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What is coral spawning?

Hard (or stony) corals reproduce by releasing their eggs and sperm all at the same time. This spawning cycle is one of nature’s most spectacular events.

Corals are a beautiful and important part of our ocean. But they can't move around the ocean floor. So, how, exactly, do they find mates? See the bizarre and beautiful phenomenon of coral spawning in this Ocean Today video. Transcript

Once a year, on cues from the lunar cycle and the water temperature, entire colonies of coral reefs simultaneously release their tiny eggs and sperm, called gametes, into the ocean. The phenomenon brings to mind an underwater blizzard with billions of colorful flakes cascading in white, yellow, red, and orange.

In ways that scientists still do not fully understand, mature corals release their gametes all at the same time. This synchrony is crucial, because the gametes of most coral species are viable for only a few hours. The “blizzard” makes it more likely that fertilization will occur.

The gametes, full of fatty substances called lipids, rise slowly to the ocean surface, where the process of fertilization begins.

When a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. Planulae float in the ocean, some for days and some for weeks, before dropping to the ocean floor. Then, depending on seafloor conditions, the planulae may attach to the substrate and grow into a new coral colony at the slow rate of about .4 inches a year.

Video Transcript

Corals are a beautiful - and important - part of our ocean. But they can’t move around the ocean floor - so, how, exactly, do they find mates?

Reef-building corals, also known as “stony” or “hard” corals, reproduce in several ways - one of the most common of which is broadcast spawning.

This bizarre and beautiful phenomenon starts when male and female corals release reproductive cells, called gametes, into the water.

Coral reefs may be separated by wide distances, so this tactic enables gametes to mix genetics and spread their offspring over a broad geographic area.

Male and female gametes combine together and form a baby coral, called a planula.

Planulae float in the water for days or weeks until they find a hard surface to which they can attach.

Along many reefs, coral spawning occurs as a synchronized event, when many coral species release their gametes around the same time.

With all the corals on a reef releasing gametes at the same time, there’s a better chance of escaping hungry predators and a greater possibility that many different genetic combinations may form.

More genetic diversity means there’s a better chance that at least some of the new corals have gene combinations that will help them survive extreme temperatures or diseases.

And with rising ocean temperatures and increasing acidification, it’s important for corals to be as resilient as possible.