Often mistaken for rocks or plants, corals are animals, and the reefs they build are created by colonies of tiny creatures called polyps. Related to sea anemones and jellyfish, their reproduction is the foundation of the coral reef ecosystem. The successful reproduction of polyps allows reefs to grow, recover from damage, and sustain the biodiversity that relies on them. Understanding how corals create new life is fundamental to appreciating these habitats.
Asexual Reproduction in Corals
The growth of a coral colony is a direct result of asexual reproduction, a process that creates genetically identical copies. The most common method is budding, where a parent polyp divides and a new polyp “buds” off from it. This can happen in two ways: intra-tentacular budding involves a polyp splitting to form two same-sized individuals, while extra-tentacular budding has a new, smaller polyp growing from the parent’s base.
Another form of asexual reproduction is fragmentation. This occurs when pieces of a coral colony break off due to physical disturbances like storms or boat impacts. If these fragments land in a suitable location, they can attach to the seafloor and grow into a new colony that is a clone of the original.
Sexual Reproduction in Corals
While asexual methods expand a colony, sexual reproduction generates genetic diversity, which is important for adapting to changing conditions. The majority of stony coral species, about three-quarters, reproduce sexually. This involves producing male and female gametes—sperm and eggs. Some species are gonochoric, meaning each colony is either male or female, like Elkhorn corals, while many others are hermaphroditic, where a single polyp produces both, a common strategy in species like Brain coral.
The most well-known strategy is broadcast spawning, where corals release large quantities of eggs and sperm into the water column. These gametes float towards the surface where fertilization occurs externally. This synchronized release increases the chances of fertilization between different colonies, even those separated by large distances.
A different strategy is brooding, which involves internal fertilization. Male corals release sperm that is captured by female corals containing eggs. The eggs are fertilized and develop into larvae within the female polyp for a period of days to weeks before being released.
The Coral Larval Journey
Following successful fertilization, a tiny, free-swimming larva called a planula is formed. These planktonic organisms are carried by ocean currents. The planula’s journey is dangerous; they face numerous predators and the vastness of the ocean means many will not survive, resulting in a period of high mortality.
The larval stage can last from a few days to several weeks. During this time, the planula swims towards the light at the surface before descending to search for a permanent home on the seafloor. Researchers believe most larvae settle within 600 meters of their parent reef, though some may travel much longer distances.
Once a suitable spot is found, the planula undergoes settlement. It permanently attaches itself to the substrate and metamorphoses from a mobile larva into a stationary polyp. This single polyp then begins to grow and will start budding asexually, laying the foundation for a new coral colony.
Environmental Cues for Spawning
The synchronized release of gametes during mass spawning events is a precisely timed process, orchestrated by environmental signals. Corals use these cues to coordinate their reproductive timing, ensuring that different colonies release their gametes simultaneously to maximize fertilization success. The long-term timing is linked to seasonal changes, particularly rising water temperatures that stimulate the maturation of eggs and sperm.
The more immediate coordination is governed by the lunar cycle. Many coral spawning events occur a few nights after a full moon in the spring. The final trigger for release is often the time of sunset, with the cover of darkness providing some protection from predators. Factors like day length, tide height, and water salinity also play a role in this reproductive timing.