How Do Corals Reproduce? Mass Spawning Explained

Asexual reproduction: budding and fragmentation

A coral colony starts with a single polyp, a tiny animal with a mouth and a ring of tentacles, as explained in is coral a plant or an animal? That first polyp grows by budding. It divides in two, or a new polyp sprouts from its side or base, and the process repeats thousands of times. Every polyp in a colony is a genetic copy of the first one.

Corals can also create entirely new colonies without sex. In fragmentation, a piece of coral breaks off, usually in a storm, and if it lands on a hard surface in good conditions, it can attach and keep growing. Branching corals such as the staghorn coral of Florida and the Caribbean spread mainly this way, and whole thickets can be made of clones of a single colony.

Fragmentation is fast and reliable, but it has a drawback: clones are genetically identical, so a disease or heatwave that kills one can kill them all.

Sexual reproduction: spawners and brooders

Sexual reproduction mixes genes and spreads young corals to new reefs. Some coral species are hermaphrodites, with each polyp making both eggs and sperm; in others, each colony is either male or female.

Corals release their young in two main ways:

• Broadcast spawning. The colony releases eggs and sperm into the water, where fertilization takes place. This is the most common method among reef-building corals; roughly three-quarters of them are thought to be broadcast spawners. In many hermaphrodite species, each polyp packs its eggs and sperm into a tiny, buoyant bundle that floats to the surface and breaks apart.
• Brooding. Eggs stay inside the polyp, where they are fertilized by sperm drifting in from other colonies. The larvae develop inside the parent and are released ready to settle, often over several months of the year. In the Caribbean, mustard hill coral and golfball coral are well-known brooders.

What is coral mass spawning?

On many reefs, colonies of dozens of different species release their eggs and sperm on the same few nights of the year, often within the same few hours. This is mass spawning. Each colony may release its gametes in a window of only 10–30 minutes, and the water can look like an upside-down snowstorm of pink, white and orange bundles.

Mass spawning on the Great Barrier Reef was first described by scientists from James Cook University in Australia, who published their findings in 1984. It happens a few nights after a full moon, usually between October and December. In the Florida Keys and much of the Caribbean, most corals spawn a few nights after the full moon in August or September. At Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico, the main spawning comes seven to 10 days after the August full moon.

Spawning together has clear benefits. It greatly increases the chance that eggs and sperm from different colonies meet, and it releases more eggs than fish and other predators can eat.

What triggers coral spawning?

Corals have no eyes or brain, so they rely on environmental signals to synchronize:

• Water temperature sets the season; spawning follows the warming of the sea in spring and summer.
• The moon sets the night; corals contain light-sensitive proteins that detect moonlight, and the dark period between sunset and the later rising of the moon a few nights after the full moon seems to be the final trigger.
• Sunset sets the hour; most species spawn a fixed time after dark.

Chemical signals in the water may also help neighbors coordinate. Studies suggest that artificial light from coastal towns can disrupt this timing in some species.

From larva to new colony

Once fertilized, an egg develops within a day into a planula, a tiny, pear-shaped larva covered in hair-like cilia that let it swim. Planulae drift with the currents for a few days to a few weeks, and some survive for two months or longer. When ready, a larva swims down to look for a place to settle, guided by chemical cues from pink crusts of coralline algae and films of bacteria on clean, hard surfaces.

After settling, the larva turns into a single polyp and starts building a limestone cup. In most broadcast-spawning corals, it must pick up its partner algae from the surrounding water. The odds are tiny: most eggs and larvae are eaten, swept away or settle in the wrong place, and many young corals are smothered by seaweed or sediment. Colonies take years to reach breeding size; staghorn coral, for example, matures at about 17 cm (7 in) tall.

Coral reproduction and reef restoration

Understanding coral reproduction is now central to saving reefs. Restoration teams use fragmentation on purpose, growing small pieces of coral in underwater nurseries and planting them out on damaged reefs. Some cut corals into tiny “microfragments”, which grow quickly and fuse together.

Other teams collect spawn on mass spawning nights, rear the larvae in tanks and settle them onto tiles before placing them on the reef, which adds the genetic variety that clones lack. Aquariums such as the Florida Aquarium have also bred several Caribbean species in tanks, including elkhorn coral in 2022, by recreating natural temperatures, sunset times and moonlight cycles. These efforts are key in places such as Florida, where heat and disease have left wild corals too few and far apart to breed well, as described in what is happening to Florida’s coral reefs?

Heat is a threat to reproduction too. Corals stressed by bleaching often produce fewer eggs or skip spawning altogether, as explained in what is coral bleaching?