Divers tend to think of corals as “sessile” – they stay put, so you know exactly where to find them. However, a new study has highlighted the ability of a mushroom coral to wander about on the seabed in search of its ideal habitat – albeit at a relaxed pace.
Mobile corals had been known about, but the way in which they move and navigate had remained “largely obscure”, according to an Australian scientific team led by Brett Lewis of Queensland University of Technology. The researchers investigated the small, free-spirited Cycloseris cyclolites to learn how and why it would go walkabout.
Experiments indicated a strong positive phototactic response to blue light, the sort the coral would expect to find in deeper-water sand-beds. It displayed far less enthusiasm for the sort of white light found in shallower surface waters, which could indicate a bleaching threat.
An overwhelming 86.7% of the mushroom corals headed towards blue light when it illuminated one end of the laboratory aquarium, while only 20% showed any keenness for white light.
The corals would move for between one and two hours to cover distances of as much as 22cm in the blue-light trials, but could be bothered to cover no more than 8mm towards white light. If blue and white lights were shone simultaneously at each end of the tank, the corals would always be drawn towards the blue.
High-res time-lapse photography revealed the combination of mechanisms employed by the coral to manage its movement. By inflating its tissues it could reduce friction, increase buoyancy to lift its core off the seabed and boost its surface area to catch currents, like a ship’s sail.
When the tissues were in turn contracted, ventral pads or ‘feet’ would contribute by adjusting interaction or friction with the substrate, allowing the coral to inch forward.
Finally, it was also able to contract and twist its lateral peripheral tissues, propelling itself forward in a style related to the pulsing swimming motion of jellyfish.
Cycloseris cyclolites seems to harness this ability when factors such as gravity, currents or waves have left it in an unfavourable, perhaps perilously shallow, location and it wants to improve its outlook – leading the researchers to conclude that corals are more “neurologically sophisticated” than previously thought. The study has just been published in PLOS One.
Thinned-out corals struggle to reproduce

Meanwhile, in a University of Queensland-led study, an international team has measured the success of a 2024 natural coral-spawning event and been concerned to find that individual corals need to be located at close quarters to reproduce successfully.
“In what came as a surprise, we saw that corals needed to be within 10m of one another, and preferably closer than that for fertilisation to take place,” said team-leader Prof Peter Mumby. “We knew corals couldn’t be too far apart, but we found they need to be closer than we’d expected.
“Climate-change impacts like bleaching are killing and reducing the density of corals, so we’re concerned that individuals may end up too far apart to reproduce successfully.”
Working in Palau in Micronesia, the researchers placed containers above 26 coral colonies on a reef as the mostly hermaphroditic corals released their eggs and sperm.

“The containers captured some of each coral’s eggs and drifted to the surface where they followed the tide,” said Prof Mumby. “Although the eggs could not escape, sperm could enter the container and fertilise the eggs.
“After an hour of drifting, the proportion of fertilised eggs was noted for each type of coral along with the distance to similar established corals.”
Fertilisation averaged 30% when corals were very close, but declined to less than 10% at a separation of 10m. By 20m it was virtually zero.
“In the future we may need to help corals continue this key part of their lives,” commented study co-author Dr Christopher Doropoulos of Australia’s national science agency CSIRO.

“Understanding the importance of local neighbourhoods provides tangible targets for interventions like coral restoration. Ideally, the density of corals would be monitored at important locations and restoration carried out to return the density back to the levels required for successful reproduction.”
The research is published in Proceedings of the National Academy of Sciences USA (PNAS).
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