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Here, underwater and aerial video recordings of multiple circling groups of basking sharks during late summer (August and September 2016\u20132021) in the eastern North Atlantic Ocean showed groups numbering between 6 and 23 non\u2010feeding individuals of both sexes. Sharks swam slowly in a rotating \u201ctorus\u201d (diameter range: 17\u201339\u2009m), with individuals layered vertically from the surface to a maximum depth of 16\u2009m. Within a torus, sharks engaged in close\u2010following, echelon, close\u2010flank approach or parallel\u2010swimming behaviours. Measured shark total body lengths were 5.4\u20139.5\u00a0m (mean <jats:italic>L<\/jats:italic><jats:sub>T<\/jats:sub>: 7.3\u00a0m\u2009\u00b1\u20090.9\u00a0<jats:sc>s.d.<\/jats:sc>; median: 7.2\u00a0m, <jats:italic>n<\/jats:italic>\u00a0=\u200927), overlapping known lengths of sexually mature males and females. Males possessed large claspers with abrasions that were also observed on female pectoral fins. Female body colouration was paler than that of males, similar to colour changes observed during courtship and mating in other shark species. Individuals associated with most other members rapidly (within minutes), indicating toroidal behaviours facilitate multiple interactions. Sharks interacted through fin\u2013fin and fin\u2013body contacts, rolling to expose the ventral surfaces to following sharks, and breaching behaviour. Toruses formed in late summer when feeding aggregations in zooplankton\u2010rich thermal fronts switched to non\u2010feeding following and circling behaviours. Collectively, the observations explain a courtship function for toruses. This study highlights northeast Atlantic coastal waters as a critical habitat supporting courtship reproductive behaviour of endangered basking sharks, the first such habitat identified for this species globally.<\/jats:p>","DOI":"10.1111\/jfb.15187","type":"journal-article","created":{"date-parts":[[2022,8,6]],"date-time":"2022-08-06T04:09:10Z","timestamp":1659758950000},"page":"1160-1181","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":31,"title":["Circles in the sea: annual courtship \u201ctorus\u201d behaviour of basking sharks <i>Cetorhinus maximus<\/i> identified in the eastern North Atlantic Ocean"],"prefix":"10.1111","volume":"101","author":[{"given":"David W.","family":"Sims","sequence":"first","affiliation":[{"name":"The Laboratory Marine Biological Association  Citadel Hill Plymouth UK"},{"name":"Ocean and Earth Science, National Oceanography Centre Southampton University of Southampton  Southampton UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Simon D.","family":"Berrow","sequence":"additional","affiliation":[{"name":"Irish Basking Shark Group Merchants Quay  Kilrush Ireland"},{"name":"Marine and Freshwater Research Centre, Department of Natural Resources and the Environment Atlantic Technological University  Galway Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ken M.","family":"O'Sullivan","sequence":"additional","affiliation":[{"name":"SeaFever Productions Woodview Park  Ennistymon Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nicholas J.","family":"Pfeiffer","sequence":"additional","affiliation":[{"name":"MERC Ecological Consultants  Galway Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Richard","family":"Collins","sequence":"additional","affiliation":[{"name":"Irish Basking Shark Group Merchants Quay  Kilrush Ireland"},{"name":"Irish Basking Shark Group  Kilkee Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kev L.","family":"Smith","sequence":"additional","affiliation":[{"name":"SeaFever Productions Woodview Park  Ennistymon Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Brianna M.","family":"Pfeiffer","sequence":"additional","affiliation":[{"name":"MERC Ecological Consultants  Galway Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Paul","family":"Connery","sequence":"additional","affiliation":[{"name":"MERC Ecological Consultants  Galway Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shane","family":"Wasik","sequence":"additional","affiliation":[{"name":"Basking Shark Scotland  Oban UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lois","family":"Flounders","sequence":"additional","affiliation":[{"name":"Basking Shark Scotland  Oban UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nuno","family":"Queiroz","sequence":"additional","affiliation":[{"name":"The Laboratory Marine Biological Association  Citadel Hill Plymouth UK"},{"name":"CIBIO, Centro de Investiga\u00e7\u00e3o em Biodiversidade e Recursos Gen\u00e9ticos, InBIO Laborat\u00f3rio Associado, Campus de Vair\u00e3o Universidade do Porto  Vair\u00e3o Portugal"},{"name":"BIOPOLIS Program in Genomics Biodiversity and Land Planning  Vair\u00e3o Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nicolas E.","family":"Humphries","sequence":"additional","affiliation":[{"name":"The Laboratory Marine Biological Association  Citadel Hill Plymouth UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Freya C.","family":"Womersley","sequence":"additional","affiliation":[{"name":"The Laboratory Marine Biological Association  Citadel Hill Plymouth UK"},{"name":"Ocean and Earth Science, National Oceanography Centre Southampton University of Southampton  Southampton UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Emily J.","family":"Southall","sequence":"additional","affiliation":[{"name":"The Laboratory Marine Biological Association  Citadel Hill Plymouth UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2022,9,8]]},"reference":[{"key":"e_1_2_9_2_1","doi-asserted-by":"publisher","DOI":"10.3750\/AIP2012.42.4.07"},{"key":"e_1_2_9_3_1","first-page":"101","article-title":"The basking shark Cetorhinus maximus (Gunnerus) in Irish waters\u2014Patterns of distribution and abundance","volume":"94","author":"Berrow S. 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