Coral reefs are the largest and most diverse form of marine life, dominating over all species of fish and plants that live in the ocean. Throughout their 240 million years of existence, they’ve fed on plankton from the water column and have supported over a million marine creatures and organisms below the surface of the water.
Many of the living reefs that we see today are estimated to be at least 6,000 years old, and perhaps what allows them to thrive despite worldwide exposure to humans and heatwaves that cause coral bleaching is their long but efficient life cycle.
While they vary greatly in shape and size, corals grow pretty much the same way. Let’s take a closer look at this fascinating process.
Life Cycle of Corals
The coral life cycle typically starts with mature corals releasing male and female gametes into the water. These cells commonly undergo sexual reproduction (although sometimes through asexual reproduction) followed by external fertilization near the surface before it sinks back to the ocean floor and creates a new coral colony.
The phenomenon can be divided into four important stages: reproduction, egg development, settlement, and budding. Let’s dive deeper into each stage.
During the first step of the coral reef life cycle, clusters of column-shaped marine invertebrates (called polyps) typically release massive amounts of sperm cells from the exterior of a reef. At the same time, millions of eggs are released from the coral.
After the release of the egg and sperm into the water, they meet and float to the ocean’s surface where they break apart and fertilize. This unique phenomenon of over a trillion germ cells rising to the surface in breathtaking synchrony is called coral spawning, and only scientists and experts can predict exactly when it will happen.
Just a few hours after fertilization, the eggs begin to divide and develop. After an egg becomes a planula larva, it can float right on the surface of the ocean for several days. The larva will drift for days up to a whole month. During this time, it will collect symbiotic algae that will help it grow until it’s ready for the next stage of development.
In some cases, the reproductive and fertilization processes take place internally. The sperm will be released through the mouth of another polyp and merge with an egg to form a larva. Once it’s mature enough, the larva will be released into the water.
A mature larva will sink into the ocean and attach itself on the ocean floor. Sometimes, it chances upon and plants itself on the top of a reef. Where it settles will determine how big and which area of the existing reef will expand.
As the larva becomes stationary (on the ocean floor or on a reef), it will go through a stage of metamorphosis and transform into a polyp or module. The polyp will then grow, divide itself in half, and make exact genetic copies of itself to eventually form a colony of growing corals.
In this longest stage of the coral’s life cycle, the coral polyp will continue to develop until it forms a mouth and tentacles. Tiny marine algae called zooxanthellae will attach itself to the settled polyp, starting a mutually beneficial symbiotic relationship that results in the production of calcium carbonate and encourages the polyp’s calcification (or hardening) process. This type of algae is also what helps reefs survive even after maturity.
The polyp will continue to grow into a column and form buds near the bottom until it starts looking like a more familiar, fully-bloomed coral. This can happen right on the seabed and around the coral to form a new reef or right on top of an existing one to make it grow even larger.
Like most living organisms, corals take a while before they can reproduce. Branching corals normally take a few years while brain corals, which grow at a slower rate, could take about eight years before reaching sexual maturity. Once they do, they can produce more gametes and eventually restart the coral’s life cycle.
Take a look at the video below for a visual example of the process described above: