Social behavior has been revealed in one of the world's strangest living things, the mint-sauce worm (Symsagittifera roscoffensis). The worm's name comes from its color, which in turn stems from the fact that it is one of the few animals that assimilates algae into its cells to provide it with food, but it seems the worms also cooperate with each other.
Mint-sauce worms live on sand beaches on Europe's Atlantic coastline. Adults get their energy requirements from the symbiotic algae Tetraselmis convolutae, which they shelter in return. The worms grow to just 3 millimeters (0.12 inches) but can form large colonies.
Professor Nigel Franks of the University of Bristol has started exploring how these colonies interact.
As photosynthesizers living in notoriously unsunny locations, mint-sauce worms take advantage of low tide by sunbathing on beaches. When the tide turns they bury themselves in the sand to avoid being swept out to sea. They've also been observed to form biofilms, mat-like structures of thousands of the worms joined together.
In Proceedings of the Royal Society B Franks and his coauthors write, “We show that individual worms interact with one another to coordinate their movements so that even at low densities they begin to swim in small polarized groups and at increasing densities such flotillas turn into circular mills.” These mills almost always rotate clockwise.
The team are the first to report this milling behavior in the worms, “Possibly because it may occur only fleetingly at a certain stage of the tidal cycle,” they say. Milling has been reported in many other creatures, however, and occurs, the authors write, “When individuals in a group are so synchronized that they follow one another nose-to-tail in a complete ring in such a way that their trajectories are almost identical and approximately circular; often there are multiple orbits nested within one another.”
Similar mills are considered maladaptive in caterpillars and ants, but Franks said in a statement. "Such social behavior helps the worms to form the dense biofilms that have been observed on certain sun-exposed sandy beaches of the East Atlantic, and to become in effect a super-organismic seaweed in a habitat where macro-algal seaweeds cannot anchor themselves,"
As the worms' density increases they go through three phase transitions, from individuals through small groups and circular mills to mats. Each transition has been seen in other species, but few exhibit all three.
The worms will crawl towards light so intense it could be damaging for their algae. The paper suggests that this could be because they take it in turns in the sun, like “Emperor Penguins who form rotating huddles as protection against extreme Arctic winds.”
S. roscoffensis is popular with biologists as a model organism for studying topics such as stem cell regeneration, but its social interactions have attracted little attention.
"Our study suggests this remarkable organism also seems to be an ideal model for understanding how individual behaviours can lead, through collective movement, to social assemblages," said Franks.