Rhythms of Marine Life

Fish have three main aims over the course of their lives: to eat, to avoid being eaten, and to reproduce. The way they approach these essential functions varies from species to species, but in the end, it is governed by rhythms.

Rhythms might be the short-term rhythm of the Earth rotating on its axis to produce the day-night cycle, or the annual cycle of the Earth orbiting the sun. As well as daily and annual rhythms, the different phases of the moon exert a powerful influence on the tides of the sea, which in turn affect all life on the coral reef. These different rhythms predict when fish are most active, when they eat, sleep, migrate, and when they breed.

Can fish tell the time?

The behavior of all animals on the coral reef is governed by an internal biological clock. This internal clock is set according to external stimuli known as "zeitgebers," or "time givers," such as the rising and setting of the sun each day or the tidal cycle. Most coral reef animals live in the tropics, where there is little annual fluctuation in day length or temperature. Yet even in these cases, monsoon seasons can cause storms to which the fish may respond by seeking deeper waters.The biological clock starts to develop early in life. In the case of fish, it is usually almost fully developed by the time they hatch from the egg.

How accurate are these internal clocks?

If an animal is taken from the wild and kept for a time in a constant environment, say, in constant light or at a constant temperature, its rhythmic behavior will gradually decrease in the absence of the vital zeitgebers to update their biological clocks. However; the internal clock is quite robust; depending on the species, it can take anything up to six weeks for a fish to lose its connection to the rhythms of the natural world completely. Therefore, to see natural behavior from fish in the home aquarium, it is necessary to replicate natural day-night fluctuations and any annual temperature cycles that the fish would experience in the wild.

What other rhythms do fish respond to?

As well as these major influences on the rhythm of their lives, fish respond to other cycles, including the need to forage when hungry and to synchronize their activities with those of their prey and the activities of rivals or mates from within their own species. While the environment imposes its own rhythms on fish, the animals themselves also undergo a life cycle from birth to death as they hatch, grow, reproduce, and die. In species with a distinct spawning season, the population is structured into year groups of different ages and
sizes. In those that spawn all year round, the cycles are less clear and the community will consist of fish of all ages and sizes.

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