Pteropoda
World wide distribution patterns

Pteropods are most abundant in Subarctic and Subantarctic waters. The Thecosomata (Limacina) are sometimes dominant components of the plankton in these areas, and may represent an important food source for whales and large fish such as cod. As Thecosomata are largely phytoplankton feeders, the effects of grazing by species like Limacina helicina antarctica can be very important in polar seas. The swarming behaviour in most Thecosomata and a few Gymnosomata like Clione limacina antarctica results in very patchy distribution patterns and varying effects on the local ecosystem. Limacina inflata, Creseis and Diacavolinia are considered to have neritic preferences; they may occur in very high abundances in coastal areas. Limacina inflata, however, is usually absent from inner neritic waters, where Creseis acicula forma acicula is most abundant. Though meso- and bathypelagic species are known to exist, most pteropods are epipelagic, and high abundances are found only in the epipelagic realm.

Pteropods are geologically a rather recent group, the oldest fossils dating from the Miocene. They probably originate from an Atlantic centre of speciation (van der Spoel and Heyman, 1983). This is still visible in the distribution patterns of some groups, such as the mesopelagic genus Peraclis, whose species have different ranges but are all found in the Northeastern Atlantic off Dakar. It is self-explanatory that these deep-living species do not reflect the influences of climate and shallow water mass configurations, inasmuch as their distribution patterns probably reflect older ocean climatic provinces. In bathypelagic species, like Limacina helicoides, climatic influences are undetectable. Restriction of this bathypelagic species to the Atlantic Ocean and its deep water outflow may indicate that it originated in the oldest (late Cretaceous) deep-sea basin, the North Atlantic Ocean. The bathypelagic Clio andreae has a much wider distribution, which may be explained by the abundance of young stages in shallow layers.

The general zoogeography of pteropods is illustrated below with some typical patterns. Species common in the epipelagic layers are influenced by climate and climate changes in addition to currents and water masses. In Clio pyramidata, a cosmopolitan species, many infraspecific forms are related to climatic belts, including cold-temperate (C. p. forma pyramidata), warm-water (C. p. forma lanceolata), polar (C. p. forma sulcata), and subpolar (C. p. forma antarctica) representatives.

Species restricted mainly to particular water masses, like Limacina lesueuri, represent another distribution type, not related to climatic belts but restricted to water masses. L. lesueuri is found in the Central water masses of all oceans, thus comprising five discrete populations. However, since this species is easily expatriated to waters outside the gyres, its pattern may resemble a warm-water belt pattern. Diacria major is even more restricted to the five Central water masses, inasmuch as it is not strongly expatriated.

In the genus Diacria both types of distributional patterns apply, serving as an example of the main biogeographic trends in pteropods. There are two species groups in the genus, the 'quadridentata group' with nine taxa (D. quadridentata distribution), and the 'trispinosa group' with seven taxa (Diacria trispinosa distribution). In the quadridentata group D. danae 'Subtropical form' is the most widespread, living in all oceans roughly in latitudinal belts at 10-30°N and S; and D. danae 'Equatorial form' is restricted to the equatorial waters of all oceans. The distribution of this species and its forms clearly follows climatic belts.

D. quadridentata is an Indo-Pacific warm-water species that does not enter the Atlantic Ocean because the African continent interrupts dispersal of really warm-water species. D. costata 'Equatorial form', an Indo-Pacific organism, shows a range comparable to, though narrower than, the preceding one. In the Pacific it gives rise to a second form, D. costata 'Central Water form' that is especially adapted to the Central Water Masses. D. schmidti schmidti is restricted to the Eastern Pacific Ocean oxygen minimum area. This is a well-established zoogeographic province with many endemic taxa. D. schmidti occidentalis is restricted to western Pacific coastal areas. D. erythra erythra is a Red Sea and western Indian ocean endemic species that probably originated in the Red Sea and subsequently invaded part of the Indian Ocean. D. erythra crassa is endemic to the Red Sea.

In the trispinosa group (Diacria trispinosa distribution) D. trispinosa forma trispinosa is the most widespread taxon, living in all oceans between 40°N and 40°S. D. trispinosa atlantica 'typical form', found in the North Atlantic between 70°N and 40°N, inhabits the transitional waters. D. trispinosa atlantica 'upwelling form' is especially adapted to the upwelling area off Northwest Africa. Upwelling areas also host isolated populations of many other planktonic and nektonic groups. Diacria rampali lives in all oceans between 30°N and 30°S, maintaining this broad distribution by expatriation around the southern tip of Africa. Proof for such expatriation can be found in shells preserved in sediments south of Africa. D. maculata is found in northwestern boundary currents of the Atlantic and Pacific Oceans, seemingly adapted to coastal water masses with a tropical origin. D. piccola has been collected so far only from sediments. D. major appears only in the Central Water masses of all oceans, and is therefore a typical water mass-linked species.

Endemism is rare in pelagic molluscs, although a few cases have been recorded. The oxygen minimum layer of the eastern tropical Pacific hosts some endemic species such as Desmopterus pacificus. This area is not physically isolated, but adaptations to the special conditions have resulted in endemism. The Mediterranean is more or less isolated as the Strait of Gibraltar functions as a partial barrier, and special endemic forms of a few species, such as Cavolinia tridentata forma tridentata, are known here. A comparable situation is found in the Red Sea where endemics like Diacavolinia flexipes occur.