title: Social behaviour and collective motion in plant-animal worms.
abstract: Social behaviour may enable organisms to occupy ecological niches that would otherwise be unavailable to them. Here, we test this major evolutionary principle by demonstrating self-organizing social behaviour in the plant-animal, Symsagittifera roscoffensis. These marine aceol flat worms rely for all of their nutrition on the algae within their bodies: hence their common name. 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. We use computer simulations to: (i) determine if real worms interact socially by comparing them with virtual worms that do not interact and (ii) show that the social phase transitions of the real worms can occur based only on local interactions between and among them. We hypothesize that such social behaviour helps the worms to form the dense biofilms or mats observed on certain sun-exposed sandy beaches in the upper intertidal of the East Atlantic and to become in effect a super-organismic seaweed in a habitat where macro-algal seaweeds cannot anchor themselves. Symsagittifera roscoffensis, a model organism in many other areas in biology (including stem cell regeneration), also seems to be an ideal model for understanding how individual behaviours can lead, through collective movement, to social assemblages.
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK nigel.franks@bristol.ac.uk.
authors.forename: Nigel R
authors.identifier: http://orcid.org/0000-0001-8139-9604
authors.initials: NR
authors.lastname: Franks
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Alan
authors.identifier: http://orcid.org/0000-0002-7734-7841
authors.initials: A
authors.lastname: Worley
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Katherine A J
authors.initials: KA
authors.lastname: Grant
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Alice R
authors.initials: AR
authors.lastname: Gorman
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Victoria
authors.initials: V
authors.lastname: Vizard
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Harriet
authors.initials: H
authors.lastname: Plackett
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK Champalimaud Neuroscience Programme, Champalimaud Centre for the Unknown, Av. Brasília, Lisbon 1400-038, Portugal.
authors.forename: Carolina
authors.identifier: http://orcid.org/0000-0002-7814-4675
authors.initials: C
authors.lastname: Doran
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Margaret L
authors.initials: ML
authors.lastname: Gamble
authors.affiliation: AnTracks Computer Vision Systems, Mountain View, CA, USA.
authors.forename: Martin C
authors.identifier: http://orcid.org/0000-0003-1402-6749
authors.initials: MC
authors.lastname: Stumpe
authors.affiliation: Department of Engineering Design and Mathematics, UWE, Bristol, UK.
authors.forename: Ana B
authors.identifier: http://orcid.org/0000-0001-9300-6986
authors.initials: AB
authors.lastname: Sendova-Franks
mesh_terms: D000818:Animals; D001267:Atlantic Ocean; D001522:Behavior, Animal; D003198:Computer Simulation; D017753:Ecosystem; D007448:Invertebrates / Q000502:physiology*; D008954:Models, Biological; D009068:Movement; D012919:Social Behavior; D013550:Swimming
keywords: circular milling; collective motion; social behaviour