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Creseis virgula (Rang, 1828b) forma virgula (Rang, 1828b)

Overview

This is a shelled pelagic snail, up to 0.6 cm long, with a transparent, uncoiled shell. The cross-section is round. The surface is smooth. The shell is long and narrow, tube-shaped and curved. The visceral mass is seen through the shell. It is a clumsy swimmer that feeds on phytoplankton and protozoa. It lives in the warm upper waters of all oceans (Creseis v. virgula 2, Creseis v. virgula line drawing).

Taxonomic Description

The shell is especially in the posterior parts distinctly curved dorsally (the angle between teleoconch and protoconch is >150°)(Creseis v. virgula 1). Directly below the rounded aperture some variable growth lines are visible and the rest the shell is smooth (Creseis v. virgula 2). The embryonic shell differs from that of other species in having no rings above the rounded posterior part. Some faint growth striae are present directly above the rounded part. The embryonic shell is only separated by a small constriction. In the posterior half of the embryonic shell a second constriction may be present. Wing glands and wing protrusions are both present, the last one touches the gland without overlapping it. The rear angle of the shell is small but not as small as in Creseis acicula. The Creseis v. virgula radula formula is 1-1-1.
The length of the shell is about 6 mm and its maximum diameter 2 mm.

Morphology and Structure

The chromosome number is 20 (2N) (Thiriot-Quievreux, 1988).

Juveniles

The protoconch is narrow but blunt at its tip(Creseis v. virgula 4), first it grows narrow then bulges outward so that the anterior part of protoconch I is wide. A rather narrow constriction occurs before the protoconch II and teleoconch are formed. The juvenile already shows the curvature of this forma (Creseis v. virgula juv., Creseis v. constricta protoc).

Reproduction

This form is a protandric hermaphrodite.

Ecology

The form is phytophagous and epipelagic. The temperature range of the species is about 15°-27.4° C and the salinity range about 36.24°/oo-36.37°/oo. A double diurnal rhythm of vertical migration seems to exist in this species with a surface maximum at 13.00-14.00 hr and one at 01.30 hr and with a mean day level at 206 m and a mean night level at 98 m.

Distribution

Its general distribution is comparable to Creseis acicula, although it is usually less abundant. In the Indo-Pacific it is more widely spread and it is more stenothermic. The only difference in distribution between virgula and conica is perhaps the more tropical character of conica, though the northern border of both coincides in the Atlantic. Its absence west off Australia is effected by the W Australian Coastal Current. The warmer southward running current east of Australia brings the species incidentally as far south as 55°S. A small population is present in the southern part of the Peru Current only. In the Caribbean Area only some vague records are known besides those concerning a small population in the Gulf of Darien. It is present in the Red Sea. See the Creseis v. virgula map.

Geological Record

This species is known from the Late Quaternary (from 140.000 BP) to present in the Red Sea. It penetrates the Adriatic already in the Pre-Boreal or Younger Dryas.

Others

Neurosecretion is described for this species.

Types

Cleodora (Creseis) virgula Rang, 1828: 316, pl. 17, fig. 2. Types not found in MHNP.
Styliola africana Bartsch, 1915: 3, pl. 34, fig. 4.
Syntypes: USNM 249794, 2 spec. (dry collection).
Type locality: Port Alfred, Cape Colong (S-Africa) Coll.: W. H. Turton.

Creseis virgula virgula