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]
Instead of examining brain activity for sleep patterns, an alternate approach is to examine any rest/activity cycles that might indicate "behavioural sleep". 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. Based on these criteria, many fish species have been observed sleeping. [1] 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. [12] [13] 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. [14] At night, Spanish hogfish, bluehead wrasse, the wrasse Halichoeres bivittatus , the cunner Tautogolabrus adspersus , and even requiem sharks, can be picked up by hand without eliciting a response. [15] [16] [17] [18] A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep. [19]
Divers can easily see fishes settling down for the night in typical shelters, such as holes and crevices, underneath ledges, amidst vegetation, inside sponges, or buried in sand. [20] Some extra protection can be derived from special secretions, such as the mucous envelope produced by several species of wrasse and parrotfish, either around the fish themselves or at the opening of their shelter. These envelopes screen the sleeping fish from predators [21] and ectoparasites. [22]
In the laboratory, periods of inactivity often alternate with periods of activity on a 24-h basis, or a near 24-h basis when the lighting conditions are constant. Circadian rhythms of activity have been documented in over 40 different fish species, including hagfish, lamprey, sharks, cyprinids, ictalurids, gymnotids, salmonids, and labrids. [23] [24]
One physiological characteristic of sleep goes by the name of "homeostatic regulation". This is the notion that animals need a more or less constant amount of sleep every day, so that if a subject is deprived of sleep one day, the amount of sleep tends to "rebound" (increase) the next few days. This has been observed in zebrafish. At night, zebrafish appear to float in the water column, either horizontally or with the head slightly up. The frequency of mouth and gill movement is reduced by almost half and they are twice as hard to arouse as during the day. If they are deprived of this sleep-like behaviour, the sleep bouts thereafter are longer and the arousal threshold is higher than usual, suggesting a rebound effect. [25] [26] Similarly, in the convict cichlid, activity decreases on days that follow an experimental disruption of the fish's normal rest behaviour at night. [27]
Many pelagic fish species, such as bluefish, Atlantic mackerel, tuna, bonito, and some sharks, swim continuously and do not show signs, behavioural or otherwise, of sleep. [29] [4] It has been argued that one function of sleep is to allow the brain to consolidate into memory the things it has learned during the animal's normal period of activity. The brain might not be able to do this while still assailed by new stimuli and new information to process. Therefore, the role of sleep would be to periodically shut down sensory input to allow the brain to form memories. Pelagic species swim in an open-water environment wherein novel stimuli is uncommon. In such species, the sensory input is so low that memory formation could take place even if the fish keeps on moving (a repetitive activity) and does not fall asleep in the traditional sense of the word. [30]
Diurnal damselfish normally sleep motionless in crevices within coral reefs at night, but three species (the green chromis, the marginate dascyllus and the whitetail dascyllus) spend the night between coral branches where they beat their fins at a rate about twice that of normal daytime swimming. This creates water currents that keep the inner zone of the coral (and thus the fish themselves) well oxygenated, at levels about four times higher than in the absence of the fish. Though the fish are active (mostly in a repetitive way), they do not respond to light or to the presence of potential predators. The researchers who documented this behaviour called it "sleep-swimming". [31]
Sleep could also be absent during specific parts of a fish's life. Species normally quiescent at night become active day and night during the spawning season. [1] Many parental species forego sleep at night and fan their eggs day and night for many days in a row. This has been observed in threespine stickleback, [32] convict cichlid and rainbow cichlid, [33] [34] various species of damselfish, [35] [36] smallmouth bass and largemouth bass, [37] [38] and the brown bullhead. [39] Some diurnal species, like the tautog Tautoga onitis, become active day and night during migration. [40] In the Mozambique tilapia, sleep has been observed in adults, but not in juveniles. [41]
Some species may be variable in the phasing of their daily activity/inactivity periods, and thus presumably of their sleep. [42] Within the same laboratory populations of goldfish, some individuals may be spontaneously diurnal while others are nocturnal. [43] Goldfish can also be diurnal if food is more available by day, or nocturnal if food is available at night. [44] Salmon are mostly diurnal when temperature is high, but become more nocturnal if temperature plummets. [45] At high latitudes, captive burbot, sculpin and brown bullhead are nocturnal in summer but become diurnal under the short photoperiod of the Arctic winter. [46] [47] In captivity, white sucker Catostomus commersonii are diurnal when living in a shoal but nocturnal when living alone. [48]
A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism and responds to the environment. Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximize the fitness of an individual. Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek χρόνος, and biology, which pertains to the study, or science, of life. The related terms chronomics and chronome have been used in some cases to describe either the molecular mechanisms involved in chronobiological phenomena or the more quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is required.
Biological rhythms are repetitive biological processes. Some types of biological rhythms have been described as biological clocks. They can range in frequency from microseconds to less than one repetitive event per decade. Biological rhythms are studied by chronobiology. In the biochemical context biological rhythms are called biochemical oscillations.
The convict cichlid is a fish species from the family Cichlidae, native to Central America, also known as the zebra cichlid. Convict cichlids are popular aquarium fish and have also been the subject of numerous studies on fish behaviour.
Cathemerality, sometimes called "metaturnality", is an organismal activity pattern of irregular intervals during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are undertaken. This activity differs from the generally monophasic pattern of nocturnal and diurnal species as it is polyphasic and is approximately evenly distributed throughout the 24-hour cycle.
Diurnality is a form of plant and animal behavior characterized by activity during daytime, with a period of sleeping or other inactivity at night. The common adjective used for daytime activity is "diurnal". The timing of activity by an animal depends on a variety of environmental factors such as the temperature, the ability to gather food by sight, the risk of predation, and the time of year. Diurnality is a cycle of activity within a 24-hour period; cyclic activities called circadian rhythms are endogenous cycles not dependent on external cues or environmental factors except for a zeitgeber. Animals active during twilight are crepuscular, those active during the night are nocturnal and animals active at sporadic times during both night and day are cathemeral.
The lake chub is a freshwater cyprinid fish found in Canada and in parts of the United States. Of all North American minnows, it is the one with the northernmost distribution. Its genus, Couesius is considered monotypic today. The genus was named after Elliott Coues, who collected the holotype specimen.
The golden shiner is a cyprinid fish native to eastern North America. It is the sole member of its genus. Much used as a bait fish, it is probably the most widely pond-cultured fish in the United States. It can be found in Quebec, and its French name is "Mené jaune" or "Chatte de l'Est".
Herotilapia multispinosa also known as the rainbow cichlid is a Central American freshwater fish of the cichlid family. It is found on the Atlantic slope of Honduras, Nicaragua, and Costa Rica from Patuca River (Honduras) south to Matina River, and on the Pacific slope of Nicaragua and Costa Rica from Guasaule River south to Tempisque River. Specimens are also reported from the Choluteca River on the Pacific side of Honduras. This species is found in lakes and swamps with muddy bottoms, where it uses its specialized teeth and only 3.5% jaw protrusion to feed mostly on algae. It is commercially important as an aquarium fish. The rainbow cichlid prefers a pH range of 7.0–8.0, water hardness of 9-20 dGH and a temperature range of 21–36 °C.
In the study of chronobiology, entrainment refers to the synchronization of a biological clock to an environmental cycle. An example is the interaction between circadian rhythms and environmental cues, such as light and temperature. Entrainment helps organisms adapt their bodily processes with the timing of a changing environment. For example, entrainment is manifested during travel between time zones, hence why humans experience jet lag.
A chronotype is the behavioral manifestation of underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. Eveningness and morningness are the two extremes with most individuals having some flexibility in the timing of their sleep period. However, across development there are changes in the propensity of the sleep period with pre-pubescent children preferring an advanced sleep period, adolescents preferring a delayed sleep period and many elderly preferring an advanced sleep period.
Sleep appears to be a biological requirement for all animals except for basal species with no brain or only a rudimentary brain. It has been observed in mammals, birds, reptiles, amphibians, fish, and, in some form, in insects. The internal circadian clock promotes sleep at night for diurnal organisms and in the day for nocturnal organisms. Sleep patterns vary widely among species, with some foregoing sleep for extended periods and some engaging in unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake.
Jürgen Walther Ludwig Aschoff was a German physician, biologist and behavioral physiologist. Together with Erwin Bünning and Colin Pittendrigh, he is considered to be a co-founder of the field of chronobiology.
Fish fulfill several criteria proposed as indicating that non-human animals experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements.
Fish intelligence is "the resultant of the process of acquiring, storing in memory, retrieving, combining, comparing, and using in new contexts information and conceptual skills" as it applies to fish. Due to a common perception amongst researchers that Teleost fish are "primitive" compared to mammals and birds, there has been much less research into fish cognition than into those types of animals, and much remains unknown about fish cognition, though evidence of complex navigational skills such as cognitive maps is increasing.
Serge Daan was a Dutch scientist, known for his significant contributions to the field of Chronobiology.
Fish are exposed to large oxygen fluctuations in their aquatic environment since the inherent properties of water can result in marked spatial and temporal differences in the concentration of oxygen. Fish respond to hypoxia with varied behavioral, physiological, and cellular responses to maintain homeostasis and organism function in an oxygen-depleted environment. The biggest challenge fish face when exposed to low oxygen conditions is maintaining metabolic energy balance, as 95% of the oxygen consumed by fish is used for ATP production releasing the chemical energy of nutrients through the mitochondrial electron transport chain. Therefore, hypoxia survival requires a coordinated response to secure more oxygen from the depleted environment and counteract the metabolic consequences of decreased ATP production at the mitochondria.
Time-place learning (TPL) is the process by which animals link events with both the location and time of occurrence. It enables them to decide which locations to visit or to avoid based on previous experience and knowledge of the current time of day. TPL presumably allows animals to maximize their chances of finding resources and avoiding predators, increasing survival chances. TPL requires spatial memory and a sense of time. The latter may be based on external time-cues (Zeitgebers), or internally generated circadian rhythms. TPL may fundamentally underlie episodic memory.
Nicholas Mrosovsky was a Canadian zoologist known for his research in the fields of homeostasis, chronobiology, and sea turtle biology. He spent his whole professional career at the University of Toronto. His laboratory was notable for its seminal investigations of the influence of behavioural arousal on circadian rhythms. He was also the founder, in 1976, of Marine Turtle Newsletter. He received a Guggenheim Fellowship in 1973, and in 1993 he was elected a Fellow of the Royal Society of Canada.