Lunar Cycles on the Reef
Life on the coral reef is not only synchronized to the 24-hour cycle of night and day and to the annual seasonal changes seen most markedly in places such as the Red Sea. The orbit of the moon is also enormously important to life in the sea. It is the moon, of course, that principally regulates the tides. As it waxes and wanes throughout the course of a lunar month, it also defines important parts of the life cycle of coral reef animals of all kinds.
How does the moon affect sea life?
In simple terms, ocean tides are caused by the gravitational pull of the moon and the sun. Twice each month, when there is a full moon and when there is a new moon, the alignment of the moon and the sun means that their gravitational pulls are combined and produce peak tidal ranges, or "spring" tides. As well as the increased water currents found during spring tides, a full moon means that the reef will be bathed in moonlight. There are several documented cases of diurnal fish, including butterflyfish and angelfish, continuing to forage busily in these conditions.
How do moon phases and tides influence the behavior of reef animals?
The biggest effect of the moon phase is on spawning behavior. Many coral reef animals show what is known as lunar or semi-lunar periodicity in their breeding efforts. In other words, their spawning peaks once every lunar cycle (usually at full moon) or twice every lunar cycle (at full and new moon). This periodicity allows them to synchronize their behavior with other members of the same species at ideal times for the survival of their eggs and young. As well as breeding, fish move around their habitat according to the tides. As tides fall, smaller animals move into slightly deeper water to avoid being trapped in rock pools, which can fluctuate widely in temperature and salinity. As they move, predators often wait for them, stationed along the route that their prey take as they escape. Those animals that do survive return to their shallow feeding grounds as the tide moves back in.
Why do so many reef species choose to spawn at full or new moon?
Many of the coral reef fishes that spawn in open water dart up from the reef into the water column to release their eggs on ebbing tides. The currents then carry the eggs away from the reef and from the huge numbers of egg-predators that live there. Once hatched, the larvae often use an incoming tide to ride in to a reef to settle. The same applies to the many invertebrates, such as coral, that use a similar tactic with the moon and tide phase. As well as using the tide to carry the spawn to safety, if many animals spawn at the same time, the huge number of eggs produced swamps the ability of predators to eat them all. This ensures that a good proportion survive to reach the comparative safety of waters away from the reef. Across the reef, the chemical signals of breeding animals of all species stimulate others to breed at the same time, enhancing this mass spawning effort.