Animals have all sorts of neat tricks to find their way in the dark. Bats have sonar, ants follow their noses, and some fishes surround themselves with electric fields that can detect nearby objects. Electrical navigation is wonderful for fishes that live in murky water, but it comes at a cost – electricity is expensive to generate and can represent almost a third of the overall energy budget in these fishes. When conditions are good, paying this cost doesn't seem to be a problem. But electrical sensing is most useful in murky and stagnant habitats that also tend to be low in oxygen. Oxygen is critical for fuelling metabolic energy production and survival in oxygen-limited environments often, therefore, depends on the ability of animals to reduce their metabolic rates. How do electric fishes balance this energetic budget and deal with the expense of electricity generation in the face of severe oxygen austerity?

A new study, led by Shelby Clarke at McGill University, Canada, has unravelled the details of this trade-off by studying the electric fish Petrocephalus degeni. The authors captured wild fish from a low-oxygen Ugandan swamp and brought them into a lakeside laboratory, where they measured metabolic rate and electricity production first under conditions of abundant oxygen and then after the fish were challenged with low-oxygen conditions.

As oxygen levels decreased in the experimental chamber, electricity production initially remained steady. However, under more severe conditions – when about 80% of the oxygen was gone – electrical activity began to decrease. The energy saved from minimizing electrical output could then be allocated to other vital processes, allowing the fish to continue to obtain enough oxygen to maintain normal metabolism until almost 90% of the oxygen was gone from the water. Amazingly, even below this critical point where the fish could not breathe as much oxygen as they required, electrical production did not cease despite its high energetic cost. Low levels of electricity persisted, perhaps representing a desperate attempt to find an escape route.

If electrical activity is constrained by oxygen supply, the authors reasoned the electric fish should get even more electric if oxygen is abundant. To test this idea, Clarke moved electric fish from their typical low-oxygen swampy habitat to a life of luxury in well-aerated aquariums. After several weeks in this housing arrangement, electrical production was indeed higher than in fish from the harsh wild conditions. However, the ability of these pampered fish to tolerate low-oxygen conditions was diminished. The authors conclude that when these electric fish have easy access to oxygen, they spend less energy on the organs used to acquire more of the gas, such as the heart, gills or blood. Instead, the energy is allocated to increased electrical capacity that presumably improves their ability to perceive their physical environment.

Like any utility company, electric fish must continually evaluate the budgetary landscape when deciding how much to invest in electricity production. And, while the mechanistic details of electrical output regulation remain to be discovered, it is clear that these fish have an impressive ability to re-organize their power system over both the short and long term, allowing them to cope with whatever conditions nature throws their way.

S. B.
L. J.
The effect of normoxia exposure on hypoxia tolerance and sensory sampling in a swamp-dwelling mormyrid fish
Comp. Biochem. Physiol. A.