ECOMORPHOLOGICAL CORRELATES OF INTERSPECIFIC PLASTICITY IN REPRODUCTIVE INVESTMENT AND EGG-PRODUCTION OF SOME CICHLID FISHES (CICHLIDAE) OFA NIGERIAN RAIN-FOREST STREAM

Interspecific variability and trends in the reproductive investment an d fecundity of four sympatric cichlid species (Thysochromis ansorgii ( BOULENGER 1901), Chromiditilapia guntheri (SAUVAGE 1882), Hemichromis fasciatus PETERS 1852, Tilapia mariae BOULENGER 1899) were studied in Ikpa River vis-a-vis certain ecological and morphological attributes. There was a positive allometric functional relationship between mean t otal length and total weight; mean body condition increased with mean length; mean ovary weight and egg weight increased with mean fish leng th and weight (minimum in ?: ansorgii and maximum in T. mariae in each case). Gonadosomatic index increased only slightly with mean fish len gth. The hierarchy of mean absolute fecundity was: C. guntheri > T. an sorgii > H. fasciatus > T. mariae. Mean absolute fecundity increased w ith mean length and weight among the substrate-spawners (T. ansorgii > H. fasciatus > T. mariae) while C. guntheri, a paternal mouth-brooder , was an ''outlier'' in these relationships. Interspecific divergence in reproductive investment, fecundity and morphometric attributes of t he cichlids (CV = 10.1-149.5%) may enhance reproductive niche partitio ning, a phenomenon which provides the fundamental biomechanism for the evolution of their distinct reproductive strategies. This explains in part, the coexistence of these cichlids in the vegetated marginal wat er biotope.