Geometric morphometric analyses of intragenus (Campylomormyrus) and intergenus hybrids in mormyrid fish

Abstract Natural hybrids within the weakly electric mormyrid fish genus Campylomormyrus have not been documented in the wild. However, recent advances have enabled the artificial production of both intragenus ( Campylomormyrus ) and intergenus ( Campylomormyrus and Gnathonemus ) hybrids. While previous studies have focused mainly on phenotypic divergence in body shape, especially electric organ discharges, ecophenotypical divergence in these hybrids remains less explored. To address this gap, we used geometric morphometrics to analyze body shape variation in intragenus and intergenus hybrids relative to their parental species. Using standardized digital photographs, we examined hybrids derived from parental species with varying genetic and phenotypic distances, including distinct snout morphologies. Our results show that hybrids are morphologically distinguishable but generally remain within the phenotypic range of their parental species. In most crosses, hybrids exhibited intermediate morphology or stronger similarity to one parent, whereas transgressive shape segregation was observed only in one cross and did not involve snout length. Snout thickness and body curvature showed more variable patterns among crosses than snout length. These findings suggest that F1 hybrid morphology is shaped mainly by intermediate expression with cross-specific parental affinity, while strongly novel morphologies appear to be rare. This study improves our understanding of hybridization in Mormyridae, its potential relevance for phenotypic diversification, and provides a basis for future work on later hybrid generations.