A coral colony stretching about 1.5 kilometers along a reef slope near the remote Cocos Islands has prompted questions about how it grew, what species it is and how long it has lived. ScienceAlert’s Jess Cockerill reports that marine biologist and diver JP Hobbs mapped the colony over 12 dives in the Indian Ocean. Its size could make it one of the largest corals found, if it is a single organism—but that remains to be determined.
The colony belongs to the genus Pachyseris, whose colonies normally reach about 2 meters in diameter, according to the report. It grows along the deep outer slope of an atoll reef, from 14 to 65 meters below the surface. Hobbs collected coral specimens at 50 points across the colony. At the end of the mapped stretch, he saw more Pachyseris beyond a 10-meter gap. He suggested the reef may once have been continuous there, and that the coral’s former length could have exceeded 2 kilometers. That is a possibility, not a measured past length.
Two separate uncertainties now shape the story. One is taxonomic: the samples could belong to a known Pachyseris species, or represent a species not yet described. The other is whether the immense formation is genetically one individual or a collection of colonies that grew together. The samples have been sent to the Queensland Museum, where marine genomicist Peter Cowman is looking forward to examining their DNA. The report does not give a result from that analysis.
The distinction matters for estimating the coral’s age. If it is one organism, it may have expanded as fragments broke away, grew elsewhere on the slope and later joined the main colony. Cowman said genetic differences that accumulated over time could potentially be used as a molecular clock. That might help estimate whether the coral is hundreds or thousands of years old, and possibly trace its origins. Those are proposed future analyses; the colony’s age has not yet been established.
Its shape may help explain how it spread. Pachyseris grows in rows of concentric, plate-like structures with sharp ridges that can break. Cowman described a possible route for expansion: a fragment falls farther down the slope, grows, then merges with the main colony. If that happened, he said, it could still be part of the same individual. Whether that process formed this particular colony is not yet known.
The location also makes the colony relevant to questions about changing reef conditions. Its depth range reaches the mesophotic, or “twilight,” zone, where light levels drop, and crosses a broad gradient of light and temperature. The report says shallow corals near the Cocos Islands have been damaged by bleaching, disease and crown-of-thorns starfish. Hobbs said conditions deeper down seem to have been favorable for long periods. The possibility that deeper reefs could provide a refuge for marine life is a question for further investigation, not a conclusion established by this colony.
For now, the map provides a reference point. Scientists can use it to survey the colony in coming years and track changes alongside bleaching events, heatwaves and other climate events. That could show how this unusual coral responds over time, but the report does not establish that its size or depth will protect it from climate change. Its genetic identity, structure and history remain open questions.