Atlanta helicinoides form B Frontier, 1966b
Overview
This is a small dextrally shelled, pelagic snail, 0.3 cm in diameter with large eyes and a single swimming fin. The shell is large, strongly flattened and keeled. There are 7 whorls. The spire is no so large, flattened, and yellowish-violet as the rest of the shell; 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 helicinoides B).
Taxonomic Description
The principle difference from the typical form is the smaller and flatter spire with more a rapidly increasing whorl diameter. Diameter of the spire varies between 0.4 and 0. 5 mm. The sculpture is slightly less pronounced than in the forma helicinoides, but there are more spiral lines than in A. fusca. The whorl formula is about 1:0.32:0.17. The eyes are depressed but show a slightly smaller base than in helicinoides. The keel is slightly higher than in A. helicinoides (Atlanta helicinoides B from lateral). For all other characters it is similar with the type form.
Shell diameter up to 3 mm.
Juveniles
The juveniles have a small, dextrally coiled shell. In the forma B (Atlanta helicinoides B SEM) young specimens show clear spiral striation on the shell (Atlanta helicinoides B), this is also seen in lateral view (Atlanta helicinoides B vel, Atlanta helicinoides B veliger, Atlanta helicinoides B 1). In the veliger SEM photograph these lines are seen on least 3 whorls. In a young specimen the protoconch I is smooth.
Reproduction
In this form the sexes are separate.
Ecology
This form is carnivorous and epipelagic.
Distribution
This form is found in the W-Indian Ocean, see the Atlanta heliciniodes group map.
Types
Atlanta helicinoides forma B Frontier, 1966: 133, fig. 22-23.
Types are not indicated.
Type locality: near Madagascar. |
