The cuttlefish has a reputation for being a rather solitary cephalopod. But new footage reveals that groups of wild cuttlefish form shoals to migrate, suggesting they are more social than we thought.
Grouping is common across the animal kingdom, providing a range of benefits including help with foraging, fending off predators and meeting to mate. In cephalopods, the behaviour is mostly associated with squids, which form schools of thousands. Cuttlefish, like octopuses, mostly prefer to explore the world on their own, and sightings of social behaviour among them are rare.
Christian Drerup at the University of Cambridge and Gavan Cooke at The Cephalopod Citizen Science Project have collected a series of reports, photos and videos from scuba divers in waters off the south coast of the UK that show 10 examples of shoaling in European cuttlefish (Sepia officinalis).
“The literature is very dogmatic about what cephalopods do and don’t do, and you kind of accept that until you see things with your own eyes,” says Cooke.
In the videos, cuttlefish could be seen travelling together in a series of formations, some in groups as large as 30. Sometimes, they formed a horizontal line, with one cuttlefish facing the other direction – possibly acting as a guard while the others slept. At other times, the cuttlefish made a spherical shape, facing outwards in all directions like an ancient Roman army testudo formation. Occasionally, the cuttlefish drifted apart before returning to a group structure.
The observations were made between 2013 and 2020 during August to September, when these cuttlefish start migrating from their nursery grounds in shallow coastal waters towards deeper waters in the English Channel and off the northern coast of France. “Shoaling allows for selfish herd defence,” says Cooke, providing safety in numbers against a range of predators along the journey. It also probably improves navigation, and may offer an opportunity for social learning.
Journal reference: Ethology, DOI: 10.1111/eth.13226
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