Creseis virgula (Rang, 1828b) forma conica Eschscholtz, 1829
Overview
This is a shelled pelagic snail up to 0.7 cm long, with a transparent uncoiled shell. The cross-section is round. The surface is smooth. The shell is slender, 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 warmer waters of all oceans in the upper water layers (Creseis v. conica 1).
Taxonomic Description
This forma changes gradually into the formae virgula and clava when its general shape is considered. Most typical is the rounded embryonic shell which does not show rings or any constriction, and the strong curvature (the angle between teleoconch and protoconch is <150°). Only a few growth striae are found above the embryonic shell. The fully-grown shell is colourless, 7 mm long, with an aperture-diameter of about 1 mm (Creseis v. conica). The anterior part of the shell is straight, only the posterior top is curved dorsally. The incomplete original description makes it difficult to detect exactly the status of this taxon. The names of the two forma are frequently exchanged. The soft parts are characterised by the wing appendages and the short columellar muscle (Creseis v. conica line drawing).
Maximum shell length is 7 mm.
Juveniles
The juveniles are characterised by the curve of the shell. The protoconch I is nearly straight, slender and conical with blunt tip. The incision between adult and embryonic shell is faint and at the level of this incision the curvature begins. Growth rate is 0.30 mm per month (Wells, 1976).
Reproduction
This form is a protandric hermaphrodite.
Ecology
This form is phytophagous and epipelagic.
Distribution
See for information Creseis v. virgula and the Creseis v. conica map.
Geological Record
This species is known from the Late Quaternary of the Red Sea and the Pleistocene of the Mediterranean.
Types
Creseis conica Eschscholtz, 1829: 17, pl. 15, fig. 3.
The types could not be traced.