Circles in the sea: annual courtship “torus” behaviour of basking sharks Cetorhinus maximus identified in the eastern North Atlantic Ocean

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 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-fin and fin-body contacts, rolling to expose the 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, 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.

Manta and devil ray aggregations: conservation challenges and developments in the field

“…We propose aggregation as the most general term, which can be used to describe any group of individuals that forms for any purpose (Johnson et al, 2002;Guttridge et al, 2009;Guttal and Couzin, 2010;Jacoby et al, 2012). Some aggregations can be social groups, within which individuals exhibit interactive behaviors such as courtship, mating (Sims et al, 2022), or cooperative feeding (Mourier et al, 2012), and dynamic associative relationships among individuals may be present (Guttridge et al, 2009;Jacoby et al, 2012;Perryman et al, 2022). In several marine species including elasmobranchs, these associations have been described as fission-fusion dynamics, where groups merge (fusion) or split (fission) depending on various factors including the costs and benefits of the size of the group (e.g., increased competition for food versus reduced predation risk) (Haulsee et al, 2016;Perryman et al, 2019).…”

“…It is well documented that many elasmobranchs form large aggregations, varying from temporary groups to structured, longterm associations (e.g., hammerhead sharks, basking sharks, blacktip sharks, and common stingrays) (Klimley, 1987;Ward et al, 2004;Croft et al, 2006;Ward et al, 2007;Croft et al, 2011;Crowe et al, 2018;Chaikin et al, 2020;Ayres et al, 2021;Sims et al, 2022). The functions of such aggregation behaviors have been hypothesized to include reproduction (Klimley, 1987;Chaikin et al, 2020;Sims et al, 2022), feeding (De-la-Parra-Venegas et al, 2011), energy conservation, (Klimley and Nelson, 1984;Economakis and Lobel, 1998;Hight and Lowe, 2007), refuge from predators (Heupel et al, 2007;McAllister et al, 2017) and social learning (Sih et al, 2009;Brown and Laland, 2011). The formation of these aggregations can be seasonal and is often linked to environmental factors and life history stages (Rohner et al, 2013;Kajiura and Tellman, 2016), food abundance (Clua et al, 2013;Hacohen-Domenéet al, 2015), critical habitats (Oh et al, 2017;Chiriboga-Paredes et al, 2022) and reproduction (Heupel and Simpfendorfer, 2005;Reyier et al, 2008).…”

Sex or snack? Drivers behind the behavioural mystery of basking shark circles revealed

“…However, in this issue, Sims et al . (2022) provide the most comprehensive characterisations of these rarely documented aggregations to date.…”

“…Harnessing the revolution in drone monitoring (Coops et al ., 2019), in combination, crucially, with underwater recordings, Sims et al . (2022) highlight key characteristics of the basking shark torus in a series of sightings off the Irish coast. Authors describe that groups numbered between 6 ‐ 23 individuals, with individuals 5.4 – 9.5 m in length, consisted of both male and females.…”

“…Since one of the first records in 1950 (Matthews, 1950), it has been recorded several more times, interpreted as courtship, mating behaviour, and group feeding. However, in this issue, Sims et al (2022) provide the most comprehensive characterisations of these rarely documented aggregations to date.…”