Limacina (Limacina) helicina (Phipps, 1774) subsp. antarctica (Woodward, 1854) forma antarctica Woodward, 1854
Overview
This is a small, shelled, pelagic thecosomatous pteropod with a left-coiled shell, 0.6 cm in diameter. The spire is very depressed. It has about 6 colourless, transparent whorls. There is no umbilical keel. Transverse striae are present. It is found in colder waters of the southern oceans where it feeds on phytoplankton, it is a mucus feeder (Limacina h.a. antarctica).
Taxonomic Description
The small, left coiled shell is composed of about 6 whorls which are more depressed than in the formae helicina and pacifica. Height of the spire is variable, in the most depressed specimens the shell is coiled in one level. The umbilicus is slightly narrow, and there is no keel. The whorls are rounded with a moderately deep suture. The shell aperture is broader than high. Transverse striation is present but not as pronounced as in the forma helicina, and is best visible on the last whorl. A faint spiral striation is dominant on the first whorls. A wing protrusion is present on the anterior wing border. The radula formula is 1-1-1, and the radula is composed of about 15 transverse rows. The operculum has an eccentric nucleus and is high conical shape.
Shell measurements: maximum diameter to 6.5 mm, while the smallest diameter is 3.5 mm.
Juveniles
The juveniles have a small left coiled shell. A special description is not available.
Reproduction
This form is a protandric hermaphrodite. Eggs are 132µm by 88µm in a gelatinous mass.
Spawning takes place at the end of the antarctic summer after the parent animals have grown throughout the southern summer. A young generation is found from May to August and starts breeding in the following winter. As in Cavoliniidae the gonad is composed of follicles. The ova are 60 µm in diameter which is half the size of the fully developed eggs in the ribbons. The formation of clusters of spermatids and sperm around feeding cells, as in Limacina retroversa, is also found in the present forma. Sperm is transferred by spermatophores.
Ecology
This form is phytophagous and epipelagic.
Distribution
The forma rangi seems to be "eurythermic" to a certain degree, while antarctica is "stenothermic" with a preference for cold waters, like the forma helicina in the N-Atlantic; the formae acuta and pacifica also seem to be "stenothermic". The forma antarctica is found in the Antarctic Ocean near the continent; its northern boundary lies in the southern part of the West Wind Drift; more to the north the forma rangi occurs, which is typical for the regions where cold and warmer water mix and for the West Wind Drift. The present form is Antarctic and circumglobal in distribution, living south of 50°S (see the Limacina h.a. antarctica map).
Types
Limacina antarctica Woodward, 1854: 207, pl. 14, fig. 41.
Types could not be traced.
Agadina (Limacina) cucullata Gould, 1852: 486.
Types are not found by Johnson (1964).
Type locality: 66°S 106°20'E, 20 miles of the Antarctic Continent.