Evolutionary Ecology of Habitat Use and Trophic Specialisation in the Endemic Fish Fauna of the Cross River Drainage

This research investigated the evolutionary ecology of habitat use and trophic specialisation in the endemic fish fauna of the Cross River drainage, Cross River State, South South Nigeria, making an original contribution to understanding how evolutionary processes have shaped the functional diversity and ecological roles of one of Africa's most biodiverse freshwater fish assemblages. The Cross River drainage harbours a high proportion of restricted-range endemic species, providing a natural laboratory for studying ecological diversification in an African riverine context. Field surveys at 15 Cross River stations documented fish community composition, habitat characteristics, and collected specimens for dietary and morphological analysis. Stable isotope analysis (delta-13C and delta-15N) was conducted on 320 specimens across 18 focal species to reconstruct trophic position and carbon source utilisation. Geometric morphometric analysis was applied to characterise body form variation associated with habitat use. Molecular phylogenetics using mitochondrial and nuclear gene sequences provided an evolutionary framework for mapping ecological trait evolution. Results revealed significant trophic diversification among congeners occupying different hydraulic habitat types, with species in high-flow rheophilic zones showing convergent morphological specialisations including dorsoventrally flattened bodies and pectoral fin modifications. Stable isotope data confirmed clear niche separation among co-occurring species, with trophic level separation of up to 1.8 trophic units within single site assemblages. Phylogenetic comparative analysis identified rapid ecological diversification within two endemic clades associated with colonisation of distinct microhabitat types. The study contributes original evolutionary ecological insight to the documentation of Cross River endemic fish diversity and provides a functional diversity framework for conservation prioritisation in the basin. Keywords: evolutionary ecology, endemic fish, Cross River, trophic specialisation, stable isotopes