The eye is such a valuable organ that it has evolved independently more than once, but it turns out animals pay a surprisingly high price for perceiving part of the electromagnetic spectrum. For one species of fish vision can consume as much as 15% of its energy intake, according to work published in Science Advances.
As a species we have relied heavily on our relatively sharp eyesight for much of our rise to global domination. Nevertheless many other species have produced remarkable refinements in their sight that far exceed our own. It has long been known that there must be a substantial cost to this sense, since animals dwelling in total darkness lose it. However, biologists have pondered how large the downsides might be.
Brains use a lot of energy, which is presumably one reason why species have not taken tool use anything like as far as we have. Our brains are estimated to use 20% of our resting metabolic rate. Naturally some of that involves processing the information that comes from our retina, but how much?
Mexican cavefish (Astyanax mexicanus) provide an excellent model to study the effects of sight. The species has varieties, known as morphs. One, as the name suggests, lives in caves in total darkness and has no eyes. The other morph inhabits surface rivers and can see.
The sighted, surface-dwelling, form of Astyanax mexicanus. Credit: Vladimir Wrangel/Shutterstock
The fact that the two morphs are closely related enough to interbreed indicates it was easy for one morph to lose its sight, while remaining largely unchanged in other ways. The process is probably recent, with the paper noting, “Intermediate cave-dwelling phenotypes with varying degrees of eye reduction have been found.”
While at Lund University Dr Damian Moran compared the morphs' energy consumption. “Our measurements... show that the visual system requires between 5% and 15% of the animal's total energy budget, depending on the age of the fish," he said in a statement. “[The Mexican cavefish] lost both eyes and visual cortex, without a doubt because of the unsustainable energy cost of maintaining a sensory system that no longer had any significance."
To analyse the metabolic rate of surface fish the team compared oxygen consumption of neural tissue to that for the whole body, finding that consumption was highest among smaller fish as they had a higher brain-size-to-body-mass ratio. The energetic cost of the eyes themselves was 15% for a 1 gram (0.04 ounce) fish, and 5% for a 8.5 gram (0.3 ounce) fish. In the nutrient-poor environment of the caves the energetic saving made by blind cavefish could be the difference between life and death.
Moran and his colleagues confirmed the findings by observing intermediary energy consumption among morphs with partial loss of eyesight and in hybrids produced by cross breeding the two most contrasting morphs. They noted that other features that can produce differences in energy consumption, such as growth rates, were very similar between the morphs.
While the loss of one sense in humans often leads to neurons being repurposed and greater capacity in other areas, it seems the cavefish have responded by having the optical processing area atrophy in order to save energy, with eyeless versions having 30% less brain mass.