Polyps, procreation and people: the strange but familiar world of coral reproduction

It would be fair to say that coral is a misunderstood animal. Yes, animal. Because although living coral looks plant-like and dead coral resembles a sort of chalky rock, the truth is more complicated. What’s really mind-blowing, however, is how much we have in common with them. So, let’s start at the beginning. How on earth is coral an animal?

Well, when you look carefully you can see that each one is comprised of thousands of tiny, soft-bodied creatures called polyps. These cute little organisms are related to jellyfish and sea anemones and live together in colonies, secreting calcium carbonate – like millions of tiny architects – to make the stony world that they live on. “It’s a three-dimensional structure that allows all the life to occur on the reef,” explains Dr Jamie Craggs of Coral Spawning International. “Just like buildings in a city.”

And within this city live tiny algae called zooxanthellae, which photosynthesise (taking sunlight, water and carbon dioxide to feed themselves and create oxygen) and provide the corals with both food and the brilliant colours which make them so beautiful. It’s a symbiotic relationship – the corals look after the algae, and the algae look after the corals.

So, they live in communities, create an environment where they have plentiful food and require clean, warm and healthy conditions to thrive. All sounds quite familiar, doesn’t it? Well, there is another, more fundamental, way that we have something in common with corals: reproduction. But surely this is nothing like ours? Well, yes and no.

How corals create new life

There’s always been something slightly mystical in the way that corals breed. The conditions need to be just right for them to be ‘in the mood’ – the temperature of the sea, the timing of sunset, a full moon… it’s complex stuff. But when the planets align, something quite extraordinary happens, which Dr Craggs describes as “a living lava lamp”. Colonies of coral release an explosion of eggs and sperm into the water, which drift to meet each other. These inseminate and produce larvae, which float in the ocean currents until they finally settle on a rock or another coral structure. Then they grow into the kind of corals we are familiar with.

Critically, this only happens once or twice a year, and corals can take many months to mature. So, it spells trouble when sea temperatures rise and corals become stressed, expelling their tiny algae. They turn white – a phenomenon known as ‘bleaching’ – and while they can sometimes recover, prolonged or repeated bleaching events leave them severely weakened and make natural recovery almost impossible. This is why marine scientists have developed alternative methods to support coral reproduction and restoration.

Cultivating coral like a gardener

This is the most common way that humans can help corals to regrow and has been successful in many places. The idea is simple, explains Dr Craggs, “They are clonal organisms – dividing and dividing to form new polyps and then new coral. So, you can break a branch off and glue it onto a new rock, in just the same way as you might take a cutting from a rose.” Crucially, it’s a low-cost, low skill, but fast way to restore reefs. But he warns that it is, however, just “creating a new plant that is a genetic clone of the parent”.

This can be a problem because, like us, corals are not all the same. Some can deal with danger better than others. “Limited genetic diversity means that we don't build in any resilience against future climate conditions,” he explains. “Because if the coral you’re harvesting from has no resilience to bleaching events, it will fail in the long term.” This problem is at the very heart of Dr Craggs’ research: how to create new species of coral that are strong enough to withstand climate change.

Recreating the magic of the ocean

Okay, we admit, this third way is not really ‘magic’ – it’s science. And it can happen in two ways – one, where eggs and sperm are caught under nets above coral reefs, then retrieved by divers and taken back to the lab to breed. It’s a long process, requiring a lot of specialist skill, money and, most of all, patience, as you wait for natural spawning to happen a couple of times a year. The other way has been pioneered by Dr Craggs himself.

He has, in the course of his career and PhD research, used a combination of his expert knowledge and targeted technologies to recreate the perfect atmosphere for corals to spawn in a lab. So, now they can be ‘tricked’ into doing so multiple times, under controlled conditions, and sperm and eggs are gathered immediately, so risk of loss or damage is minimal.

Then a kind of coral IVF procedure takes place and new ‘test tube baby’ coral larvae develop after just a few days, after which they are returned to the aquarium to grow before being transported to the reef. Breeding corals in this way, Dr Craggs can identify and combine the most resilient species, creating entirely new ones that may be able to survive all but the most aggressive and damaging circumstances.

So, yes, we humans have a bit more in common with coral than we might ever realise – and, like we sometimes do, it too needs extra help on the reproductive front. But our connection with corals is in far more than just surprising similarities. Because when corals survive and the ocean thrives, life is better for everyone. Not only the millions of people who depend on coral reef ecosystems for food, income and coastal protection, but every single person on earth.

“With every second breath we're taking, we are getting oxygen from the ocean,” stresses Dr Craggs. “It's generated from systems in the ocean and has a direct effect on human health as well as ocean health. We need healthy oceans for healthy people.”

Learn more about our partnership with Coral Spawning International and Nature Seychelles.