SCIENTIFIC NAME Astyanax mexicanus

TAXONOMY PHYLUM: Chordata CLASS: Actinopterygii ORDER: Characiformes FAMILY: Characidae

Physical description

Credit: Kim Caesar/Nature Publishing Group

The Mexican tetra is a small fish (growing to a maximum overall length of 12 cm) widely distributed in northeastern Mexico and south Texas. Females tend to be larger and fuller-bodied than males and have a straight rather than slightly curved edge to their tail fins. The species consists of at least 29 different cave populations in addition to its surface-dwelling form, all of which are interfertile1. The surface-dwelling fish is silvery in color with a black band that extends along each side to the tail. The tail and lower fins of some individuals may be tinted yellow or red. In contrast, fish from blind cave populations have no eyes and are devoid of pigmentation, with pinkish-white-colored bodies. Their lack of eyes also causes some craniofacial differences between cave fish and surface fish2.

Research résumé

A. mexicanus are particularly well-suited to studies of evolutionary development because the surface- and cave-dwelling populations can be compared side-by-side to examine how regulatory genes mediate the evolution of development1. More than once over the past 2–3 million years3, surface-dwelling A. mexicanus have invaded caves, giving rise to multiple cave-adapted lineages1,4. In this way, each cave fish lineage represents a replicate experiment, providing a unique opportunity to study the developmental mechanisms underlying parallelism and convergence4.

Several different cave fish populations have independently evolved a cluster of specific adaptations in response to the perpetual darkness of their habitat. Some of these changes are constructive, such as an enhanced feeding apparatus, including larger jaws and a greater number of taste buds and teeth, and a system of mechanoreceptors called neuromasts that are larger and greater in number and have longer cupulae than those of surface fish1,5. Cave fish also gained adaptive behaviors including attraction to vibrations, increased exploratory and food searching behavior, increased olfactory capabilities and a special feeding posture6. These enhancements likely developed in compensation for their blindness, which is the result of eye degeneration during development. Cave fish embryos form small primordial eyes, but 40 h after fertilization, the lens enters apoptosis, and cell death spreads to the neural retina, triggering the progressive degeneration of the entire eye7.

Other regressive changes in cave fish include loss of pigmentation, which results from a loss of melanophores and a reduced capacity to synthesize melanin1,2, and loss of sleep, schooling and aggressive behaviors3,6. A recent study showed that oscillation of clock genes in cave forms of A. mexicanus are substantially altered8, and cave forms do not exhibit a circadian rhythm in metabolism unlike surface forms4.

Several analyses have been done to determine the genetic underpinnings of trait differences between surface- and cave-dwelling forms of A. mexicanus, and especially of eye loss1,2,5,7, with results indicating that multiple genetic modifications have contributed to an overall defect in lens biology. A genome3 and a developmental transcriptome9 of A. mexicanus have recently been reported, which will aid future research in identifying candidate genes underlying various behavioral and morphological trait differences.