Ecology: Biology of Interaction. 5.17. Adaptive Biorhythms
Influence of sunlight on many biological phenomena is mediated by daily changes in illumination intensity, changes in day length, and associated seasonal alternation. Dynamics of lunar gravitation and lunar phases also set periodic changes. These and other factors give rise to adaptive biological rhythms: daily, tidal, seasonal, multiyear, and others.
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5.16. Water Balance of Organisms
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction, Chapter 5. Autecology and Fundamentals of Environmental Science
5.18. Photoperiodism
5.17. Adaptive Biological Rhythms Biological rhythms are regular, cyclically repeating changes in the intensity and nature of biological processes and phenomena. Daily Rhythms Circadian rhythms (from Latin circa - around, dies - day) are biological rhythms with an approximate 24-hour period. Daily (circadian) rhythms are a phenomenon inherent in the vast majority of living organisms. They are caused by the Earth's rotation around its axis. They are set by external environmental conditions (primarily the alternation of light and darkness), although they persist as free-running rhythms even when isolated from the external environment. The duration of free-running rhythms usually differs slightly from 24 hours. Most organisms have an internal "clock" - a timekeeping mechanism. This clock allows the organism to anticipate changes in environmental conditions and regulate its activity accordingly. External conditions that set daily rhythms are called zeitgebers (from German Zeitgeber - "time giver"). The main zeitgeber for daily rhythms is the alternation of light and darkness; for lunar rhythms, it is the rhythm of lunar illumination or the corresponding tidal rhythm. Daily rhythms are an important adaptation to environmental conditions, allowing: - synchronization of the organism's activity with daily environmental changes; - preventing direct confrontation between different competitors (some are active during the day, others at night); - synchronization of the activity of different systems within the organism. Daily rhythms are characteristic of cells, tissues, organs, and whole organisms, but in complex animals, there is a "master clock" that synchronizes all local rhythms. In mammals, this master clock is the suprachiasmatic nucleus of the hypothalamus. Lunar Rhythms Lunar rhythms (circalunar, from Latin luna - moon) are associated with lunar illumination or the corresponding tidal rhythm (in the latter case, they are also called circatidal). Example: Nereis, a marine polychaete worm, reproduces during specific lunar phases. The Pacific grunion (Leuresthes tenuis) spawns on beaches during the highest (syzygy) tides; larvae must be ready to enter the open sea with the next high tide - two weeks later. Seasonal Rhythms Circannual (from Latin annus - year) rhythms are related to seasonal changes. Seasonal environmental cyclicity is the most important ecological factor for organisms living in extratropical latitudes. The main signal that sets seasonal rhythms is the change in day length (photoperiod). Circalunar rhythms are rhythms with an approximate lunar period. Seasonal rhythms of organism activity are related to photoperiodism (regulation of seasonal phenomena by day length - more on this in the next section). Physiological Mechanisms of Circadian Rhythms The molecular basis of the circadian clock is feedback loops involving special "clock" proteins. In Drosophila, the period and timeless genes are well-studied; their products form protein complexes that inhibit their own transcription. This cycle takes about 24 hours. Additional Materials: Educational Model: Circadian Rhythm (Internal Biological Clock)
5.16. Water Balance of Organisms
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction, Chapter 5. Autecology and Fundamentals of Environmental Science
5.18. Photoperiodism