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In this study, we explored variation in sperm morphometry of the African cyprinid Barbus neumayeri Fischer, 1884 (Neumayer’s barb) across seven sites with a wide range in dissolved oxygen, from hypoxic swamps to intermittent normoxic streams to well-oxygenated rivers. We explore whether fish physiological condition (K) or hypoxia can affect the reproductive traits, and whether condition–hypoxia dependence of sperm traits including head length (LH), head width (WH), flagellum length (LF), and hydrodynamic ratio (HR) vary across sampling sites. Significant differences were found in fish total length (P = 0.0212), as well as in K, left and right testis masses, total gonad mass, and gonadosomatic index (P < 0.0001 for all traits). Total gonad mass was lower in hypoxic sites than in well-oxygenated sites. Interestingly, the left and right testes from normoxic environments were double the size of testes from hypoxic environments. Despite little variation in sperm flagellum length, sperm heads were longer in swamps than in streams or rivers, giving the sperm head a more hydrodynamic shape. This variation in HR may be beneficial in the more stagnant waters of the swamp compared with the other environments. Future studies are necessary to understand whether variation in sperm morphology correlates with sperm swimming performance and male reproductive capacity.

Population level response to hypoxia has become an issue of global significance because of increased frequency and intensity of hypoxic events worldwide, and the potential for global warming to exacerbate hypoxic stress. In this study, we sequenced two nuclear intronic regions and a single mitochondrial region across seven populations of the African cyprinid, Barbus neumayeri from two river drainages in Uganda: the Rwembaita Swamp-Njuguta River System and the Dura River. We then examined two indices of population structure, G ST and Jost's D, to detect links between oxygen availability and genetic variation and to determine if population genetic structure was associated with (i) dissolved oxygen regime (hypoxia or normoxia), (ii) geographical distance, or (iii) a combination of dissolved oxygen regime and geographical distance. Our results indicate that over a large scale (between drainages), geographical distance significantly affects the genetic structure of populations. However, within a single drainage, dissolved oxygen regime plays a key role in determining the genetic structure of populations. Within the Rwembaita-Njuguta system, gene flow was high between locations of similar oxygen regimes, but low between areas characterized by divergent oxygen regimes. Interestingly, G ST analyses appear to yield less realistic measures of population structure than Jost's D, suggesting that caution must be taken when interpreting and comparing the results from different studies. These results support the idea that aquatic dissolved oxygen can act as a selective force limiting gene flow among populations of aquatic species and therefore should be considered when implementing conservation plans and assessing environmental impact of human activities.

This study described the behavior of the swamp-dwelling African cyprinid Barbus neumayeri in response to laboratory acclimation to normoxic and hypoxic conditions to detect costs associated with hypoxia exposure. Behavioral observations were conducted every two weeks over a four-week acclimation period in both normoxic and hypoxic conditions, and feeding trials were conducted after six weeks of acclimation. Gill ventilations were shallower in normoxia-acclimated fish than in hypoxia-acclimated individuals, and gill ventilation rate declined over the hypoxia acclimation. There was no effect of hypoxia acclimation on routine activity; however, individuals acclimated to hypoxia showed a depression in feeding rate relative to normoxia acclimation. This decline in feeding activity under hypoxia acclimation may account, at least in part, for the lower condition of the hypoxia-acclimated fish. These findings suggest that hypoxia exposure does not significantly impact routine activity levels of these swamp-adapted fishes, but seems to depress higher energy activities such as feeding rate.

This study evaluated whether the African cyprinid Barbus neumayeri from Rwembaita Swamp (low-oxygen) and Njuguta River (high-oxygen) in the Kibale National Park, Uganda differed in traits related to aerobic and anaerobic metabolic potential. Haematocrit was measured as an index of blood oxygen-carrying capacity, and tissue activities and isozyme composition of lactate dehydrogenase (LDH) were measured as indices of tissue anaerobic capacity. To address whether site-dependent differences were acute responses v. longer-term adjustments to environmental conditions, these variables were measured in fish sampled shortly after collection and after laboratory maintenance under well-aerated conditions. In fish sampled in the field, those from the low-oxygen site had significantly higher haematocrit, but this difference disappeared after long-term laboratory maintenance. In contrast, fish from the low-oxygen site had higher liver LDH activities than fish from the high-oxygen site, and this difference persisted during laboratory maintenance. Polymorphism was detected at both the LDH-A and LDH-B loci, and genotype frequencies for LDH-B differed significantly between collection sites. These results demonstrate physiological, biochemical and genetic differences in B. neumayeri from habitats differing in dissolved oxygen availability and suggest both acute and long-term responses to local environmental conditions.

Adaptive change in one character may affect associated, but functionally unrelated, characters in adaptive or nonadaptive ways. For example, adaptive change in gill size (i.e., larger gills in hypoxic habitats) may alter adjacent feeding structures and therefore have indirect effects on feeding behavior. This study examines whether differences in total gill filament length among populations of the African cyprinid Barbus neumayeri correlate with variation in trophic structures and feeding performance. Morphological measurements revealed larger total gill filament length in populations from low-oxygen habitats. Differences in external and internal morphology of the trophic apparatus correlated with those found in gills and suggest a reduction in food capture and processing ability in fish with larger respiratory apparatus. In paired feeding trials on a novel prey type, large-gilled fish spent more time feeding than small-gilled fish of the same body size without ingesting more food. This suggests less efficient food uptake in large-gilled fish from hypoxic habitats. We suggest that variation in gill size facilitates the broad habitat distribution of B. neumayeri but may limit the success of phenotypes crossing physicochemical gradients.

To test whether patches of papyrus swamp contribute to diversification of populations of non-air-breathing fishes, the gill morphology of Barbus neumayeri was compared between a papyrus swamp and several tributaries which diVered in oxygen regime. Total gill filament length diVered among sites and was negatively related to dissolved oxygen availability, supporting strong selection pressure for low-oxygen tolerance in the swamp interior. Among recaptures of marked B. neumayeri over a 4·5-year period among the focal swamp 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. This movement rate would theoretically be suYcient to homogenize gene frequencies among populations. However, randomly amplified polymorphic DNA (RAPD) markers indicated significant genetic diVerentiation among sites and no relationship between genetic diVerences and geographical distances among sites suggesting habitat-specific selection pressures on dispersers, rather than insuYcient dispersal. ? 1999 The Fisheries Society of the British Isles

ummary Wetlands, dominated by papyrus (Cyperus papyrus), are extensively distributed in east and central Africa. However, the use of papyrus swamps by fishes is limited to species that can tolerate the low oxygen conditions that can prevail in the dense swamp interior. In this study, we ask whether the patchy distribution of papyrus swamps may contribute to variation among fish populations in traits related to oxygen uptake. A series of paired experiments was used to compare the respiratory behaviour of a small water breathing cyprinid Barbus neumayeri from the dense interior of a papyrus swamp in western Uganda, to populations from nearby well-oxygenated rivers and streams. The respiratory behaviour of the swamp population differed from the other populations. Several lines of evidence suggest that B. neumayeri from papyrus swamps are more tolerant of low oxygen conditions than populations from nearby river and stream sites, including lower thresholds for aquatic surface respiration in the swamp population, lower gill ventilation rates in the swamp population and higher loss of equilibrium in fish from the river/stream sites. This small scale geographical variation in behaviour among B. neumayeri populations supports an earlier study that documented variation in gill morphology among sites that differed in oxygen availability, and supports the significance of papyrus swamps in promoting diversification among populations. Resume Les zones humides, dominees par les papyrus (Cyperus papyrus), sont tres largement reparties en Afrique orientale et centrale. Cependant, l'utilisation des marais a papyrus par des poissons se limite aux especes qui peuvent supporter les conditions pauvres en oxygene qui prevalent dans les zones plus denses des marais. Dans cette etude, nous demandons si la distribution eparpillee des marais a papyrus peut contribuer a une variation, entre les populations de poissons, des caracteristiques liees a l'absorption d'oxygene. Une serie d'experiences couplees a servi a comparer le comportement respiratoire d'un petit cyprinide, Barbus neumayeri, venant de l'interieur dense d'un marais a papyrus dans l'ouest de l'Ouganda, a celui de populations provenant des cours d'eau voisins bien oxygenes. Le comportement respiratoire de la population du marais etait different de celui des autres populations. Plusieurs preuves suggerent que les B. neumayeri provenant des marais a papyrus supportent mieux des conditions pauvres en oxygene que ceux des rivieres et cours d'eau voisins, comme par exemple des seuils plus bas pour venir respirer a la surface chez la population des marais, des taux plus bas de ventilation branchiale chez cette merae population, et une perte d'equilibre plus grande chez les poissons des cours d'eau. Cette variation geographique sur une petite echelle du comportement entre les populations de B. neumayeri vient appuyer une etude anterieure qui documentait la variation de morphologie des branchies entre des endroits qui differaient quant a la quantite d'oxygene, et renforce la signification des marais a papyrus pour la promotion de la diversification entre les populations.

I examined the effects of extreme hypoxia and water level fluctuations on the distribution and movement of an air-breathing clariid catfish, Clarias liocephalus, in a papyrus swamp in western Uganda. Monthly records of the distribution and relative abundance of C. liocephalus across 28 swamp stations were used to examine seasonal trends in habitat use and movement. My results suggest that dissolved oxygen did not directly limit use of, or dispersal through, the papyrus swamp. C. liocephalus were found at most stations in the dry season, and there was no significant relationship between the number of stations used per month and mean monthly dissolved oxygen levels. In addition, there was no evidence for a significant effect of oxygen on the relative abundance of fish among stations during the driest months, or during peak flood conditions. However, relative abundance was positively correlated with water depth during the dry season. Fish from deeper habitats exhibited fewer injuries and lower rates of disappearance than fish from shallower waters. This may reflect the risk of aerial predation for surfacing fish. A comparison of habitat use and movements of C. liocephalus with the only other fish species found in the papyrus swamp, Barbus neumayeri, supports the idea that respiratory mode can affect the use of hypoxic papyrus swamps. B. neumayeri, is a small water-breathing cyprinid that uses aquatic surface respiration in response to severe hypoxia. The air-breathing capabilities of C. liocephalus permitted more widespread use of swamp waters than B. neumayeri. This study demonstrates that an understanding of the respiratory mode of fish species may be useful in predicting patterns of habitat use.

In seasonally flooding fresh waters, dissolved oxygen and fluctating water levels combine to create strong seasonal changes in habitat availability and dispersal pathways for fishes. In this study we demonstrate how respiratory mode can affect the use of and dispersal through hypoxic papyrus swamps for a small cyprinid fish, Barbus neumayeri, a species which uses aquatic surface respiration (ASR) in response to severe hypoxia. Monthly records of the distribution and relative abundance of B. neumayeri across 28 stations in a papyrus swamp in Uganda were used to examine seasonal patterns of habitat use and movement. The distribution of B. neumayeri was very restricted during the drier months. Most fish were captured in open pools and channels with large areas of open water surface and higher oxygen. Movement of B. neumayeri was limited to short periods during the wet season when peak water levels produced high levels of oxygen and pathways for movement. Three lines of evidence suggest that these patterns relate to its respiratory mode. These include: the positive :relationship between dissolved oxygen and the number of stations used per month, the positive relationship between fish density at dry season stations and dissolved oxygen, and the absence of fish from stations with. little open water surface area during the dry season. Significant differences in the gill morphology between B. neumayeri from the papyrus swamp and those from the main river suggest that papyrus swamps may contribute to the diversification of B. neumayeri by limiting movement and demanding specialization for extremely hypoxic waters.