![]() ![]() Mozambique tilapia are motionless at the bottom at night, with a lower respiratory rate and no eye movement, and they do not respond as readily as during the day to electrical currents or food delivery. The typical sleep posture of the brown bullhead is with the fins stretched out, the tail lying flat on the bottom, the body inclined to one side at an angle of 10-30 degrees to the vertical, the cardiac and respiratory frequencies much slower than normal, and much less sensitivity to sound and to being touched. Based on these criteria, many fish species have been observed sleeping. The following four behavioural criteria are characteristic of sleep in birds and mammals and could be extended to fishes: (1) prolonged inactivity (2) typical resting posture, often in a typical shelter (3) alternation with activity in a 24-h cycle (4) high arousal thresholds. Instead of examining brain activity for sleep patterns, an alternate approach is to examine any rest/activity cycles that might indicate "behavioural sleep". On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care. For example, zebrafish, tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark) Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response. However, other fish do seem to sleep, especially when purely behavioral criteria are used to define sleep. There is also doubt about certain blind species that live in caves. Some species that always live in shoals or that swim continuously (because of a need for ram ventilation of the gills, for example) are suspected never to sleep. ![]() In birds and mammals, sleep is defined by eye closure and the presence of typical patterns of electrical activity in the brain, including the neocortex, but fish lack eyelids and a neocortex. Whether fish sleep or not is an open question, to the point of having inspired the title of several popular science books. Yet this oscar is behaviorally quiescent at night, lying unresponsive on the bottom with its eyes turned downward, and might be said to sleep. Sleep can be defined in birds and mammals by eye closure and typical electrical patterns in the neocortex, but fish lack eyelids and a neocortex.
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