Brienomyrus niger

Sonja studies a species of electric fish called Brienomyrus niger, from the family Mormyridae, which lives in turbid waters in Nigeria. Mating is triggered during the rainy season. Male fish can be differentiated from female fish by the shape of the anal fin, which changes shape and becomes very flexible during the mating season. This allows the male fish to use his tail to manipulate the female to allow mating to take place. Sonja is trying to determine if it is hormonal differences between the male and female fish that allow the male to flex his tail in these complicated movements.

There are other physical changes that happen to males during mating. The male fish's voice changes - this is an electric discharge that can be measured - and its gonads triple in size. In controlled laboratory experiments, Sonja adds a sex hormone to tanks containing female fish. This hormone seems to turn female fish into male fish, at least externally. Taking careful measurements, Sonja compares data on treated and untreated fish to find out if the changes created by the added testosterone correspond to differences between the genders.

Sonja uses only wild specimens, purchased from a vendor in Nigeria, to conduct her experiments. The vendor supplies fish at least 4-5 inches long in batches of between 20 and 200. They are sent in bags of water with air that are put in an insulated box to maintain a constant temperature. When a shipment of fish first arrives, it has to be kept in quarantine, separate from other fish in the laboratory for two weeks. This avoids the possibility of any kind of infection spreading among the batches of specimens. Sonja sorts the quarantined fish into three groups: juvenile, subadult, and adult. She prefers fish from the last two categories for her studies. Subsequently, all the fish are put in a tank together. The large number of electric fish in the tank results in high conductivity of the water. Sonja slowly adds extra water over time to balance the water to 150 units of connectivity. She measures each fish carefully: weighing it, measuring its body height, depth, and fin lengths, and using an oscilloscope to measure the electrical impulses from each fish (individuals can be differentiated by their electrical discharges). Some individuals are placed in a control tank.

Over the next eight weeks Sonja adds an antigen, specifically testosterone, to the water in the test tank. She wears gloves to avoid absorbing any of the hormone into her own body. The fish absorb the testosterone, probably through their gills. After eight weeks, Sonja repeats her measurements, and also measures each fish's electrical discharge. Normally, the electrical discharge in males and females is distinctly different, but after females are given the hormone treatment, they have the same electrical output as males. She also X-rays each fish every week to discern any changes in its anal fin bones. After four weeks, a clear thickening of the bones is apparent. The movements of the male fish's anal fin during the mating ritual require extra muscles. In order to create more surface area to which extra muscles can be attached, the bones themselves grow longer. Sonja continues to X-ray each fish to confirm any changes in bone structure. She draws 50 microliters of blood from each fish to measure its testosterone level and then tests the water for testosterone and other chemicals. (Sonja's hypothesis is that there should be a high concentration of testosterone in the fish tissue and a measurable loss of testosterone in the water.) She also studies the histology of the fishes' fins, using staining methods to determine whether there are more muscles attached to the bones. In order to confirm that new muscles have formed, she uses BRDU, a chemical that marks newborn cells using DNA.