As aquarists, we love our beautiful coral aquariums. The color, structure, and diversity corals can bring to a tank is unparalleled. As proud aquarists, our aquariums also provide great tools for education for family and friends as well as ourselves of course. Have you wondered about how these beautiful corals have come about? In this guide, we provide you with everything you have ever wondered about coral spawning.

Understanding Coral Spawning

Many corals will spread across the reef asexually through fragmentation. Yet this results in clones of the same colony and does not increase the diversity or gene pool of the reef. Therefore, there is a second method of reproduction, this is sexual reproduction. Coral polyps will simultaneously release egg and sperm bundles that they’ve spent months growing into the ocean for external fertilization. This happens in a mass event annually, known as coral spawning. This spawning cycle is one of nature’s most impressive underwater spectacles, resembling the inside of a snow-globe!

As the gametes (eggs and sperm) slowly rise upwards to the surface, the process of fertilization begins. When a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. These planulae will continue to float in the ocean for several days, even weeks in some cases, before descending to the ocean floor. Depending on the conditions of the seafloor, the planulae may attach to the surface and with any luck, slowly grow into a new coral colony.

When Does It Happen?

Firstly, not all corals, or all coral reefs, have these mass spawning events, or spawn at the same time, or rely on the same environmental cues for mass spawning. For the most part, coral spawning occurs once a year, aligning with cues from the lunar cycle and the water temperature. For some species they rely instead on chemical cues. In a phenomenon still not fully understood by scientists, entire colonies of mature corals will release their gametes  all at the same time. It is crucial that these colonies are in sync, as the gametes of most coral species are only viable for a few hours. This underwater blizzard of gametes therefore makes it more likely that fertilization will occur. This incredible evolutionary adaptation gives corals the best chance for widespread reproduction.

Where Is The Best Place To Observe Coral Spawning?

The world’s largest synchronized coral spawning event happens along the Great Barrier Reef. In 2020, scientists observed the main spawning event took place overnight on December 5th. Some corals were observed to have spawned earlier in November, but the majority of corals released their spawn in December, thanks to perfect water temperatures and a late November full moon. Amazingly, more corals were seen spawning this year than in previous years. This is incredibly impressive, highlighting the giant effort and resilience of coral species after three mass coral bleaching events in five years. 

“Year after year, this shows that despite the pressures on the Reef from climate change, there is still hope for the future” 

Dr. David Wachenfel, Chief Scientist, Great Barrier Reef Marine Park Authority, quoted in

Can Corals Spawn In Your Aquarium?

For many aquarists, achieving coral spawning is one of the ultimate goals in reef husbandry. In the past this was rarely observed in reef tanks but for many experienced, and increasing for relatively new tank owners, spawning in aquarium corals is becoming more frequently observed. Even more excitingly, for many aquarists, their reports of spawning events are truly novel. For some coral species, their reproduction has never been witnessed or documented before, including in the wild. These observations serve as critical resources for increasing our understanding of the nature of how these species sexually reproduce. 

Have you ever observed coral spawning in your tank? We’d love to hear from you! Please let us know about your experience and send us your pictures! As always you can also tag us across our social media channels. You can contact us via our website here, or find us on Facebook, Twitter and Instagram!