SCHAACK, S.*; CHAPMAN, L.J.: Cascading effects of hypoxia on the functional morphology, ecology, and diversification of an African cyprinid.

Adaptive change in one character may affect associated, but functionally unrelated, characters in an adaptive or non-adaptive way. For example, in some fish species, individuals living in low oxygen environments have larger gills than conspecifics living in well-oxygenated water. Adaptive change in gill size may alter adjacent feeding structures and therefore have indirect effects on behavior or other fitness components. This study investigates whether variation in gill size of the African cyprinid Barbus neumayeri correlates with variation in associated trophic structures, foraging ecology and fitness-related characters in two populations living in different oxygen regimes. Our different populations of B. neumayeri offer a unique system to examine functional-morphological tradeoffs. Reciprocal transplant cage experiments showed lower survivorship for small-gilled fish moved to hypoxic swamp waters. Large-gilled fish, however, do not show lower survivorship in stream cages, though they may suffer a competitive disadvantage if their large gills impact associated feeding structures. To investigate this question, we compared trophic morphology and feeding efficiency of fish from small- and large-gilled populations. In addition, we evaluated differences in diet by quantifying prey abundance and selection for each group. Finally, we looked at fitness components in natural populations, including growth rates and condition factor. Our results demonstrate differences in functional morphology, feeding efficiency, diet, and some fitness components between the two populations. We argue that variation in gill size has facilitated the broad habitat distribution of B. neumayeri but may limit the success of phenotypes crossing physico-chemical gradients leading to diversification in this species.