A coral is a large colony of individual animals, known as polyps, that are related to sea anemones and jellyfish. These polyps build the massive structures we call reefs. Over their lifespans, corals undergo a complex life cycle. This process creates new, genetically diverse individuals that establish new colonies, and also enables existing colonies to expand and repair themselves. The cycle represents a journey from a single, free-swimming organism to a vast, stationary reef.
Coral Reproductive Processes
Corals reproduce in two primary ways: sexually and asexually. The most common method of sexual reproduction is broadcast spawning, where entire communities synchronize the release of their gametes—eggs and sperm—into the water. This event is tied to environmental cues like the lunar cycle and water temperature to maximize fertilization. These gametes are released in buoyant bundles that float to the surface, where they break apart for external fertilization.
A different form of sexual reproduction is practiced by brooding species. In this case, only the sperm is released into the water. The sperm is taken in by female polyps containing eggs, where internal fertilization occurs. The resulting larvae then develop within the protection of the parent polyp before being released.
Asexual reproduction creates new colonies without exchanging genetic material. One method is budding, where a polyp divides itself to create a genetically identical new polyp. Another form is fragmentation, which happens when pieces of a coral colony break off due to storms or other disturbances. If these fragments land in a suitable location, they can grow and establish new colonies genetically identical to the original.
From Larva to Polyp
Following successful fertilization, the zygote develops into a free-swimming larva known as a planula. This larval stage is the only time the organism is motile and can disperse to new locations. The planula drifts with ocean currents for a period that can last from several days to a month.
The planula’s journey is not passive; it actively seeks a suitable place to settle. The larva uses environmental cues to find a hard substrate free of excess sediment or algae. Chemical signals from certain types of crustose coralline algae are attractive to many planulae, indicating a healthy environment. Light and sound can also play a role in guiding the larva to a promising location.
Once a suitable spot is found, the planula attaches itself to the substrate and undergoes metamorphosis. The larva changes shape, becoming a flattened disc from which a mouth and tentacles appear. The organism secretes a cup-like skeleton of calcium carbonate at its base, cementing it permanently to the surface. This newly formed, stationary organism is now a polyp, the foundational unit of a new coral colony.
Colony Formation and Growth
The single founder polyp marks the beginning of a new coral colony. Growth begins through asexual budding, where the initial polyp replicates itself by dividing. This produces a new, genetically identical polyp that remains connected to the original. This process repeats, with new polyps budding from older ones, gradually increasing the size of the colony.
As the polyps multiply, they collectively construct the calcium carbonate skeleton. Each polyp secretes this material from its base, building upon the existing structure. The shape and pattern of this growth are determined by the coral’s species, resulting in diverse forms like massive, boulder-like corals or branching varieties. The skeleton provides support and protection for the soft-bodied polyps.
This process of budding and skeletal secretion continues over years, allowing the colony to expand. The shared tissue connecting the polyps, known as the coenosarc, allows the colony to function as a single organism. Nutrients and resources are shared across the colony through this living tissue, supporting the growth of the entire structure.
Attaining Maturity and Symbiosis
As the colony grows, it eventually reaches maturity and gains the ability to reproduce sexually. At this stage, its polyps begin to develop their own gametes to participate in future spawning events. This marks the transition from a juvenile colony focused on growth to an adult colony that contributes to the next generation.
A defining aspect of most reef-building corals is their symbiotic relationship with microscopic algae called zooxanthellae. While some larvae acquire these symbionts, many polyps take them in from the environment shortly after settling. These algae live within the coral’s tissues, where they photosynthesize and convert sunlight into energy.
This partnership is highly beneficial to the coral’s survival. The zooxanthellae can provide the coral with up to 90% of its nutritional needs, fueling the process of skeleton building. In return, the coral provides the algae with a protected environment and compounds for photosynthesis. This relationship also gives corals their vibrant colors, as the algae’s pigments are visible through the coral’s transparent tissues.