The cost of eyes

A study reporting Mexican fish living in caves save energy by completely lacking eyes was published this week inScience Advances.

A species of cavefish found in Mexico has two different forms, or morphs. One ofthe morphs lives in surface rivers in bright sunlight, has large eyes and a well developed visual system, while the other lives in a cave and completely lacks eyes.

Researchers from Lund University, Sweden, wanted to know for sure why the cave dwelling morph of the cavefish has lost its optic system. They have investigated a possible explanation: the energetic cost of the visual system.

To study the effect of having eyes on the energy used by the fish, they had to measure both the energy usage of the whole fish, and that of the eyes on their own. In the same way that human energy consumption would be measured by using a treadmill and an oxygen mask, the team studied the fish swimming in a flowing tank, or flume, and monitored the oxygen content of the water. The changes in oxygen content allowed them to calculate the oxygen consumption of the entire fish, and therefore their metabolic rate.

To calculate the energy usage of the visual system, they had to remove the eyes from the fish, and the lens from the eyes. The eye was stretched eye inside out over a pin and inserted into a small chamber of flowing fluid. When kept in good conditions, the eye was able to be kept alive for many hours, allowing the researchers to measure the oxygen consumption of the retina per minute.

Combining these two sets of results enabled the team to calculate the proportion of the total energy budget of the whole fish taken up by the optic system.

Eric Warrant, lead researcher, didn't expect the energy cost would be so great. In smaller fish, where the eyes make up a larger proportion of the fish's body, the eyes and their associated visual system used up to 15% of the total energy budget of the fish.

As there is not much food available to the fish that live in the caves, using energy for a visual system that is not needed could make the difference between life and death.

Professor Warrant and his group have confirmed that the visual system of animals is a very expensive thing to have. If a species no longer requires its visual system then it pays, in evolutionary terms, to allow that system to regress because you can potentially save a lot of energy. He says that this a "very compelling reason" as to why the cave dwelling morph of the cavefish no longer has eyes.