Papyrus swamps, hypoxia, and faunal diversification: variation among populations of Barbus neumayeri

To test whether patches of papyrus swamp contribute to diversification of p opulations of non-air-breathing fishes, the gill morphology of Burbus neuma yeri was compared between a papyrus swamp and several tributaries which dif fered in oxygen regime. Total gill filament length differed among sites and was negatively related to dissolved oxygen availability, supporting strong selection pressure for low-oxygen tolerance in the swamp interior. Among r ecaptures of marked B. neumayeri over a 4.5-year period among the focal swa mp and connected stream and river sites, 93% of fish were recovered at the site of capture. Some of the individuals that moved crossed physicochemical gradients and traversed long distances within the swamp/stream system. Thi s movement rate would theoretically be sufficient to homogenize gene freque ncies among populations. However, randomly amplified polymorphic DNA (RAPD) markers indicated significant genetic differentiation among sites and no r elationship between genetic differences and geographical distances among si tes suggesting habitat-specific selection pressures on dispersers, rather t han insufficient dispersal. (C) 1999 The Fisheries Society of the British I sles.