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

对鱼类来说，水中溶解氧的可用性会限制栖息地质量和扩散途径。 缺氧沃茨对某些鱼类的作用与缺水相似，对鱼类的活动造成障碍，或限制了对替代生境的选择。 缺氧导致鱼类进化出各种形态和生理适应（例如，呼吸空气的器官，大鳃的发展）。 这些适应有相关的成本和收益，应该会影响鱼类的能力，通过脱氧沃茨殖民，栖息或分散。 尽管人们对脱氧的生理和形态适应的演变很感兴趣，但对缺氧沃茨作为屏障或生物过滤器的作用及其对鱼类多样化的影响还没有很好的了解。 这项研究计划认为，缺氧湿地在鱼类区系多样性的进化作用，通过解决的问题，缺氧是否作为一个强大的选择力，促进东非fishes.Oxygen的种群之间的变化是普遍存在的热带淡水沃茨，特别是在密集的湿地，洪泛区池，和淹没的森林。 在东非，广泛的缺氧湿地主要由纸莎草港呼吸空气的鱼类，如非洲肺鱼。 也有一些非空气呼吸（水呼吸）鱼类，这些鱼类往往是具有非常有效的摄氧机制的物种（例如，非常大的鳃表面积）或栖息在沼泽边缘的物种，在那里与开放的沃茨的相互作用提高了水中的氧含量。 尽管在密集的湿地中鱼类的数量很少，但这些栖息地可能对维持鱼类多样性非常重要。 对于呼吸空气的鱼类来说，湿地不太可能限制它们的扩散。 然而，对于不能忍受水中低氧条件的水呼吸鱼类来说，大的沼泽分界线可能会隔离种群并导致多样化和物种形成。 即使对于一些可以在密集沼泽中生存的水呼吸鱼类，栖息地的利用和扩散仍然可能受到物种中溶解氧可用性和氧吸收效率的限制。 这可能导致沼泽和开放水域种群之间的差异（即，当一个种群生活在稠密的沼泽中，而其他种群生活在含氧充足的溪流和河流中时，同一物种的相邻种群可能在形态学或遗传组成上存在差异）。 L.查普曼和他的同事们证明，三种非洲水呼吸鱼类的沼泽种群比附近氧气充足的沃茨中的同类种群有更大的鳃。 对其中一种鱼类的进一步研究表明，沼泽鱼类与附近富氧溪流和河流鱼类在呼吸行为和生理特征上存在差异。 然而，控制这些模式的种群间变异的进化机制是未知的。 本研究计划采用在乌干达的实地研究和实验室研究相结合，在佛罗里达大学，以研究在何种程度上缺氧有助于呼吸水东非鱼类的呼吸生物学的地方间变异，以及他们的呼吸生物学是否限制栖息地的使用和性能。 该计划的重点是一个物种，从每一个栖息在缺氧沼泽和良好的氧气湖泊和/或河流的三个主要谱系的水呼吸鱼类：Barbus neumayeri（一种小鱼），Pseudocrenilabrus victoriae（一种慈鲷）和Gnathonemus victoriae（一种电鳗鱼）。 本研究通过评估遗传和环境影响对呼吸特征以及氧应激下的生理和行为表现的相对贡献，扩展了这些物种的早期研究。 一个主要的问题是呼吸特征的变化有多大（例如，沼泽和开放水域种群之间的遗传差异（例如鳃大小、血红蛋白浓度）是遗传差异的结果，而有多少变异是由于表型可塑性（不涉及遗传变化的对环境的反应的特征变化）？ 该项目还考虑了变异是否是适应性的，鳃形态的变化是否影响非功能相关的特征（即，大的鳃是否限制了邻近的肌肉，例如那些参与进食的肌肉？），和表型（大鳃鱼和小鳃鱼）之间相互作用的潜力。 这些问题的评估将提供深入了解环境斑块（沼泽开放水域马赛克）如何促进非洲鱼类的进化多样性，并提高我们对适应性表型可塑性与当地适应的理解。

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.