Daily Rhythms in a Timeless Environment: Circadian Clocks in Astyanax mexicanus

The prevalence of circadian clocks in most organisms on the planet implies they are an adaptive trait; however, only a few studies provide evidence to support this. Animals living in aperiodic environments, such as caves, provide a unique opportunity to address this issue by removing what is thought to be the primary environmental pressure for circadian clocks—the environmental light-dark cycle. Astyanax mexicanus is at the forefront of our understanding of circadian clocks in constant darkness, thanks to studies detailing the extent of the function in cavefish of the core molecular circadian clock and outputs, such as activity and metabolic rhythms. Here we review these studies in detail and put them into context with the wider understanding of clocks in cave animals, to determine the role of circadian clocks in timeless environments.