Abstract
Studies of the evolution of cave animals mostly omit two important facts. Firstly, the focus is on eye regression and loss of melanin pigment, but the reduction of the many other traits, which have become functionless because of the absence of light as a transporter of information is often not considered. In Astyanax cave fish, these traits are various types of body colour, circadian rhythm, pineal organ, dorsal light reaction, schooling and visually released aggressive behaviour. Secondly, the conspicuous variability of regressive traits is rarely considered. Variability in eyes, pigmentation and aggressive behaviour develop at both the population and individual levels. It is enhanced by left–right asymmetry, because the eye rudiments of one specimen may diverge in size and show divergent structural differentiation. Asymmetry of the eye was described for brotulid, poeciliid and pimelodid cave fish (Wilkens, 2010), and also for cave isopods and mysids (Kosswig, 1940).
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