Pteropoda
Vertical patterns and migration

Many pteropods undergo more or less pronounced diel vertical migrations, feeding at night near the surface and migrating to greater depths during the day (Stubbings, 1938; Humphreys and Myers, 1968; van der Spoel, 1973b; Kobayashi, 1974; Wormuth, 1981). Reverse migrations, with shallower daytime occurrences, have been reported in a few cases (Stubbings, 1938). Most species are influenced by specific hydrographic conditions so that day and night levels are different from place to place. Migration ranges vary in different species; for example, Diacavolinia species migrate daily only over some ten metres, whereas Clio pyramidata migrates over hundreds, even up to 1500, metres (van der Spoel, 1973b).

Irregular bottom topography is often responsible for remarkable deviations from the normal vertical pattern. For example, deep-sea species may occupy shallower layers above sea mounts and in other shallow waters. Neritic species like Creseis acicula may descend to the bottom at night in shallow areas where intermediate depths are lacking. Deviations from normal vertical distributions may also be induced by productivity and temperature changes (van der Spoel and Schalk, 1988; Schalk and van der Spoel, 1988). When temperatures at depth are slightly higher than the usual 0-4°C, as in the Banda Sea, normally bathypelagic species may appear in the epipelagic zone. Furthermore, diel vertical migrations may be suppressed altogether when productivity is above normal (Schalk, 1988).

Although most species are epipelagic, several are typically mesopelagic (see Table Pteropoda): e.g. Clio recurva, Peraclis spp., and some Gymnosomata, such as Thliptodon and Cephalobrachia. The Thecosomata Limacina helicoides, Clio chaptali, and Clio andreae, and the Gymnosomata Massya and Schizobrachium are bathypelagic. Clio andreae and Limacina helicoides undergo ontogenetic vertical migration, juveniles occurring higher in the water column than the adults.