Responses of the Plio-Pleistocene freshwater gastropods of Kos (Greece, Aegean Sea) to environmental changes.

Abstract

In the Plio-Pleistocene freshwater gastropods of Kos, three different kinds of faunal responses to the changing environment can be referred to: 1) varying species numbers as responses of the fauna as a whole, 2) evolutionary changes in shell morphology, and 3) non-hereditary modifications in shell colour as a reaction to varying salinity. Evolutionary changes in shell sculpture must be explained as an expression of adaption to certain environmental factors, which, however, are still unknown. Nevertheless, some extrinsic forces important for gastropod evolution can be determined. Separating mechanisms within the basin caused splitting of populations, and the populations separated from eacch other had different evolutionary trends (microgeographical differentiation, eg Mikrogoniochilus minutus). Micro-allopatry can also be observed in Rhodopyrgula rhodiensis from the Pliocene of Rhodes. Some more wide spread populations were split by the separation of the eastern Kos lake from inland waters in central Kos (Melanopsis gorceixi, Theodoxus doricus), and in the latter species they became reconnected, when there was subsequent contact between these waters. A similar development seems to have occurred in the Rhodian Viviparus rhodensis. On the other hand, lack of separating mechanisms within the basin led to gradual evolution along non-splitting lineages (eg Prososthenia sturanyi and large portions of the Viviparus and Melanopsis lineages). Evolutionary rates increased when the populations became relatively small (bottleneck effect). In the case of Viviparus brevis by the shrinkage of lake size.-Author