Morphology and field behavior of pteropod molluscs: feeding methods in the families Cavoliniidae,Limacinidae and Peraclididae (Gastropoda: Thecosomata)

From 1983 to 1985 we studied the in-situ feeding behavior of fourteen species belonging to nine genera of thecosomatous pteropods, one in the family Limacinidae, seven in the family Cavoliniidae, and one in the family Peraclididae. All the euthecosomes we examined fed with a large spherical mucous web. Peraclis reticulata (D'Orbigny), however, uses a funnel-shaped mucous sheet, resembling that of other pseudothecosomes. Although the feeding mechanisms are similar in all the animals we examined closely, we found great differences in the size and morphology of the external part of the mantle. In species of Diacria and Cavolinia the external mantle attains its greatest complexity, and appears to be used for the sorting or rejection of food and for flotation. Species of the other five cavoliniid genera we studied have greatly reduced external mantles and sink slowly while feeding. Species of Limacina have no external mantle but appear to arrest sinking by setting their mucous webs. P. reticulata, which also appears to be neutrally buoyant, has an extensive external mantle that completely covers the shell. We conclude that feeding with a large mucous web explains the presence of fragments of large, fast moving prey in the stomach contents of both pseudothecosomes and euthecosomes. At present, it is impossible to decide whether thecosomes should be regarded as carnivorous trappers of prey or as suspension feeders. The use of an external mucous web for feeding is probably common to all thecosome pteropods and is the fundamental andaptation that enables them to live holoplanktonically.