Cavefishes.

Life in caves means life in perpetual darkness. This has two dramatic effects on animals: it eliminates the need to see and reduces the availability of food as there is no local photosynthesis. Food availability for cave dwellers is often seasonal, episodic or unscheduled. Like other cave animals, fish species adapted for cave life exhibit a suite of sensory, morphological, physiological and behavioral traits that are shared among species from phylogenetically distant families. Most cave fishes are entirely or partially blind. The most extreme of them, the obligatory cave-dwelling fish that spend their entire life in caves, are the subject of this primer. At present, over 200 such cavefish species have been described, and all of them have evolved independently from surface ancestors. Thus, each cavefish species is a replicate of the same natural experiment, testing the evolutionary response of a sighted surface fish to the absence of light and the limitations on food in a subterranean environment. The evolutionary responses converge on loss of eyes and pigmentation and the augmentation of other senses, such as taste, smell or mechanosensation, as well as a more efficient metabolism, changes in feeding behavior, altered activity levels, loss of circadian rhythmicity and increased wakefulness. However, not all of these troglomorphic traits are present in every cavefish species.