As life choices go, it might seem extreme, but for blind Mexican cavefish, the choice to forgo sight was probably a no-brainer: eye running-costs are high in the dark, oxygen-poor cave waters where the fish make their homes. In addition, many truly sightless cavefish also make do with very little sleep, requiring as little as 5–20% of the sleep required by their surface-dwelling cousins. But Alex Keene, from Florida Atlantic University, USA, highlights that ‘little was known about the neural mechanisms underlying this dramatic behavioural shift’. Intrigued by the phenomenon, Keene and graduate student James Jaggard began investigating the neural mechanisms that have led cavefish populations to reduce their dependence on sleep.
Explaining that sleep is defined as occurring when animals are slow to respond to physical stimuli, Jaggard focused on the Pachón cavefish population – which have dramatically reduced their eyes – filming the fish for 24 h to determine how long they sleep. Categorizing fish as asleep when they ceased moving for 60 s or more, Jaggard noticed that the cave fish took fewer sleep bouts, only achieving a total of ∼1 h of sleep a day, compared with the ∼7 h taken in more frequent bouts by the sighted relatives above ground. But what mechanism has caused the cavefish to cut their sleep so dramatically?
As sensory systems – such as vision, hearing and smell – are known to play a significant role in regulating sleep, Jaggard and Keene decided to inactivate the fish's lateral line – which senses water flow and the presence of prey – to find out whether that system affected the fish's sleep pattern. Bathing the fish in the antibiotic gentamicin, which damages vibration-sensitive hair cells in the hearing systems of mammals, Keene recorded the fish's sleep patterns and was impressed to see that the cavefish now slept as much as their surface cousins. This suggests that enhanced sensory input underlies the evolutionarily derived sleep loss in Pachón cavefish, and the team adds, ‘these findings reveal a wake-promoting role for the lateral line’. However, when Jaggard tested the impact of gentamicin on other sleepless cavefish populations (Molino, Tinaja, Los Sabinos and Chica cavefish), none of them gained more sleep time, suggesting that each of the subterranean populations has independently evolved distinct mechanisms for regulating sleep.
Another question that intrigued Jaggard and Keene was why cavefish miss out on sleep. As sleep loss could extend the time available for foraging in their barren cave homes, the duo starved Pachón fish and discovered that the hungry animals increased the amount of sleep to ∼6 h day−1. So, the fish may be able to modulate the amount of sleep that they take depending on food availability to maximise foraging opportunities when food is available.
Finally, Jaggard tested which flow sensitive receptors on the surface of the Pachon fish are responsible for keeping the fish awake longer, by selectively coating the flow sensors on different portions of the body with adhesive. Recording how much the fish slept, Jaggard could see that the flow sensors on the fish's head and trunk were essential for regulating sleep, as the treated fish slept more. And when he compared the amount of time that the fish indulged in sleep between individuals – which can vary significantly – it was clear that the fish that slept the least had the greatest numbers of flow sensors.
Summing up, Keene says, ‘The evolution of enhanced sensory capabilities contributes to sleep loss in cavefish’, and he is eager to selectively disable the lateral line sensors in order to learn more about their contribution to sleep regulation in these extraordinary fish.
