Coral Spawning on the Ningaloo

Vibrant Corals seen on one of our snorkelling sites. Photo Elle Gillett

Coral reefs are a captivating and unique marine ecosystem. With the vibrant colours, the diversity of life and pristine waters, it’s understandable how those who come across them are left in awe! But there is something else that is unique about corals and that is the way that they reproduce.

Each year, the waters of the Ningaloo become covered in clouds of reds and pinks, following a natural phenomenon known as coral spawning. We have recently been lucky enough to witness this spawning event take place on our reef here in Coral Bay.

The way these corals spawn is a fascinating event which is vital to the health of the reef ecosystem. But what exactly is coral spawning?

Corals are often mistaken for rocks or plants due to their sessile (immobile) nature, but are in fact animals and reproduce in peculiar ways. Although coral polyps wriggle around a fair bit to catch their prey using their tentacles, they can’t exactly hop-up and move around to find a mate, which can make reproducing rather difficult! Most cnidarians (corals, jellyfish, anemones) reproduce asexually, which means they can produce new polyps, replicating themselves and making their colonies grow larger. To produce new colonies in new areas, however, requires a bit of a different strategy.

Once a year along the Ningaloo, usually 5-10 days after the full moon in March/April, when the water temperatures are just right (around 27⁰c), the entire colony of corals simultaneously release millions of tiny eggs and sperm bundles (spawn) into the water column. This phenomenon, which only occurs at night, is known as coral spawning and happens all at once to avoid most of the spawn being eaten by neighbouring fish and maximises their chance of fertilisation. The spawn can last a few hours or up to a couple of days. The underwater world becomes a blizzard of colourful specs floating within the water.

During the spawning, the eggs and sperm float to the ocean’s surface, where they fertilise and develop into small larvae (called planula). With assistance from the ocean currents and tides, these larvae will, in time, settle in a suitable place on the reef, metamorphosing into a small coral polyp. This polyp will reproduce asexually, growing new polyps as fast as it can to increase the size of this new coral colony.

Not only is this mass spawning event crucial for the reef to spread new colonies across other areas, but vital to other species that rely on these ecosystems. The coral spawn provides food for many reef fish, and swarms of plankton are drawn to the reef by the spawn, which then attracts our large filter feeders, the Whale Sharks.

There is still little known about this phenomenon, with scientists still unsure just how the corals release the eggs and sperm bundles all at the same time. They have a rough idea of when the spawning event will take place, but cannot put it to an exact time or date.

This unique and incredible natural event is vital for the future of our coral reef ecosystems. Understanding these spawning events gives scientists a clearer picture of the health of a reef and helps with ongoing (and increasing) conservation efforts. If you want to discover the incredible corals and learn more about their unique reproductive habits, jump on board our glass-bottom boat and snorkel tour on Nhanya Ku to immerse yourself in the wonderful and weird world of the corals.

Clouds of pink coral spawn on the ocean's surface (James Gilmours AMIS, 2022)