Plant-animal worms round themselves up in circular mills on the beach

Ana B. Sendova-Franks*, Nigel R. Franks, Alan Worley

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

4 Citations (Scopus)
196 Downloads (Pure)


Collective motion is a fascinating and intensely studied manifestation of collective behaviour. Circular milling is an impressive example. It occurs in fishes, processionary caterpillars and army ants, among others. Its adaptive significance, however, is not yet well understood. Recently, we demonstrated experimentally circular milling in the marine plant-animal worm Symsagittifera roscoffensis.We hypothesized that its function is to gather the worms and facilitate the dense films they form on the beach to promote the photosynthesis of their symbiotic algae. Here, we report for the first time, to our knowledge, the occurrence of S. roscoffensis circular mills in nature and show that it is by no means rare. The size and behaviour of circular mills in their natural environment is compatible with our earlier experimental results. This makes S. roscoffensis a good study system for understanding the proximate and ultimate mechanisms of circular milling.

Original languageEnglish
Article number180665
Number of pages5
JournalRoyal Society Open Science
Issue number7
Early online date25 Jul 2018
Publication statusPublished - Jul 2018


  • Circular mills
  • Collective behaviour
  • Collective motion


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