Evolution of the fast start response in the cavefish Astyanax mexicanus

The extreme sensory environment of subterranean caves presents an adaptive challenge for troglobitic organisms. Although caves provide distinct selective pressures (e. g., darkness, food scarcity), advantages often come with living in isolated places. The cave-adapted (hypogean) form of the teleost Astyanax mexicanus lacks predators in its natural environment, unlike its ancestral surface (epigean) form that inhabits the nearby rivers. This species provides a unique opportunity for comparative studies because A. mexicanus has invaded the cave environment multiple times, providing us with independent comparative adaptive experiments. Here, we examine kinematic parameters that characterize the escape responses of various forms of A. mexicanus. We demonstrate that populations differ in details of fast start performance, but that surface fish do not perform better than cavefish in escape responses. Our results are more consistent with genetic drift or divergent selection among caves than with relaxation of selection in determining the evolution of escape behavior in this system.