| Abstract: | Extreme habitats are characterized by the presence of physio‐chemical stressors, but also differ in aspects of the biotic environment, such as resource availability or the presence of competitors. The present study quantifies variation in trophic ecology of a small livebearing fish (Poecilia mexicana, Poeciliidae) across four different habitats that included nonsulphidic and sulphidic surface waters, as well as a nonsulphidic and a sulphidic cave. Resource use in different habitat types was investigated using gut content analysis. Populations diverged in resource use from a diet dominated by algae and detritus in nonsulfidic surface habitats to a diet including invertebrate food items in the other habitats. Poecilia mexicana in cave habitats further exhibited a higher dietary niche width than conspecifics from surface habitats. The condition of P. mexicana was analysed using storage lipid extractions. Fish from sulphidic and cave habitats exhibited a very poor condition, suggesting resource limitation and/or high costs of coping with extreme conditions. Finally, divergence in resource use was correlated with variation in viscerocranial morphology. A common garden experiment indicated both a genetic and plastic basis to the morphological variation observed among field populations. It is suggested that the morphological diversification is an adaptation to the differential use of resources among populations. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 517–528. |
