A novel fish behavior: “Floating” of Paramisgurnus dabryanus loaches

The interplay between sleep and ecophysiology, behaviour and responses to environmental change in fish

Evidence of behavioural sleep has been observed in every animal species studied to date, but current knowledge of the behaviour, neurophysiology and ecophysiology associated with sleep is concentrated on mammals and birds. Fish are a hugely diverse group that can offer novel insights into a variety of sleep-related behaviours across environments, but the ecophysiological relevance of sleep in fish has been largely overlooked. Here, we systematically reviewed the literature to assess the current breadth of knowledge on fish sleep, and surveyed the diverse physiological effects and behaviours associated with sleep. We also discuss possible ways in which unstudied external factors may alter sleep behaviours. For example, predation risk may alter sleep patterns, as has been shown in mammalian, avian and reptilian species. Other environmental factors - such as water temperature and oxygen availability - have the potential to alter sleep patterns in fish differently than for terrestrial endotherms. Understanding the ecological influences on sleep in fish is vital, as sleep deprivation has the potential to affect waking behaviour and fitness owing to cognitive and physiological impairments, possibly affecting ecological phenomena and sensitivity to environmental stressors in ways that have not been considered.

A vertebrate family without a functional Hypocretin/Orexin arousal system

The Hypocretin/Orexin signaling pathway suppresses sleep and promotes arousal, whereas the loss of Hypocretin/Orexin results in narcolepsy, including the involuntary loss of muscle tone (cataplexy).1 Here, we show that the South Asian fish species Chromobotia macracanthus exhibits a sleep-like state during which individuals stop swimming and rest on their side. Strikingly, we discovered that the Hypocretin/Orexin system is pseudogenized in C. macracanthus, but in contrast to Hypocretin-deficient mammals, C. macracanthus does not suffer from sudden behavioral arrests. Similarly, zebrafish mutations in hypocretin/orexin show no evident signs of cataplectic-like episodes. Notably, four additional species in the Botiidae family also lack a functional Hypocretin/Orexin system. These findings identify the first vertebrate family that does not rely on a functional Hypocretin/Orexin system for the regulation of sleep and arousal.