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Watch Spawning Corals Synchronize With the Night Sky

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When the moon, sun and ocean temperatures all align, an underwater “snowstorm” occurs. Corals put on a massive spawning spectacle by sending tiny white spheres floating up the water column all at once.


Thank you to Surfshark VPN for supporting this PBS video.

Once a year, something astounding happens at Australia’s Great Barrier Reef. It lasts barely half an hour.

If you jumped into the water at this very moment, it’d be like swimming through a snow globe, hundreds of kilometers across.


But these “snowflakes” are actually packets of eggs and sperm of coral. Corals might look like colorful rocks or undersea gardens, but they’re actually animals. A coral is a colony of hundreds of thousands of tiny individual animals called polyps.

Each of these flower-shaped polyps has a mouth and tentacles. Polyps secrete calcium carbonate that creates their skeleton. It gives them structure and anchors them to a rock or the seafloor.

Since they can’t move to find a partner and mix up the gene pool, most warm-water corals practice “broadcast spawning.”

But with such a short window to meet up, they have to sync it just right. The warming summer waters cue the right month. The light from a waning moon cues the right day, and the setting sun cues the exact minute. Good luck out there!

These bundles contain the coral’s gametes — its sperm and eggs. But the gametes don’t mix in there. The bundles float to the surface and burst open.

Sperm search out a new egg. Only one of these guys will get in. Look familiar?

Once fertilized, it starts dividing and transforms into this adventurous larva called a planula. The planula swims through the sea, searching for a place to settle down.

Chemical and light sensors on its backside guide the planula to the perfect spot. It wants what we want: a stable foundation, plenty of sunlight, and room to grow. The planula cements itself into place and morphs into a polyp.

As it grows, it absorbs algae called zooxanthellae from the surrounding water. See these green dots? They live inside the polyps.

The algae give the coral nutrition and its brilliant colors. Then something curious happens: The polyp clones itself. It grows copies right out of its side, that then bud their own clones. Through broadcast spawning and cloning, corals create the massive reefs we’re familiar with.

But reefs are in danger, and that’s not just a problem for the corals.They’re vital ecosystems that provide food and shelter for a quarter of marine life, like fish, crustaceans and sea turtles. Climate change is the main culprit.

When ocean waters warm up too much, stressed polyps expel their colorful and nutritious algae. This is coral bleaching.

When reefs die and spawning season comes, it’s harder and harder for the eggs and sperm to find each other.

So, researchers at the California Academy of Sciences in San Francisco have replicated the delicate spawning conditions in a lab.

Lights mimic moon cycles, and heaters simulate the change of seasons. Their goal is to discover the best ways to grow corals, so more scientists can help restore them to the oceans.

An underwater blizzard is a thing of beauty, even more so when you consider how this snowstorm can replenish a delicate and threatened ecosystem.

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