Annual courtship ‘torus’ behaviour of basking sharks identified in the eastern North Atlantic Ocean

Circles in the sea: Annual courtship ‘torus’ behaviour of basking sharks Cetorhinus maximus identified in the eastern North Atlantic Ocean

David W. Sims, Simon D. Berrow, Ken M. O’Sullivan, Nicholas J. Pfeiffer, Richard Collins, Kev L. Smith, Brianna M. Pfeiffer, Paul Connery, Shane Wasik, Lois Flounders, Nuno Queiroz, Nicolas E. Humphries, Freya C. Womersley, Emily J. Southall

ABSTRACT:

Groups of basking sharks engaged in circling behaviour are rarely observed and their function remains enigmatic in the absence of detailed observations. Here, underwater and aerial video recordings of multiple circling groups of basking sharks during late summer (August and September, 2016–2021) in the eastern north Atlantic Ocean showed groups numbering between 6 and 23 non-feeding individuals of both sexes. Sharks swam slowly in a rotating ‘torus’ (diameter range: 17–39 m) with individuals layered vertically from the surface to a maximum depth of 16 m. Within a torus, sharks engaged in close-following, echelon, close flank-approach or parallel swimming behaviours. Measured shark total body lengths were 5.4 – 9.5 m (mean L_T, 7.3 m ± 0.9 S.D.; median 7.2 m, n = 27), overlapping known lengths of sexually mature males and females. Males possessed large claspers with abrasions that were also seen on female pectoral fins. Female body coloration was paler than that of males, similar to colour changes seen 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-fin and fin-body contacts, rolling to expose ventral surfaces to following sharks, and breaching behaviour. Toruses formed in late summer when feeding aggregations in zooplankton-rich thermal fronts switched to non-feeding following and circling behaviours. Collectively, our observations explain a courtship function for toruses. This study highlights northeast Atlantic coastal waters as critical habitat supporting courtship reproductive behaviour of endangered basking sharks, the first such habitat identified for this species globally.

Journal of Fish Biology, DOI: 10.1111/jfb.15187

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