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Limacina (Limacina) helicina (Phipps, 1774) subsp. helicina (Phipps, 1774) forma pacifica Dall, 1871

Overview

This is a small, shelled, pelagic thecosomatous pteropod with a left-coiled shell, 0.2 cm in diameter. The spire is depressed. It has about 6 colourless, transparent whorls. There is no umbilical keel. Transverse striae are absent. It is found in colder waters of the North Pacific Ocean where it feeds on phytoplankton and it is a mucus feeder.

Taxonomic Description

The small, transparent shell is composed of about 6 whorls, the spire is flatly coiled which is expressed in the small height/width ratio of 0.75. The umbilicus is moderately wide and deep, but not as wide as in the forma helicina, the umbilical keel is absent. The height of the shell aperture is only slightly greater than its width. Transverse striation is completely absent and only some faint growth lines are present. The protrusions on the wing borders are present on the anterior border. For the rest of the characters, this forma resembles the forma helicina. Intermediates are found between the present forma and acuta, living in the area between that of pacifica and acuta. These intermediates are characterised by a height/ diameter ratio varying between 0.9 and 0.7, and by the fact that the striation may be present or absent.
Shell measurements: diameter to 2 mm, height/diameter ratio 0.768.

Juveniles

The juveniles have a small left coiled shell. A special description is not available.

Reproduction

This form is a protandric hermaphrodite.

Ecology

This form is a phytophagous and epipelagic.

Distribution

The forma pacifica is found in the Pacific Ocean, between 25° and 60°N, and along the American West Coast between 20° and 60°N. This forma is not subarctic, though the temperature and salinity range differs only slightly from that of subarctic environments, see the map. The vertical distribution range is at daytime from 0 or 180 to 400 or 500m, at night from 0 to 1000 or 350m.

Types

Limacina pacifica Dall, 1871: 138.
Lectotype: USNM 56767 (dry collection) Paralectotypes: USNM 665801, 9 fragments (dry collection).
Type locality: Monterey (restricted; it was also Neah Bay and off Peninsula Alaska). Coll.: Dall.

Limacina helicina helicina pacifica