Exploring potential drivers of brain size variation in the electric fish Brachyhypopomus occidentalis.

Characterizing the factors that shape variation in brain size in natural populations is crucial to understanding the evolution of brain size in animals. Here, we explore how relative brain size and brain allometry vary with drainage, predation risk and sex in natural populations of the electric knifefish Brachyhypopomus occidentalis. Fish were sampled from high and low predation risk sites within two independent river drainages in eastern and central Panamá. Overall, we observed low variation in brain-body size allometric slopes associated with drainage, predation risk and sex category. However, we observed significant differences in allometric intercepts between predation risk sites. We also found significant differences in relative brain mass associated with drainage, as well as significant differences in absolute brain mass associated with drainage, predation risk and sex category. Our results suggest potential constraints in brain-body allometry across populations of B. occidentalis. However, both drainage and predation risk may be playing a role in brain mass variation among populations. We suggest that variation in brain mass in electric fishes is affected by multiple extrinsic and intrinsic factors, including geography, environmental complexity, social interaction and developmental or functional constraints.

Hypoxia-induced plasticity in the metabolic response of a widespread cichlid.

The African cichlid, Pseudocrenilabrus multicolor victoriae is a eurytopic fish that exhibits high levels of developmental plasticity in response to dissolved oxygen availability. In this study, F1 offspring from three sites in the Mpanga River drainage of Western Uganda characterized by different dissolved oxygen (D.O.) regimes were reared under normoxic or hypoxic conditions. After 1 year, enzymes were measured to determine the tissue metabolic capacity of four different tissues: muscle, heart, brain and liver. The enzymes measured were pyruvate kinase [PK], lactate dehydrogenase [LDH], citrate synthase [CS], and cytochrome C oxidase [CCO], and an additional two, malate dehydrogenase (MDH) and fructose 1,6-bisphosphatase (FBPase), were examined in the liver only. Individuals reared under hypoxia exhibited elevated levels of LDH and CCO in the heart; and depressed activity levels of brain CS and liver CCO and MDH relative to normoxia-reared sibs. Results from this study demonstrate that long-term exposure to hypoxia during development can induce changes in the metabolic capacities of P. multicolor. This flexibility may be important in facilitating persistence in variable and/or novel environments, and in the face of increasing global hypoxia.

Population differentiation of the African cyprinid Barbus neumayeri across dissolved oxygen regimes.

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.

Natural variation in enzyme activity of the African cichlid Pseudocrenilabrus multicolor victoriae.

This study describes the metabolic capacities of the African cichlid Pseudocrenilabrus multicolor victoriae from four sites in Uganda, East Africa. Fish were captured during the dry season, from two aquatic systems in different regions (Lake Nabugabo and Mpanga River). Within the Lake Nabugabo region, individuals were sampled from Lake Kayanja (normoxic) and Lwamunda Swamp (hypoxic); within the Mpanga River system, individuals were sampled from Bunoga and Kahunge (characterized by seasonal variation in dissolved oxygen (D.O.)). Enzyme activity levels of pyruvate kinase, lactate dehydrogenase, citrate synthase, and cytochrome C oxidase were measured in four tissues: white skeletal muscle, heart, brain, and liver. Two additional enzymes were measured in the liver, malate dehydrogenase and fructose 1,6-bisphosphatase. Regional differences between enzyme activities in most tissues were evident; however, little variation was observed between two sites within a region despite differences in D.O. In general, P. multicolor from the Mpanga River system displayed greater anaerobic enzyme activity in white skeletal muscle, lower gluconeogenic enzyme activity in the liver, and an overall higher enzyme activity in the heart and brain tissues than fish from the Nabugabo region. The latter may reflect a long-term adaptation to low-oxygen conditions at the metapopulation level in the Nabugabo region.

Oxygen limitation and tissue metabolic potential of the African fish Barbus neumayeri: roles of native habitat and acclimatization.

BACKGROUND: Oxygen availability in aquatic habitats is a major environmental factor influencing the ecology, behaviour, and physiology of fishes. This study evaluates the contribution of source population and hypoxic acclimatization of the African fish, Barbus neumayeri, in determining growth and tissue metabolic enzyme activities. Individuals were collected from two sites differing dramatically in concentration of dissolved oxygen (DO), Rwembaita Swamp (annual average DO 1.35 mgO2 L(-1)) and Inlet Stream West (annual average DO 5.58 mgO2 L(-1)) in Kibale National Park, Uganda, and reciprocally transplanted using a cage experiment in the field, allowing us to maintain individuals under natural conditions of oxygen, food availability, and flow. Fish were maintained under these conditions for four weeks and sampled for growth rate and the activities of phosphofructokinase (PFK), lactate dehydrogenase (LDH), citrate synthase (CS), and cytochrome c oxidase (CCO) in four tissues, liver, heart, brain, and skeletal muscle. RESULTS: Acclimatization to the low DO site resulted in lower growth rates, lower activities of the aerobic enzyme CCO in heart, and higher activities of the glycolytic enzyme PFK in heart and skeletal muscle. The activity of LDH in liver tissue was correlated with site of origin, being higher in fish collected from a hypoxic habitat, regardless of acclimatization treatment. CONCLUSIONS: Our results suggest that the influence of site of origin and hypoxic acclimatization in determining enzyme activity differs among enzymes and tissues, but both factors contribute to higher glycolytic capacity and lower aerobic capacity in B. neumayeri under naturally-occurring conditions of oxygen limitation.

Effects of long-term hypoxia on enzymes of carbohydrate metabolism in the Gulf killifish, Fundulus grandis.

The goal of the current study was to generate a comprehensive, multi-tissue perspective of the effects of chronic hypoxic exposure on carbohydrate metabolism in the Gulf killifish Fundulus grandis. Fish were held at approximately 1.3 mg l(-1) dissolved oxygen (approximately 3.6 kPa) for 4 weeks, after which maximal activities were measured for all glycolytic enzymes in four tissues (white skeletal muscle, liver, heart and brain), as well as for enzymes of glycogen metabolism (in muscle and liver) and gluconeogenesis (in liver). The specific activities of enzymes of glycolysis and glycogen metabolism were strongly suppressed by hypoxia in white skeletal muscle, which may reflect decreased energy demand in this tissue during chronic hypoxia. In contrast, several enzyme specific activities were higher in liver tissue after hypoxic exposure, suggesting increased capacity for carbohydrate metabolism. Hypoxic exposure affected fewer enzymes in heart and brain than in skeletal muscle and liver, and the changes were smaller in magnitude, perhaps due to preferential perfusion of heart and brain during hypoxia. The specific activities of some gluconeogenic enzymes increased in liver during long-term hypoxic exposure, which may be coupled to increased protein catabolism in skeletal muscle. These results demonstrate that when intact fish are subjected to prolonged hypoxia, enzyme activities respond in a tissue-specific fashion reflecting the balance of energetic demands, metabolic role and oxygen supply of particular tissues. Furthermore, within glycolysis, the effects of hypoxia varied among enzymes, rather than being uniformly distributed among pathway enzymes.