Sexual dimorphism and species divergence between a habitat generalist and a habitat specialist

Sexual dimorphism, the phenotypic differences between males and females within a species, is widespread in the animal kingdom. This study investigates the extent of sexual dimorphism and species divergence in external morphology in two closely related freshwater fish species, Devario malabaricus and D. micronema, in Sri Lanka. Devario malabaricus, a habitat generalist, inhabits a wide range of aquatic environments, while D. micronema, a habitat specialist, resides in shaded rainforest streams. The study reveals size differences between the two species: D. malabaricus is consistently larger than D. micronema. However, there are no size differences between the sexes within each species. Several additional morphological traits, such as head length, interorbital width, predorsal length and postdorsal length display divergence between the species as well as between the sexes within each species; these patterns are also consistent across populations. The sexually dimorphic traits are similar between the two species despite their distinct ecological habitats. However, Devario malabaricus exhibit stronger sexual dimorphism compared to D. micronema, supporting the prediction of a positive relationship between the extent of sexual dimorphism and ecological generalism. The study highlights the importance of considering sexual dimorphism in morphometric comparisons in taxonomic studies of Devario and suggests that divergent selection between the sexes contributes to morphological variation in this group.