Seasonal Changes on Coral Reefs
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At the equator there is very little variation in day length or temperature over the course of the year. As a result, the animals that live on coral reefs at very low latitudes show few regular annual patterns in their growth or behavior. But with each mile traveled north or south away from the equator, the influence of the seasons increases. Although warm weather and bright conditions are still the norm at the Tropic of Cancer and the Tropic of Capricorn, the days are noticeably longer and warmer in summer than in winter, with up to two-hour changes in day length and 18°F(10°C) temperature differences over the course of the year.These differences affect the coral reef animals at higher latitudes by promoting seasonal behavior.
Do coral reef fishes breed all year round or do they have breeding seasons?
Perhaps the majority of coral reef fishes spawn throughout the year, taking advantage of the consistently good conditions that are found on most reefs to produce as many offspring as they can. But even among those that do breed year round, there are often peaks of spawning activity, when the fishes synchronize their greatest effort with ideal conditions.
In the northern Red Sea, coral reef fishes experience perhaps the greatest seasonality. Here, fish make use of the warm waters of the summer months to breed. Temperature levels are extremely important for the development of both eggs and young; development and growth are faster in warmer waters. So the adoption of breeding seasons by tropical marine fishes is likely to be related both to this and to higher food availability. Also, the faster metabolism of the adults during the warmer months enables them to produce plenty of eggs rapidly, compared to other, leaner periods of the year.
If fish do have breeding cycles, when are these?
Across the globe, annual spawning peaks have been recorded in the Caribbean, the Red Sea, and the Indo-Pacific. As a rule, spawning activity peaks around spring, both in the Northern and Southern Hemispheres. Caribbean fish, for example, are most likely to breed between February, March, and April. It has been suggested that this is to coincide with annual peaks in plankton levels for the resulting larval fish to feed on, but this link remains unproven.
How do the fish know when these seasons are?
Even though they are not as pronounced in the tropics as elsewhere in the world, changes in day length keep the fishes activity coordinated. The onset of spring stimulates them and causes them to come into breeding condition. This works because the fish have a so-called "photoinductible" phase in their brain, making them highly sensitive to the tiny changes in daylight and day length that occur at this time of year. During spring, the days lengthen along with the slight day-on-day increases in temperature. These cues are backed up by the spawning behavior of many other animals on the reef. Even low levels of hormones in the water are thought to promote breeding activity in members of the same species or even in completely different ones.