Strange Green Worms Form 'Superorganism' as they Sunbathe

Strange green flatworms -- commonly known as "mint-sauce worms" due to their bright green color -- behave more like plants than they do animals, "feeding" off the sun's rays. In fact, adult worms survive entirely on the nutrients produced by photosynthesizing symbiotic algae living within their bodies. But the peculiarities of this plant-animal species don't end there. Curiously, a new study has found these worms collect together in enormous groups, forming a kind of "superorganism."

The three millimeter-long worms, scientifically known as Symsagittifera roscoffensis, are found along much of Europe's Atlantic coast. When enough of these tiny organisms gather together in shallow water on sheltered sand beaches to sunbathe, they start swimming around in a circle. When the tide is high, however, they bury themselves in the sand. They are often thought of as a plant-animal hybrid for having both plant and animal characteristics. 

In the latest study, researchers from the University of Bristol investigated this swirling mass, revealing that it forms a sort of vortex that ultimately attracts more individual worms into the grouping. Such large-scale social behavior may be the key to the tiny animals' survival and odd eating habits.

"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," Professor Nigel Franks, study lead author from Bristol's School of Biological Sciences, said in a news release. "Our study suggests this remarkable organism also seems to be an ideal model for understanding how individual behaviors can lead, through collective movement, to social assemblages."

Many other organisms, such as army ants, display a similar method of circular milling, as the swirl is called. Their study was recently published in the Proceedings of the Royal Society B.

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