxygyrus keraudreni swimming).

Taxonomic Description

Shell planorboid, consisting of a transparent cartilaginous, slightly flexible material and encircled by a cartilaginous keel (Oxygyrus keraudreni). This keel ends abruptly near the aperture and is composed of two ridges so that it is double at its base. The juvenile shell, in adults covered with cartilaginous substance, is calcareous and covered with pronounced longitudinal, wavy sculpture lines (Oxygyrus keraudreni sculpture). The transparent, colourless or brownish shell parts are always surrounded by the keel, though in the first whorls it is not more than a small rim. The keel and transparent shell show numerous transverse growth lines. The calcareous juvenile shell becomes absorbed after it is covered by the permanent shell. The shell aperture is triangular and pointed on the outer side and concave on the inner side. The umbilicus is wide, and all whorls are visible in apical view. The eyes (type C) are characterised by the broad, darkly pigmented retinal part which nearly encloses the entire lens. The tentacles are very short (Oxygyrus keraudreni 2). Radula with tricuspoid median plate with a large central cusp and two lateral, variable cusps, intermediate plates with comb and additional cusp. The Oxygyrus keraudreni radula formula 2-1-1-1-2 with about 80 transverse rows (Oxygyrus keraudreni rad) The Oxygyrus keraudreni radula has a tricuspoid median teeth, the central cusp being larger than the lateral ones. The Oxygyrus keraudreni operculum is rounded trapezoid (Oxygyrus keraudreni operculum) with a small gyre in the anterior-inner portion of last whorl; the centre of gyre is small (Oxygyrus keraudreni ABRS, Oxygyrus keraudreni oral, Oxygyrus keraudreni apical, Oxygyrus keraudreni.umb.)
Maximum shell diameter 10 mm.

Juveniles

The Oxygyrus keraudreni larve have a small, dextrally coiled shell without a keel. The Oxygyrus keraudreni veliger shell is calcareous, coiled in one plane and involute. The first 1.5 whorls are spiral turrate. The first whorls are dark brown, the later ones are more reddish. There are 8, increasing to more than 20, wavy spiral sculpture lines (Oxygyrus keraudreni struct). The shell diameter is 120-600 µm. The operculum is transparent.

Reproduction

In this species the sexes are separate. Organogenesis of this species is described by Thiriot-Quievreux (1971a). Though reproduction in the Mediterranean is not restricted to certain seasons the highest number of specimens is found in spring and late summer.

Ecology

This species is a carnivore and epipelagic. Predators include the fishes: Alepisaurus ferox and A. brevirostris.

Distribution

It is a very common species found in the Atlantic and Indo-Pacific Oceans. It penetrates the W Mediterranean. Only a few records are known for the Pacific, but it probably is regularly distributed between 40°N and 30°S, see the Oxygyrus keraudreni map. Sampling is insufficient and only 7% of the ocean area between 40°N and 60°S is carefully sampled and only 20% incidentally. This species does not avoid offshore waters and it penetrates deeply into the N-Indian Ocean. It can be concluded that it is ubiquitous between 40°N and 30°S and is found in all types of water. The distribution pattern of this species is not influenced by currents.

Types

Atlanta keraudrenii Lesueur, 1817: 391, pl. 2.
Syntypes: MHNP, 2 spec.
Type locality: South Atlantic Ocean. Coll.: Eydoux and Souleyet.

Oxygyrus keraudreni