Atlanta helicinoides forma helicinoides Souleyet, 1852
This is a small dextrally shelled, pelagic snail, 0.2 cm in diameter with large eyes and a single swimming fin. The shell is strongly flattened and keeled. There are 7 whorls. The spire is large, and like the rest of the shell yellowish-violet, it shows spiral sculpture. The operculum has a clear spire. It is a carnivore predating on relatively large zooplankton. This species occurs in warm waters of the Atlantic and Indian Oceans (Atlanta h. helicinoides 1).
Taxonomic Description [after Richter, 1987]
The shell is small and the whorls are close together (Atlanta h. helicinoides 2). The spire is large and high with clear spiral sculpture (Atlanta h. helicinoides protoc2) on the upper and underside (Atlanta h. helicinoides). Inner whorlsides in spire are decalcified. The whole shell is light yellowish-violet, the spire is slightly darker and the keel base is somewhat darker. The aperture is broad oval. The formula of the whorls is about 1:0.50:0.20. The operculum (type C) is broad, monogyre with a clearly concave underside. The eyes (type C) have a dorso-ventrally flattened and laterally broadened pigment mantle without a dorsal window. The pigment continues at the inside far forwards. The radula shows sexual dimorphism (Atlanta h. helicinoides radula): in the males it resembles Atlanta inflata, the lateral teeth are, however, higher and stronger, the median tooth is small with a hollow border and a small median cusp, the laterals are broadly compressed, unicuspoid, the additional teeth are long and slender but shorter than the laterals.
Shell diameter 2 mm.
The juveniles have a small dextrally coiled shell. Young (Atlanta h. helicinoides young 2) specimens show clear spiral striation that completely covers the Atlanta h. helicinoides juvenile. This is also seen (Atlanta h. helicinoides juv 2) from lateral. In the Atlanta h. helicinoides veliger SEM photograph these lines are seen on the Atlanta h. helicinoides protoconch they cover at least 3 whorls. In a Atlanta h. helicinoides young specimen protoconch I is smooth. The Atlanta h. helicinoides living veliger shows six broad velar lobes.
In this form the sexes are separate.
This form is carnivorous and epipelagic.
The helicinoides group, such as Atlanta peresi occurs between 40°N and 30°S. It is remarkable that the abundance in the N-Atlantic is clearly greater than in the S-Atlantic Ocean. In the Atlantic and Pacific Oceans the group seems to avoid equatorial waters. Atlanta helicinoides is cosmopolitan, but records of Atlanta h. helicinoides specimens are scarce, see the Atlanta heliciniodes group map.
Atlanta helicinoides Souleyet, 1852: 384 (1841) pl. 20, figs. 23-30.
Lectotype: MHNP, shell dissolved (alcohol collection). Paralectotype: MHNP 4 specimens, shells dissolved (alcohol collection).
Type locality: China Sea Coll. Eydoux and Souleyet.