Pneumoderma atlanticum (Oken, 1815) subsp. atlanticum (Oken, 1815) forma atlanticum (Oken, 1815)
Overview
This is a naked pelagic snail, up to 1.3 cm long, with a rather transparent body, the visceral mass is seen through the body wall. The colour is purplish. It is oval to conical in shape. There are two lateral arms each with 80 suckers, well visible in the Pneumoderma a.a. atlanticum drawing by Pruvot-Fol. It is a good swimmer that hunts shelled pteropods as food and lives in warm waters of all oceans at all depth (Pneumoderma a.a. atlanticum drawing).
Taxonomic Description
Body is cylindrical, purple to brown. The posterior gill is hexagonal, with long fringed branches, each with about 7 ramified fringes. Lateral gill is short, without fringes. The posterior footlobe is not long. The lateral footlobes are horseshoe-shaped, a median foot tubercle is present, and the penis has no sucker. Only two lateral sucker arms present, with 40 to 80 suckers each (Pneumoderma a.a. atlanticum). The suckers are short stalked and subequal in size. In young specimens, of 6 to 8 mm, 28 are present and in larvae of 3 to 4 mm only 10 to 14 suckers are found. The Pneumoderma a.a. atlanticum radula formula is 4-1-4 or 4-0-4 as the median plate may be absent in older specimens. The median plate is tricuspoid. The hook sacs are relatively long.
Body length up to 13 mm.
Morphology and Structure
The nervous control of larval cilia is described by Mackie et al. (1976)
The chromosome number is 24 (2N) (Thiriot-Quievreux, 1988)
Juveniles
Only the very young juveniles have a shell. This shell is a bowl-shaped protoconch I, without structure. On this half globe a small rim is found that can be considered protoconch II as it is separated by a shallow incision.
Reproduction
This species is a protandric hermaphrodite.
Ecology
This is a carnivore species feeds on Thecosomata and Chaetognatha. Predators include the fishes Alepisaurus ferox and Alepisaurus brevirostris.
Distribution
This species was considered by Tesch (1950), and by several other authors, as including most of the other taxa described in this genus. To their opinion, the species was a wide spread cosmopolitan species preferring warmer surface waters. Some subspecies, like Pneumoderma atlanticum subsp. atlanticum do not avoid equatorial waters. It is found in the entire tropical and subtropical Atlantic and Indian Oceans, and also in the Mediterranean Sea, see the map of Pneumoderma a.a. atlanticum map. In the Pacific Ocean, it is restricted to the Panama area and S-Pacific while in the N-Pacific the subsp. pacificum occurs.
Types
Pneumodermis atlantica Oken, 1815: 327.
Types not located.