FRESHWATER SNAILS OF THE BULINUS TRUNCATUS/TROPICUS COMPLEX IN KENYA: TETRAPLOID SPECIES

Abstract

Studies in recent years on chromosome number in freshwater snails belonging to the Bulinus truncatus/tropicus complex demonstrated the occurrence in Kenya of one diploid species, B. tropicus, and 2 tetraploid species, B. truncatus and B. permembranaceus. Further observations are now described on the morphologies of the tetraploid species, and their distributions. Observations were made on B. truncatus from 26 localities (11 newly reported) and B. permembranaceus from 26 localities (7 newly reported), in respect of chromosome number, egg proteins, enzymes (5 systems), shell (10 variables measured), male genital system (normal or aphallic) and radula (1st lateral tooth, size and mesocone shape). B. permembranaceus differs most clearly from B. truncatus in egg proteins, enzyme-loci and in lacking aphallic individuals. Mesocone shape is generally less angular in B. permembranaceus. The shell of B. permembranaceus grows larger, has the columella more commonly concave and its spire is proportionally higher and more acute. To distinguish these tetraploid species from the partly sympatric B. tropicus, observations on chromosome number and biochemical characters are necessary. The tetraploids are allopatric, B. truncatus being found rarely as high as 1900 m altitude, whereas B. permembranaceus occupies the altitude range 1940–2760 m. B. truncatus may be expanding its distribution in Kenya in man-made waterbodies, while B. permembranaceus possibly is restricted by adaptation to cool conditions and by interaction with B. tropicus. Differences between B. truncatus and B. permembranaceus, both morphological and biochemical, indicate their origins lie in independent episodes of tetraploidy.