People tend to rely on their eyes to understand the world, but some fish don't have eyes or even light for illumination. A population of Astyanax mexicanus, or ‘cavefish’, thrive deep in caves where they navigate and find food in total darkness. A team of researchers based at the University of Florida, USA, led by James Liao, looked beyond this obvious difference to understand the hidden mechanisms behind the cavefish's success in exploring the world without eyes. They wanted to know if the cavefish nervous system adjusted to the lack of eyes by amplifying other senses, specifically the fish sense of ‘touch’, which detects water flow through structures called neuromasts distributed across the fish's body. The researchers compared how the neuromasts differ between the blind cavefish and another sighted population of A. mexicanus, which live in streams on the surface of the planet and use their eyes to explore.
Instead of comparing how the adult fish responded to flowing water, the team compared larval fish from each species to rule out the possibility that any differences between the sighted and blind fish were due to learning as they developed. They found that the distribution of neuromasts along the sides of the fish larvae differs between the two populations: the blind cavefish have more neuromasts closer to their heads than the fish from a river at the surface. However, each neuromast was made up of a similar number of the flow-sensitive hairs that get tugged as the water moves past them – similar to the hair cells in our inner ears, which sense air pressure changes and allow us to perceive sound.
To determine whether the nervous system of the blind cavefish also adapted to the dark environment, the researchers measured the electrical signals produced by a single sensory neuromast when the larvae were still. When not swimming, the electrical signals produced by the blind cavefish neuromasts were stronger than those produced by their sighted relatives, indicating that the baseline of communication between the neuromast and the brain is higher. In addition, when the team vibrated the hairs of a neuromast to mimic the water flowing past, the response was again stronger in the cavefish than the surface fish.
The researchers then determined how the nervous system of the blind cavefish communicated while the larvae simulated swimming. By recording electrical signals from neuromasts, they found that when the surface fish are swimming, their neuromasts relay even fewer signals than when they are still, which allows the larval fish to ignore water flow across their body when generating their own movement. However, the blind cavefish's neuromasts continued to relay signals when swimming.
Next, the team tested whether the surface fish's lack of sensitivity as they swim is dictated by the brain. They located neurons in the brain that send signals to the neuromast flow sensors and experimentally silenced the neurons. Without signals from the brain, the neuromasts of the surface-dwelling fish relayed signals similarly to the cavefish: they continued to be active when the fish were swimming. Suppressing the sensory signals produced by neuromasts during swimming can lead to more efficient swimming for the surface dwellers, but for the blind cavefish, more sensitive neuromasts are likely to be beneficial. And, when the researchers tested two other populations of blind cavefish that live in other cave systems, they found that the nervous systems of both populations adopted a similar strategy to increase their sensitivity to flowing water while swimming.
Fish with eyes rely on their sight to navigate while swimming. Without eyes, cavefish feel their way through the water by continuing to signal waterflow even when the fish are swimming. This extra attention likely requires more energy, but it's worth the cost in the darkness.
