Implications of hypoxia for the brain size and gill morphometry of mormyrid fishes

The mormyrids are well known for their remarkable electrogenic and electrolocation capabilities and exceptionally large cerebellum that may account for much of their total oxygen consumption. Mormyrids living in oxygen-deficient waters may use oxygen efficiently, protecting the brain from hypoxia damage; and/or brain size may be reduced. This study compares the TFL, gill lamellar density, gill lamellar area, total gill surface area, and brain size of two species of mormyrids from extremely hypoxic waters in Uganda (Gnathonemus victoriae and Petrocephalus catostoma) to two open-water species (Mormyrus kannume and Gnathonemus longibarbis) from the same region. In addition, interdemic variation was considered by comparing swamp populations of G. victoriae and P. catostoma to open-water populations of the same species. Total gill surface area of all species fell within the upper range for freshwater fishes. However, there were both intraspecific and interspecific differences in gill characters. Interdemic comparisons showed larger gill size in swamp-dwelling populations. Brain size varied among species; larger brains were characteristic of species from well-oxygenated waters. Large gill surface area may permit survival of mormyrids in oxygen-stressed environments; however, mechanisms compensating for hypoxia seem to be inadequate to support a brain size as large as that seen in fish from well-oxygenated waters.