Pteropoda
Variation

At the turn of the century the typological species concept and morphological studies dominated taxonomy. Pfeffer (1880) and Boas (1886), however, already payed attention to the concept of variability in pteropods. Many species are represented by one or more subspecific taxa. With the advent of the theory of geographic speciation (Mayr, 1963), geographic variation became very important (van der Spoel, 1967). Isolation is rare and problematic in the marine pelagic realm where all water masses tend eventually to mix and isolated basins are essentially lacking. So geographically isolated subspecies are only rarely found, though the Mediterranean and Red Sea are relatively isolated areas where independent taxa may develop. In the late sixties the cladistic and phylogenetic approach was stimulated by the theory of Hennig.

Variation is common in planktonic animals with broad distributions ranging over different climatic belts and ocean basins. Species with a large north-south range tend to reflect the influences of climate, and species that occur in various water masses usually differ in the different environments.

Clio pyramidata exhibits shell-shape variations: narrow pyramidal in polar seas, broad lanceolate in the tropical waters. Its embryonic shell differs in size: the warmer the water the smaller the volume of protoconch I. Thus, two different trends of variation are involved, the protoconch showing a purely eco-phenotypical temperature-dependant variation, and shell-shape showing clinal variation depending on selective pressure in different water masses and restricted gene-flow between populations. Restricted gene-flow smooths out strong differences, producing gradual clines, whereas absence of gene-flow yields stepped clines. In Clio steps in the cline occur at boundaries between major water masses (van der Spoel, 1967). Thus, shell shapes of Clio pyramidata can be used as hydrographic indicators, and embryonic shell dimensions can serve as ecological indicators (Furnestin, 1978, 1979).

Intraspecific variability in South Atlantic specimens of L. helicina has been discussed extensively (Tesch, 1946; van der Spoel, 1967, 1976; Dadon, 1984a, b, 1989, 1993). Associated with climatic belts (van der Spoel, 1967), an ecocline is found along its latitudinal range in the South Atlantic, affecting differences in both morphology and life cycle strategy (Dadon, 1993).

Diacria costata is a typical example of water mass-related variation (Van Leyen and van der Spoel, 1982). In Central waters the shells are smaller than in Equatorial waters. An example of more locally restricted variations is in the shell-color of Diacria trispinosa, with a pattern different in upwelling areas from that in the open ocean.

In Cymbulia peroni size variations mark the difference between the Atlantic and Mediterranean populations (Furnestin, 1979). In soft-bodied Gymnosomata these relatively minor variations are more difficult to assess.