Swamps and Faunal Diversification: Interdemic Variation in the Respiratory Ecology of East African Fishes

University Of Florida, Gainesville FL

Investigators

Abstract

For fishes, the availability of dissolved oxygen in the water can limit habitat quality and dispersal pathways. Oxygen-scarce (hypoxic) waters play a similar role to the absence of water for some fish species, creating a barrier to movement or limiting selection of alternative habitats. Oxygen scarcity has led to the evolution of varied morphological and physiological adaptations in fishes (e.g., development of air-breathing organs, large gills). These adaptations have associated costs and benefits that should affect the ability of fishes to colonize, inhabit, or disperse through deoxygenated waters. Despite great interest in the evolution of physiological and morphological adaptations to deoxygenation, the role of oxygen-scarce waters as barriers or biological filters and their impact on fish diversification is not well understood. This research program considers the evolutionary role of oxygen-scarce wetlands in fish faunal diversification by addressing the question of whether or not hypoxia acts as a strong selective force contributing to variation among populations of East African fishes. Oxygen scarcity is widespread in tropical fresh waters, particularly in dense wetlands, floodplain pools, and flooded forests. In East Africa, extensive hypoxic wetlands dominated by papyrus harbor air-breathing fishes like the African lungfish. Some non-air-breathing (water-breathing) fishes also occur, and these tend to be species with extremely efficient oxygen uptake mechanisms (e.g., very large gill surface area) or species inhabiting the edge of the swamp where interaction with open waters elevates the oxygen content of the water. Despite low numbers of fish species within dense wetlands, these habitats may be very important in the maintenance of fish diversity. For air-breathing fishes, wetlands are not likely to limit dispersal. However, for water-breathing fishes that cannot tolerate low oxygen conditions in the water, large swampy divides may isolate populations and lead to diversification and speciation. Even for some water-breathing fishes that can survive in dense swamps, habitat use and dispersal may still be limited by dissolved oxygen availability and oxygen uptake efficiency in the species. This may result in differences between swamp and open-water populations (i.e., neighboring populations of the same species may diverge in morphology or genetic makeup when one population lives in dense swamp and other populations live in well-oxygenated streams and rivers). Earlier studies by L. Chapman and colleagues demonstrated that swamp populations of three water-breathing African fishes have larger gills than populations of the same species in nearby well-oxygenated waters. Further studies on one of the species revealed differences in respiratory behavior and physiological characters between fish from swamp and fish from nearby well-oxygenated streams and rivers. However, the evolutionary mechanisms controlling these patterns of interdemic (between-population) variation are unknown. This research program uses a combination of field studies in Uganda and laboratory studies at the University of Florida to examine the degree to which oxygen scarcity contributes to interdemic variation in the respiratory biology of water-breathing East African fishes, and whether their respiratory biology is limiting habitat use and performance. The program focuses on one species from each of three major lineages of water-breathing fishes that inhabit both hypoxic swamps and well-oxygenated lakes and/or rivers: Barbus neumayeri (a minnow), Pseudocrenilabrus multicolor victoriae (a cichlid) and Gnathonemus victoriae (an electric mormyrid fish). This research expands on earlier studies of these species by evaluating the relative contribution of genetic and environmental influences on respiratory characters as well as physiological and behavioral performance under oxygen stress. A major question is how much of the variation in respiratory traits (e.g., gill size, hemoglobin concentration) between swamp and open-water populations is the result of genetic differences, and how much of the variation is due to phenotypic plasticity (change in characters in response to the environment that does not involve genetic change)? The project also considers whether the variation is adaptive, whether change in gill morphology impacts non-functionally related characters (i.e., do large gills constrain adjacent muscles such as those involved in feeding?), and the potential for interactions between phenotypes (large- and small-gilled fish) in the field. The evaluation of these questions will provide insight into how environmental patchiness (the swamp-open water mosaic) fosters evolutionary diversification of African fishes, and enhance our understanding of adaptive phenotypic plasticity versus local adaptation.