Subterranean Fishes of the World

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2022

Nature

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Like a sleepy person searching for the bathroom in the dark, eyeless cavefish navigate using surrounding walls as a guide. Cavefish in the genus Sinocyclocheilus live in freshwater caves in parts of China, and lost their ability to see — and often their eyes — over millions of years of evolution in the constant darkness. Bing Chen at Guangxi University in Nanning, China, and her colleagues collected cavefish of 26 species and analysed sensory organs called neuromasts, which detect movement and changes in pressure in the water. Neuromasts are found along fishes’ bodies and heads, but the researchers found that the cavefish had more neuromasts on one side of their heads — either the right or the left — than on the other. The asymmetry was pronounced in species that had no eyes or very small eyes. When the researchers tested the ability of the cavefish to swim freely, eyeless species tended to stick to the wall on the side of their body that had more neuromasts. The asymmetric neuromast system evolved independently in distantly related cavefish species, suggesting that wall-following is a successful survival strategy for creatures living in perpetual darkness.

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