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. Yet this oscar is behaviorally quiescent at night, lying unresponsive on the bottom with its eyes turned downward, and might be said to sleep.[1]
Whether fish sleep or not is an open question, to the point of having inspired the title of several popular science books.[2][3] 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. 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.[4] There is also doubt about certain blind species that live in caves.[5]
However, other fish do seem to sleep, especially when purely behavioral criteria are used to define sleep. For example, zebrafish,[6] tilapia,[7] tench,[8] brown bullhead,[9] and swell shark[10] 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. On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care.[11]
^Reebs, S.G. (2008-2014) Sleep in fishes. Retrieved 24 July 2014.
^Feldman, D. (1989) When do fish sleep? And other imponderables of everyday life. Harper and Row, New York.
^Weis, J.S. (2011) Do fish sleep? Fascinating answers to questions about fishes. Rutgers University Press, New Brunswick.
^Kavanau JL (July 1998). "Vertebrates that never sleep: implications for sleep's basic function". Brain Res. Bull. 46 (4): 269–79. doi:10.1016/S0361-9230(98)00018-5. PMID 9671258. S2CID 6626805.
^Parzefall, J. (1993): Behavioural ecology of cave-dwelling fish; pp. 573–606 in: Pitcher, T.J.(ed.), The Behaviour of Teleost Fish; London: Chapman&Hall.
^Zhdanova, I.V., Wang, S.Y., Leclair, O.U., and Danilova, N.P. (2001) Melatonin promotes sleep-like state in zebrafish, Brain Research 903: 263–268. Yokogawa T, Marin W, Faraco J, Pézeron G, Appelbaum L, et al. (2007) Characterization of Sleep in Zebrafish and Insomnia in Hypocretin Receptor Mutants, PLOS Biology Vol. 5, No. 10, e277 doi:10.1371/journal.pbio.0050277 and criticism and rebuttal, at PLoS Biology
^Shapiro, C.M., and Hepburn, H.R. (1976) Sleep in a schooling fish, Tilapia mossambica, Physiology and Behavior 16:613–615
^Peyrethon, J., and Dusan-Peyrethon, D. (1967) Étude polygraphique du cycle veille-sommeil d'un téléostéen (Tinca tinca), Compte-Rendus de la Société de Biologie 161: 2533-2537
^Titkov, E.S. (1976) Characteristics of the daily periodicity of wakefulness and rest in the brown bullhead (Ictalurus nebulosus), Journal of Evolutionary Biochemistry and Physiology 12:305–309.
^Nelson, D.R., and Johnson, R.H. (1970) Diel activity rhythms in the nocturnal, bottom-dwelling sharks Heterodontus francisci and Cephaloscyllium ventriosum, Copeia 1970: 732–739.
^Reebs, S.G. (2002) Plasticity of diel and circadian activity rhythms in fish, Reviews in Fish Biology and Fisheries 12: 349–371.
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