The colour pattern of the caudal fin, a useful criterion for identification of two species of Tilapia and their hybrids

The present work reports on some chromatic characters and their change with size in two species of Tilapia (T. zillii and T. guineensis) and their first generation hybrids. Successful reciprocal hybridizations between T. zillii and T. guineensis were performed in concrete tanks. The hybrids obtained were viable. The colour patterns of the hybrids and their parents were registered during a rearing cycle of 12 months. Between 2 and 13 cm standard length (L s ), the hybrids were found to be heterogeneous and three phenotypes were observed. The type (1) phenotype had a fully yellowish caudal fin without dots, the type (2) phenotype had a bicoloured caudal fin with the upper part clear yellowish and the lower part dark yellowish and without dots and the type (3) had a similar bicoloured caudal fin, but with dots. Above 13 cm L s . the hybrid population was homogenous and all specimens had a bicoloured caudal fin with dots. In T. zillii, all specimens >14 cm L s had a greyish caudal fin with dots while all T. guineensis >13 cm L s were characterized by a bicoloured caudal fin without dots. A multivariate analysis of the morphometric and meristic characters did not allow a clear separation of all groups. The study showed that the external morphology of the hybrids was intermediate to that of the parental species.