MORPHOLOGICAL AND GENETIC DIFFERENTIATION OF 11 POPULATIONS OF THE AFRICAN CATFISH CHRYSICHTHYS-NIGRODIGITATUS (SILUROIDEI, CLAROTEIDAE), WITH CONSIDERATION OF THEIR BIOGEOGRAPHY

Eleven samples of Chrysichthys nigrodigitatus and one sample of C. mau rus were studied using morphometrics and enzyme polymorphism. Samples from opposite areas in the range of the species were most differentiat ed morphologically (Dagana in Senegal and Selingue in Mall, on one han d, and Bas Kouilou in Congo, on the other hand). In the sample from Ba s Kouilou, most specimens overlap those from Ebrie Lagoon in Cote d'Iv oire. The populations from Cote d'Ivoire are genetically the most vari able and have the highest number of alleles in common with the sample of C. maurus (ancestral alleles). The samples from the limits of the s pecies' range (Dagana, Bas Kouilou) are those that have the least poly morphism (both monomorphs) and the fewest alleles in common with C. ma urus. On the basis of these results we present a hypothetical scenario for the colonization of basins by populations of C. nigrodigitatus af ter the last glaciation. Colonization started in the area between Cote d'Ivoire and Benin. From this region of origin, some populations colo nized the basins to the west via the coastal area up to Senegal. Indep endently, other populations colonized the basins to the east via the c oastal area up to Zaire. The colonization of the Niger most likely occ urred through its Lower Delta.