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Pterosoma planum Lesson, 1827 (Crosse emend., 1827)

Overview

This is a large shelled pelagic snail, up to 4 cm long, with an almost completely transparent body. The body can never withdraw into the shell (Pterosoma planum 1, Pterosoma planum 3). The shell is very flat conical with a high keel. Only the broad visceral nucleus is darkly pigmented. The large eyes, with black retina, and the mouth organs are clearly seen. The intestine is frequently full and visible. The body is long-oval and flattened with one swimming fin. It hunts large prey such as fish, shelled pteropods and other heteropods. It lives in the upper layers of the warm waters of the Indo-Pacific. (Pterosoma planum 1).

Taxonomic Description

The body is dorso-ventrally depressed into an oblong disk with an attenuated tail. The cutis is thick, transparent, with many tubercles all over the body (Pterosoma planum). The proboscis, of moderate length, is found ventrally on the disk-shaped body and forms a separate cylindrical part. The swimming fin is more or less quadrangular with a relative small sucker in both sexes. The visceral nucleus is flattened and clearly shows the whorls of the first shell parts, it is placed on short stalk nearly central on the disk-shaped body. There are 6 to 8 gills. The swimming fin is found opposite to the insertion of the stalk on the body. The shell is strongly flattened, nail-shaped over the median line and the depressed keel connects the aperture margin and embryonic shell. The aperture is oval and pointed. The embryonic shell consists of about 3.5 whorls with some transverse striae on the second whorl. The Pterosoma planum eyes are broad based with a slightly oblique retina, only the left tentacle is present; in very large specimens the right tentacle is also developed. Eyes and tentacles are covered by the tuberculated frontal margin of the body-disk. The Pterosoma planum 2 radula formula is 2-1-1-1-2. The Pterosoma planum radula details shows tricuspoid median plate.
Body length up to 80 mm.

Juveniles

The juveniles have a small dextrally coiled shell. The Pterosoma planum veliger shows about three smooth whorls. The Pterosoma planum protoconch I is smooth, the protoconch II shows some transverse ribs.

Reproduction

In this species sexes are separate.

Ecology

This species is carnivorous and epipelagic.

Distribution

This species is restricted to the Indo-Pacific Oceans and is most abundant in the Indian Ocean. The distribution pattern is comparable to other species of this family. Most typical is the rare occurrence in the Tasman Sea and the continuous distribution through the central Pacific Ocean, which indicates that it is a real warm water species, see the Pterosoma planum map.

Types

Pterosoma planum Lesson (Crosse emend.), 1827: 415, pl. 10, fig. A.
Types could not be located.
Type locality: Equatorial waters between Moluccas and New Guiney. Coll.: Lesson.

Pterosoma planum