A coral colony off Australia’s Cocos Islands runs 1.56 km, as long as 100 buses

A single coral colony off Australia’s remote Cocos (Keeling) Islands stretches more than a kilometre and a half along the outer reef, about as far as a hundred buses parked nose to tail. Divers had known for decades that the reef slope was carpeted with the same plate-like coral. Only when someone swam the whole thing with a GPS did it become clear that the patches might all belong to one living structure.

That makes it one of the largest coral colonies ever recorded, and it raises a question biologists cannot answer yet: is this one animal or many? Genetic tests now under way will decide whether the reef holds a single organism or a crowd of neighbours that fused into one.

What the divers measured

The coral is a rose coral of the genus Pachyseris, according to the Queensland Museum, which announced the find with Adelaide University and James Cook University. It runs 1.56 kilometres from end to end and averages 36.5 metres across, covering roughly 56,940 square metres of seafloor. That is close to eight football pitches. Further sections continue for another 200 to 300 metres beyond the main body.

The colony clings to the outer reef slope between 14 and 65 metres down. At the shallow end, recreational divers can reach it easily. At the deep end, it enters what marine biologists call the mesophotic zone, the dim band of reef below about 30 metres where sunlight fades and few divers go.

The first person to notice it was not a scientist. Local dive operator Dieter Gerhard spotted large stretches of the coral more than 30 years ago. What nobody had done was follow it to the end.

How they did it

Lead researcher JP Hobbs of Adelaide University spent months mapping the colony with colleagues from the Queensland Museum and James Cook University. The method was simple and slow. Hobbs swam along the edge of the coral, and when his air ran low before he reached the end, he marked his stopping point with GPS so the next dive could pick up from there.

It took 12 dives to trace the full outline. Each dive added another stretch of length, and the measurements of width along the way gave the area estimate. The work was funded by Parks Australia as part of a wider effort to understand the Cocos (Keeling) Islands Marine Park, and the results appear in the Raffles Bulletin of Zoology, a journal published by the Lee Kong Chian Natural History Museum in Singapore.

Why divers and not drones or satellites? A coral sheet that drops from 14 to 65 metres cannot be photographed from above. Below a few tens of metres, light is too weak for aerial or satellite imagery to tell one coral from another, so someone has to be in the water.

How it compares with other giants

The most famous recent record holder was announced in the Solomon Islands. A National Geographic expedition found a colony of Pavona clavus at a site called the Three Sisters, measuring 34 metres wide and 32 metres long and thought to have been growing for some 300 years. Its lead scientist, Molly Timmers, compared it with the previous record holder, a coral called Big Momma in American Samoa: “While Big Momma looked like a huge scoop of ice cream plopped down on the reef, this newly discovered coral is as if the ice cream started to melt.”

The Cocos coral is a different kind of giant. The Solomon colony is a tall, rounded mass. The Pachyseris on Cocos is a thin, spreading sheet. Measured by length, it is about 46 times longer, and its footprint is roughly 50 times larger. Measured by volume or age, nobody can say yet which one wins.

Australia already holds one record for clonal sprawl in the sea. In Shark Bay, on the Western Australian coast, genetic testing showed that seagrass samples taken up to 180 kilometres apart all came from a single plant, covering about 200 square kilometres of seafloor. The Cocos coral is tiny by comparison, but it would be one of the largest animal colonies ever measured.

What it doesn’t settle

The headline number is a length, not proof of a single organism. A coral colony is made of thousands of tiny polyps that are clones of each other, all budded from one original larva. But corals also break, and broken fragments can regrow as separate colonies that share the same genes. Different colonies can also grow into each other and fuse. A sheet 1.56 kilometres long could be any of these things.

That is what the genetic tests are for. If samples taken along the colony carry identical DNA, the coral is one genetic individual, though even then it may have spread by fragments rather than by continuous growth. If the DNA differs, the “colony” is a patchwork of neighbours.

There is a further complication. A genome study of Pachyseris speciosa, a common species of the genus, found that it is actually several lineages that look identical but live at different depths. Researchers led by Pim Bongaerts reported that these lineages overlap across shallow and mesophotic reefs while remaining genetically separate. A coral that spans 51 metres of depth on Cocos is exactly where such hidden splits could show up.

The age of the colony is also unknown. The announcement does not give a growth rate or an estimate of how long it has been there, and any figure would be a guess until the genetic and growth data are in.

Why Cocos matters

The Cocos (Keeling) Islands are two low coral atolls made up of 27 islands, roughly midway between Australia and Sri Lanka, with a population of around 600 people. They have a special place in reef science. On 1 April 1836, HMS Beagle arrived to take soundings of the atoll, and Charles Darwin, on board, found support there for his theory of how atolls form, later published as The Structure and Distribution of Coral Reefs.

The waters around the islands became part of two new Australian marine parks approved in March 2022, which together cover about 744,000 square kilometres around Christmas Island and the Cocos (Keeling) Islands. The coral survey is part of the work to find out what those parks actually protect.

Common questions about the giant Cocos coral

What is the largest coral colony in the world?

The Solomon Islands colony of Pavona clavus, 34 metres wide and 32 metres long, was widely reported as the largest single coral. The Cocos Pachyseris colony is far longer at 1.56 kilometres, but genetic tests still have to show whether it is one colony or several that fused.

Is a coral colony one animal?

A colony is made of many polyps, each a small animal, all cloned from one founding larva and connected by living tissue. Biologists usually treat the whole colony as one genetic individual. Fragmentation and fusion can blur that picture.

Where are the Cocos (Keeling) Islands?

They are an Australian external territory in the Indian Ocean, roughly midway between Australia and Sri Lanka. The territory is made of two coral atolls with 27 islands, and only two of them are inhabited.

How deep does the Cocos coral grow?

It grows on the outer reef slope between 14 and 65 metres deep. The deepest parts reach the mesophotic zone, the low-light band of reef below about 30 metres.

What comes next

The next answer will come from the lab. The team is now analysing DNA from samples along the colony to establish whether it is a single genetic individual, a clone spread by fragments or several colonies grown together. That result will decide whether the Cocos coral enters the record books as one of the largest single animals ever measured or as a remarkable crowd of them.

Tags: marine biology, Adelaide University, Cocos (Keeling) Islands, coral colony, coral reefs, Pachyseris