|Limacina helicina (Phipps, 1774)
Phylum Mollusca / Class Gastropoda / Subclass Opisthobranchia / Order Thecosomata / Family Limacinidae
The shelled pteropod Limacina is a rare visitor to surface waters of central California, but when present, can be incredibly abundant. This swimming snail has a distinct coiled unpigmented shell. Although not dense by benthic gastropod standards, the shell adds enough negative buoyancy that frenetic flapping of the large pair of wings (derived from the foot tissue) is necessary to keep the animal from sinking. Production of a large mucous web with a diameter up to 6 cm also provides buoyancy. As is typical in thecosome pteropods, the web’s primary function is for trapping tiny planktonic prey. Ventral to the mouth and wings, and also derived from the foot, is the posterior footlobe. Cilia on the footlobe and a pair of lateral footlobes aid in transporting food particles collected by the mucus to the mouth. The food-laden web is ingested and a new one quickly deployed.
Limacina are common in arctic and high latitude temperate waters, where they may have a shell diameter up to 15 mm. Individuals in the southern part of the range (central and southern California) usually are between 1 to 3 mm. With their dark pigmentation they appear as dark specks when examining the contents of a plankton tow. The dark pigment can also stain the flesh of predatory fish. During daylight hours Limacina tend to migrate to deeper waters (down to 100 meters). Occasionally scuba divers may be treated to massive surface swarms during the day. Lurking in the midst of such swarms you can often find the gymnosome pteropod Clione, the arch-nemesis of Limacina that specializes in feeding on the tasty flesh.
All images in the JelliesZone © David Wrobel and may not be copied or used in any form without permission.