Coral polyps, the tiny animals that build vast reef structures, use two distinct reproductive strategies to maintain their colonies and establish new reefs. Corals are marine invertebrates belonging to the phylum Cnidaria, closely related to sea anemones and jellyfish. The individual coral is called a polyp, a sac-like animal only millimeters in size, anchored within a calcium carbonate skeleton. Reproduction is necessary for these sessile organisms, allowing existing colonies to grow and repair damage, and facilitating the dispersal of genetic material to colonize new reef ecosystems.
Asexual Methods of Colony Growth
Corals primarily use asexual reproduction to increase the size of an existing colony, creating genetically identical copies. This process ensures the local maintenance and expansion of a successful individual. The most common method of colony expansion is budding, where a new polyp forms without the exchange of gametes.
Budding occurs in two main ways. Intratentacular budding is when a parent polyp divides down the middle of its oral disk to create two new polyps. Extratentacular budding is when a new polyp sprouts from the tissue between two existing polyps. Both processes result in a larger, single colony composed of physically connected clones. Fragmentation is a natural mechanism for establishing entirely new colonies. This occurs when a piece of the parent colony, often broken by a disturbance like a storm, settles on a new substrate and begins growing separately.
Sexual Reproduction: Broadcast Spawning and Brooding
Sexual reproduction allows corals to mix genetic material, introducing diversity and facilitating long-distance dispersal. Corals accomplish this through two different methods: broadcast spawning and brooding. Approximately 75% of stony corals are broadcast spawners, releasing their gametes—eggs and sperm—into the water column for external fertilization.
Broadcast Spawning
Broadcast spawning relies on synchronization to ensure fertilization success, as corals cannot move to find a mate. This is often a spectacular, synchronized event known as mass spawning, occurring annually or semi-annually, typically a few nights after a full moon. Environmental cues, including rising water temperature, day length, and the lunar cycle, trigger the simultaneous release of gamete bundles. This synchronized timing maximizes the chance that eggs and sperm from different colonies will meet and successfully fertilize.
Brooding
Brooding is a contrasting strategy where fertilization is internal, and eggs are retained within the female polyp. Male corals release sperm into the water, which female polyps take in to fertilize the eggs inside the gastrovascular cavity. Because development occurs internally, brooding species release fewer larvae, but these larvae are often larger and more developed. Brooders tend to release their larvae over several months, often around the new moon, making them less reliant on a single, synchronized mass spawning event.
The Life Cycle: From Larva to New Reef
The product of successful sexual fertilization is the planula larva, the motile, free-swimming stage of the coral life cycle. Planulae are small, ciliated organisms that function as the primary agent of dispersal. For broadcast spawners, planulae spend days to weeks developing as they are carried by ocean currents, traveling great distances from the parent reef.
The planula’s journey ends when it is competent to settle, meaning it is capable of transforming into a sessile polyp. To ensure survival, the planula must find a suitable, hard substrate, guided by specific chemical cues. Larvae are attracted to biofilms and especially to crustose coralline algae (CCA).
Upon selecting a site, the planula attaches to the substrate and undergoes metamorphosis, transitioning from a mobile larva into a single, stationary founder polyp. This initial polyp begins to secrete a calcium carbonate skeleton, establishing the base of the new colony. The founder polyp then starts to divide asexually through budding, rapidly adding new, genetically identical polyps to grow into a large, reef-building colony.