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Monica Folgueira, Steve Wilson's lab, UCL
Confocal micrograph of a cavefish embryo at around five days post-fertilisation. The embryo has been stained with an antibody against a calcium-binding protein (in green) to show different neuronal types and their processes in the nervous system. This staining also reveals taste buds, which are located around the mouth and along the body of the cavefish.
How do cavefish get around in the dark?
Fish have a number of sensory systems on their bodies to help them move through the water. The cavefish Mexican tetra (Astyanax mexicanus) has a seeing and a blind form; the latter lives in dark environments, and relies on other senses. The lateral line is a sensory system, used by fish and some amphibians, that is responsible for detecting vibrations, movement and pressure changes in the surrounding waters.
As well as the lateral line, the blind cavefish has specially adapted traits that its sighted relation (dwelling nearer the surface) does not, including a greater number of neuromasts (mechanoreceptors) along its body. Both the blind and seeing varieties have taste buds in the lips and oral cavity, but the blind cavefish has also developed them in the lower jaw and has an increased number of taste buds along its body. Studies have shown that not only does the blind cavefish possess more taste buds but the taste buds themselves are more efficient. They are associated with significantly more axons to efficiently transmit this sensory information than these cells in the seeing cavefish.
Why do blind cavefish develop eyes but then lose them?
All cavefish, seeing and blind, start life with eyes, but the eyes of the blind cavefish do not fully develop. Eyes are of little or no use in the dark cave environment, so the blind cavefish relies on other senses, including taste buds, mechanoreceptors and neuronal structures, which have evolved to become more powerful. Disuse causes the partially developed eyes to degenerate over time.
If the cavefish eyes are useless, why hasn't evolution selected against the presence of them altogether? This evolutionary mystery puzzled Darwin and we have no comprehensive answer even today.
However, it is thought that a gene key to eye development is negatively linked to a gene responsible for the fish's heightened other senses. The greater usefulness of the latter, scientists believe, has led to an evolutionary trade-off. Evolution has selected for the gene that develops other senses, in turn down-regulating and switching off the equivalent gene for the eyes - but not until later stages of the fish's growth. If so, loss of eye development is a side-effect of the blind cavefish's superior other senses.