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Over the course of millions of years of evolution, cavefish have developed several adaptations that help them survive in the dark. Some of these changes would be detrimental to their sighted, surface-dwelling ancestors, but are vital to the survival of the cavefish. Of the 80 known species of cavefish that have evolved from surface-dwelling fish, the Mexican cavefish is the only one with a remaining surface-dwelling relative and is the most often studied.
Skin and Eyes
Cavefish are blind and, in most cases, no longer possess even vestigial eye structures. One possibility for this adaptation is that the cavefish expanded other sensory organs and needed both the surface area of the skin and the processing power of the brain for the sensory enhancements, leading to the disappearance of the eyes altogether.
Cavefish also lack pigment in the skin; in the light, they appear pink in color. Albinism could be fatal to surface-dwelling fish, leaving them without protection from ultraviolet light, but that poses no issue for cavefish. Meredith Protas, a graduate of Harvard University, studied albinism in cavefish and discovered it's caused by the same gene that produces albinism in humans.
They Can't Hear You
Conventional wisdom states that, when one sensory organ fails, the other senses are enhanced. In at least two species of cavefish, the opposite holds true. Daphne Soares, a neuroscientist at the University of Maryland, discovered cavefish only hear lower frequency noises, up to about 800 Hz. Ambient cave noise is normally around 1 kHz, a higher frequency than the cavefish can hear. This adaptation allows the cavefish to effectively filter out unnecessary background noises in his environment. Soares also found the cavefish had lower densities of hair cells in the hearing organs, which are necessary for audio reception.
The Lateral Lines
Lateral lines are nerves running along either side of an aquatic vertebrate's body. Similar in function to antennae in insects, lateral lines allow the fish to sense pressure changes and movement in the water. The lateral lines are enhanced by structures called neuromasts, specialized hair cells running along the lateral line to enhance sensory abilities. Cavefish rely heavily on vibration attraction behavior to sense the location of potential prey, and will move towards vibrations in the water. This behavior could lead surface-dwelling fish toward a predator and into possible danger, but with few or no cave-dwelling predators, this behavior is essential to the survival of the cavefish.
Putting Their Teeth Into It
The Astroblepus pholeter, a cave-dwelling catfish in Ecuador, possesses structures called denticles running along the dorsal midline of the body. Each denticle has a rod covered with enamel, and a nerve at the base surrounded by muscle tissue. This unique adaptation assists the fish with the detection of water flow and helps him survive in fast-flowing, turbulent waters. The denticles, which are normally associated with cutting, protection or drag reduction in surface-dwelling fish, help the A. pholeter grasp substrate and keep his nose pointed into the flow of water.
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