History of mollusc community types and faunal dynamics in continental saline ecosystems of the South Mediterranean Quaternary

摘要

Quaternary continental aquatic ecosystems characterised by poly/eusaline waters, and without a connection to the Mediterranean Sea, are dominated by the potamidid gastropod Potamides conicus (Blainville) and the cardiid bivalve Cerastoderma glaucum (Poiret). Metasaline environments are characterised by monotypic occurrences of C. glaucum. The primarily freshwater-dwelling thiarid gastropod Melanoides tuberculata (Muller), in contrast, colonised oligo/mesosaline habitats, similarly to its precursors from the Upper Messinian Lago-Mare ecosystems. In case of the establishment of saline conditions in former freshwater environments, characteristic taphocoenoses document the dynamic development in the context of progressive evaporation: the Melanoides palaeocommunity from a Holocene ecosystem at In Ecker (S Algeria) is accompanied by freshwater inhabitants, which did not reach their adult size of complete development due to the temporal character of their habitats. Oligo/mesosaline conditions are reflected in oxygen isotope signatures of the Melanoides shells. High variations in carbon isotope values indicate changes in the composition of the diet during ontogeny, and may support the interpretation that the ecosystem developed progressively. In comparison to Neogene and fossil Quaternary assemblages, the composition of extant eusaline communities is considerably more diversified: larger North African lakes in Libya and Egypt are characterised by neozoan elements originating from the Mediterranean fauna. The dynamic development of these faunas was predominantly supported by anthropochory in the context of fish cultures. The mode of early ontogenetic development played an important role with regard to the introduction of gastropods to continental environments, since the entire spectrum of euryhaline coastal gastropods with planktotrophic larval development is absent from these ecosystems. The preadaptational loss of planktotrophy enabled Potamides to establish persisting populations. Apart from its brooding mode, Melanoides is characterised by parthenogenesis, and thus a single individual, introduced to a continental ecosystem, can establish a population.