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A circadian rhythm is a pattern of activity governed by an internal clock with a period of roughly 24 h. The actual onset of activity may be shifted each day (the clock may be “reset”) by some external stimulus or Zeitgeber (German for “time giver”) such as sunrise. The need for an external resetting mechanism becomes obvious when one realizes how much day length changes during different seasons. Tides and feeding events can also serve as Zeitgebers. Activity rhythms in many teleosts can become established (entrained) if a meal is provided at a fixed time each day. Fish then develop an activity rhythm that anticipates the time of feeding, even in the absence of food and in constant light (Spieler 1992). In the absence of a Zeitgeber, such as during experimental conditions of constant light or darkness, rhythms are often maintained at slightly more or less than 24 h and are referred to as free running.
Free-running rhythms, involving either diurnal activity and nocturnal inactivity or the converse, have been demonstrated in a number of fishes, including hagfishes, swell sharks, anguillid eels, minnows including Goldfish, salmonids, suckers, South American knifefishes, burbots (Gadidae), killifishes, moronid temperate basses, and wrasses (Boujard & Leatherland 1992; Reebs 1992, 2002; Gerkema et al. 2000). Many fishes that show such patterns also exhibit considerable inter- and intraindividual variation in the rhythms (Reebs 2002).
Normally distinct activity cycles can be disrupted by experimental additions of predators or by the removal of resting structure. Distinct cycles also often break down during the breeding season and when fish migrate. Many strongly diurnal reef fish species spawn late into evening twilight (Sancho et al. 2000b), and normally diurnal minnows, Yellow Perch, and gobies spawn at night. The adaptive function of such breakdowns in periodicity is not understood. More obvious is the adaptiveness of a loss of activity rhythms in species that demonstrate parental care. Eggs and larvae must be guarded and fanned throughout the diel cycle, not just when the parents are normally active. Studies of several species, including catfishes, sticklebacks, centrarchid sunfishes, cichlids, and damselfishes indicate that parental care is also provided during the time period when adults would normally be inactive (Reebs 1992, 2001).
Circadian rhythms control many other aspects of fish behavior, morphology, and physiology. Many functions are under neuroendocrine control. The pineal organ on the dorsal surface of the brain secretes the hormone melatonin, which has a direct effect on the seasonal control of reproduction, sexual maturation, development, and growth, as well as shorter term effects on coloration, locomotor activity, and social behavior (see The endocrine system). Melatonin is secreted on a circadian basis, maximally at night and minimally during the day. This rhythm, which is entrained by light and temperature detected by the pineal, is maintained even in cultured pineal tissue removed from a fish (Zachman et al. 1992). Photoreceptors sensitive to changing light and involved in circadian regulation also occur in the parapineal organ, parietal eyes, and deep brain (Foster et al. 2006). Secretion of hormones, such as prolactin, estradiol, progesterone, cortisol, testosterone,thyroxine, and triiodothyronine also follow endogenous (internally generated) circadian, semilunar, or lunar periodicities that are in turn affected by day length, temperature, and other hormone concentrations. Changing the light or temperature regime, or injecting a fish with hormones or hormone precursors, will cause changes in swimming activity and rest, temperature and salinity selection, reproduction, fat deposition, weight gain, and other aspects of growth. Hence the light–dark cycle can affect the timing of a neural pacemaker or clock, which in turn determines the timing of neural and hormonal cycles, which then entrain cellular rhythms in tissues, all governing the activity and behavior of the fish (Meier 1992).
The physical location of the clock (or clocks) in fishes remains a mystery. In mammals, a region in the brain, specifically the suprachiasmatic nucleus of the hypothalamus, serves as an endogenous oscillator (master clock). No direct analog of the suprachiasmatic nucleus has been found in fishes, although the hypothalamus has neural connections to light-receiving structures and other features that make it a candidate region for such a role, and the pineal has also been implicated in the control of many circadian rhythms in fishes (Boujard & Leatherland 1992; Holmqvist et al. 1992).
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