Ecology and morphological characters of different populations of Poecilia sphenops vandepolli (Cyprinidontidae)
Studies on the Fauna of Curaçao and other Caribbean Islands , Volume 32 - Issue 1 p. 102- 130
In the Netherlands Antilles, opposite the Venezuelan coast, the cyprinodont fish Poecilia sphenops vandepolli is found in fresh water, in brackish water, in the sea and in supersaline water. When comparing the populations from fresh water, sea water and supersaline water some significant morphological differences were found, e.g. in size, in depth of the body and of the caudal peduncle, in length of the head, and in the number of rays in the pectoral and caudal fins and the number of lateral scales. In raising experiments, however, it could be shown that these differences are phenotypic. The characteristics of the subspecies or varietas arubensis as described by VAN LIDTH DE JEUDE (1887) proved also to be phenotypic. Optimum growth was found in seawater. The adaptation to fresh water after transfer from sea water or supersaline water is quicker than in the opposite direction; this concerns specific weight adaptation, growth resumption and the change of preference for the new salinity after transfer. With respect to these characteristics fresh water is more favourable than sea water or brine. The inland migration after rainfall is not caused by the fresh water itself, but by an organic compound that is found in inland water, whether fresh or saline, and also in rain water after it has been in contact with the soil. From the fact that mollies also are attracted by IJsselmeer water, just as elvers are, it seems likely that mollies and elvers are attracted by the same organic compound. This behaviour of the molly causes irregular migrations from sea to inland waters which prevent the inland populations from developing into separate forms, races or subspecies.
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Feltkamp, C. A., & Kristensen, Ingvar. (1970). Ecology and morphological characters of different populations of Poecilia sphenops vandepolli (Cyprinidontidae). Studies on the Fauna of Curaçao and other Caribbean Islands, 32(1), 102–130.