Cavefish

Abstract

What are cavefish? Like other cave-dwelling animals, fish that live in caves have often lost their eyes and body colour, but the evolutionary mechanisms underlying these reductions are unknown. There are 86 species of cavefish that belong to 18 different families, and many of them have lost their eyes independently. Comparing cavefish to their free-living ancestors can thus help us understand convergent evolution. However, the ancestors of most cave species are already extinct, with the notable exception of the Mexican Blind Cavefish. What is the Mexican Blind Cavefish? In 1819, the French anatomist Georges Cuvier described the Mexican tetra (Astyanax fasciatus mexicanus). Astyanax belongs to the teleost order Characiformes, which also includes the piranha, the neon tetra or the pencil fish. What is so special about them?Astyanax comprises both eyed, surface-dwelling and eyeless, cave-dwelling populations. Each cavefish population shows a different degree of eye and pigment degeneration. Recent data indicate that some cavefish populations independently evolved from eyed surface fish at different times. Why are they blind? One hypothesis is that losing the eyes could have selective advantages for living in a cave. Darwin thought that reduced eye size and covering the eye with a flap of skin could be advantageous by preventing inflammation and diseases. Others have proposed that eye loss is economical, because the visual system consumes a lot of energy. An alternative explanation suggests that mutations have accumulated in genes required for eye formation, because selection on the visual system is relaxed in darkness. There is also the possibility of an evolutionary trade-off, such that functional eyes were sacrificed to allow the development of more ‘useful’ characters, such as more taste buds and an enhanced lateral line system. What about the other senses? Cavefish have evolved enhanced chemo- and mechanosensory systems. Extra taste buds help them find food more quickly in complete darkness. Their enhanced mechanosensory lateral line system helps them to perceive their environment. These enhanced senses also might have aided the fish in maintaining their complex spawning dance in darkness. How did they lose their eyes? The signaling molecule sonic hedgehog (Shh) is involved in eye, jaw, and taste bud development in the Mexican cavefish. During embryonic development, Shh expression is expanded in the cavefish compared to surface fish, causing eye degeneration and an increase in the number of taste buds. This pleiotropic effect supports the trade-off theory. How did they lose their color? It has been proposed that the loss of dark pigment cells is a consequence of the accumulation of neutral mutations under relaxed selection. A recent study has shown that independently evolved Astyanax cave populations have retained pigment cell precursors but cannot synthesize the pigment, possibly due to damage of the same pigment-synthesizing pathway. Do cavefish sleep at night? The sleeping behavior of cavefish is less obvious than that of their surface relatives, but in the lab they are less active at night. Their pineal organ is functional and can detect light. Although cavefish seem to have a circadian rhythm in the lab, it is not clear if they have a daily rhythm in a dark cave. Is there a cavefish genome project? Not yet, but comparing the genomes of cavefish and surface fish might reveal the causes and consequences of microevolutionary change at the genetic level as well as mechanisms of convergent evolution. A genome project could also help understanding macroevolution. While Astyanax has a ‘generalized’ body shape, its relatives, e.g. piranhas, have evolved a variety of unique morphological characters.