Cuvierina columnella (Rang, 1827a) forma atlantica Van der Spoel, 1970a
Overview
This is a shelled pelagic snail, up to 1 cm long, with a completely transparent, bottle-shaped shell. The greatest shell width occurs at 1/3 of the shell length from posterior. The surface has very faint growth lines. In the juvenile stage the shell has a protoconch that points as a needle below the bottle-shaped adult shell, and is closed off by a septum. The visceral mass is seen through the shell. It is a good swimmer that feeds on phytoplankton and protozoa. It lives in the warm upper waters of the Atlantic Ocean, where it can occur in mass blooms (Cuvierina c. atlantica 1).
Taxonomic Description
Shell bottle-shaped, with the greatest diameter relatively close to the caudal end. A clear constriction is present below the reniform aperture. The caudal septum is convex (Cuvierina c. atlantica drawing, Cuvierina c. atlantica 2, Cuvierina c. atlantica shell). Shell usually not hyaline, with a smooth surface. In contrast to C. columnella forma columnella the increased shell diameter at the caudal end does not show a distinct swelling. The radula formula is 1-1-1. The (Cuvierina c. atlantica 4) median tooth is unicuspoid.
Shell length varies between 7.0 and 10.0 mm and this forma is larger than the forma columnella with a shell length between 4.0 and 8.0 mm.
Juveniles
The juvenile is distinctly different from the adult. It has a slender conical shell (protoconch II) with an oval protoconch I. A week incision separates both protoconchs, near the incision thicker growth lines are sometimes found and fine growth lines are seen on the protoconch II. The animal first fills the whole shell. When the teleoconch (bottle-shaped) starts to develop, a septum is formed between protoconch II and the teleoconch, the body detaches from the protoconch I and the columellar muscle is attached near the septum. A skinny or minute stage develops in which the adult shell reaches its final size (Cuvierina c. atlantica stages). Only in exceptional cases does the protoconch II stay fixed to the shell(# Cuvierina c. atlantica 3).
Reproduction
The forma is a protandric hermaphrodite.
Ecology
The form is omni- or phytophagous and epipelagic. Copepod naupli, tintiniids, thecate dinoflagellates, Globigerina and centric diatoms were found in the gut. Horizontal swimming speed is 4.0 mm/sec, upward swimming is 30.2 mm/sec and sinking speed is 62.5 mm/sec. (Davenport and Bebbington, 1990).
Distribution
This form is restricted to the warm waters of the Atlantic Ocean, see the Cuvierina c. atlantica map. Most typical is the occurrence in the Sargasso Sea and the Caribbean Area. The influence of the Labrador Current on the northern boundary is only small; the Gulf Stream incidentally transports specimens northward which explains the rare record south of Iceland.
Types
Cuvierina columnella forma atlantica Van der Spoel, 1970: 120, fig. 20.
Holotype: ZMUC (alcohol collection).
Type locality: 37°05'N 54°34'W Coll.: CDAE stat. 299.
Styliola sinecosta Wells, 1974: 293, figs. 1, 3a-c.
Holotype: NMNS 69251 Paratypes: NMNS 69252 Up to 69255, 11 spec.
Type locality: 13°11'N 59°41'W, off Barbados. Coll.: F. E. Wells.