Evidence of interactions between white sharks and large squids in Guadalupe Island, Mexico

Shark-cephalopod interactions have been documented in trophic ecology studies around the world. However, there is little information about the encounters between white sharks Carcharodon carcharias and squids in the eastern North Pacific Ocean. Here we provide evidence of interactions between white sharks and large squids in the waters of Guadalupe Island, Mexico. Through the use of non-invasive techniques, we found the presence of evident scars made by large squids on the body of the white sharks, mainly on the head and trunk, of at least 14 sharks recorded during August–December in the years 2008, 2012, 2013, 2017 and 2019. The mean length of the white sharks was 3.7 m (SD ± 0.6; total length), although the majority of the sharks with scars were adult and subadult males (n = 9; 64%). One of these males was photographically recaptured during the same season in which the individual showed new scars, confirming that the squid-white shark interaction likely occurs near Guadalupe Island. Our results highlight the importance of the twilight zone for white sharks and the use of shared habitat and trophic interactions between squid and white sharks, in which future ecosystem studies should consider both species for management and conservation purposes.


Results
A total of 14 white sharks were detected with unusual scars on their body during August-December of the years 2008, 2012, 2013, 2017 and 2019 (Table 1; Fig. 1). The average size (TL) of the white sharks was 3.7 m (SD ± 0.6), in which nine sharks (64%) were males (seven subadults; two adults) and five were females (36%; three subadults; two adults). Regarding sexual maturity, the majority of white sharks observed with scars were subadults (n = 10; 71%), although adults were also observed (n = 4; 29%). The shark WS11 was first observed with a scar on its trunk on 13 October 2017, which was detected again with the same scar during its recapture in August and September 2019. A few weeks later, this shark was registered with new scars that were first recorded in September, October and November (Fig. 1a,b), which indicates that this interaction likely occurred in Guadalupe Island between September 9-21 (Table 1).  www.nature.com/scientificreports/ The scars observed on the white sharks were double or single layers with multiple sucker marks around the shark's head and in the trunk between the dorsal and pectoral fins (Fig. 1). Wounds near the shark's mouth and trunk suggest a defensive response from squid to the white shark, although this could not be confirmed. The pattern and shape of the scars matches the suction cups on the arms and tentacles of cephalopods as observed in other shark species 17 . The estimation of the diameter of the most evident sucker marks in the shark WS11 ( Fig. 1a) allowed us to determine that the left side scar (≈ 20 cm long) had suction cups that measured 1-3 cm, while the length of the right side scar measured ≈ 60 cm long. Even though the cephalopod species could not be confirmed, observed marks from all analysed sharks could suggest an encounter with taxa such as the jumbo squid D. gigas, the neon flying squid O. bartramii, or the giant squid A. dux based on their potential distribution, arms and tentacles length, as well as the estimated size of the scars and suckers 12,16-18 .

Discussion and conclusions
Guadalupe Island is an important feeding site for white sharks in the eastern North Pacific Ocean. The simultaneous presence of pinnipeds, yellowfin tuna (Thunnus albacares), other elasmobranchs and squids is an indicator of prey availability for the white shark during subadult and adult stages. Previous studies using stable isotopes have estimated the contribution of prey such as pinnipeds and D. gigas in the muscle of white sharks analysed in Guadalupe Island 5 ; while this species of squid was also found as a prey item in the stomach of a 4.6 m female caught near California 4 . In the present study, the use of non-invasive techniques provided evidence of recent interactions between the white shark and large cephalopods. These observations highlight the importance of the mesopelagic zone for white sharks in terms of feeding, since subadult and adult sharks remain in deep waters below 100 m during the day 9,19 , similar to what has been recorded in other areas such as the SOFA 6,7 . In addition, the usual depth range of the probable species of cephalopods during daytime is 300-600 m for O. bartramii 20 and 200-300 m for D. gigas 15 , which is consistent with the patrolling depths reported for the white shark in the area 7,9 and coincides with the bathymetry of Guadalupe Island.
Squid interactions have been widely observed in other large marine predators such as sperm whales Physeter macrocephalus 21,22 . These interactions have been confirmed by the presence of squid beaks in the stomachs of sperm whales, in addition to the scars observed on the head and body of these cetaceans 21,22 . The fact that squid cause these marks on sharks suggests an extremely aggressive encounter between predator and prey, in which the defensive scars protrude on the head, gills and body of the white shark. The suction power of the arms and tentacles of large squids is likely to deform the structure of the shark dermal denticles and hence the scars, and in some cases generate open wounds depending on the intensity of the embrace 23 .
The consumption of cephalopods could be essential in the diet of the white shark by allowing a quick digestion and absorption due to the large amount of proteins and low lipid content present in this group of invertebrates 3,[24][25][26] . In addition to the protein content, it has been suggested that some cephalopod species such as D. gigas contain high contents of essential fatty acids for shark reproductive processes, such as gonadal maturation and embryonic development, which could favour spermatogenesis in subadult males and pregnancy in adult females [24][25][26][27] .
The presence of scars caused by large squid in 14 sharks during the study period could seem a low number compared to the estimated number of white sharks in Guadalupe Island, which has been calculated to be 120 sharks 28 . However, the presence of scars could not be visible in all white sharks that prey on squid, as this will depend on the intensity of the interaction, effectiveness of the attack and the handling of the prey. The frequency of these interactions and their relevance in the trophic ecology of the white shark can be studied by tagging both sharks and squid, as well as complementary studies on fatty acids and stable isotopes of carbon and nitrogen 18,24,26 .
The white sharks from Guadalupe Island have a segregated migration in which subadult and adult males arrive in August and adult females in late September 8,9 . In this study, most squid scars were observed in subadult male sharks throughout the season, suggesting that their arrival to Guadalupe Island could be related to the consumption of squids and tunas, while the Northern elephant seals Mirounga angustirostris arriving in December would complement the diet of adult sharks in a dietary shift related to their maturity 4,5,9 . This emphasizes the importance of habitat use for both white sharks and squids, where future conservation-related studies should include the potential impacts of both prey and predators distribution, mainly in terms of oceanographical changes related to temperature, productivity and minimum oxygen layers 6,9,[14][15][16]26 . This type of research will also be able to elucidate evolutionary aspects of the interaction between sharks and cephalopods, since both lineages are ancient and their study could provide relevant information on the ecological importance of both taxa in the dynamic exchange of matter and energy in marine ecosystems 6,12,18,26 .

Materials and methods
Guadalupe Island is located 240 km west from the closest point to the occidental coast of Baja California, Mexico (Fig. 2). This volcanic island is the nucleus of a biosphere reserve that presents a high diversity of seabirds, pinnipeds, bony fishes, elasmobranchs, marine invertebrates, and seaweeds 29 . As it is not on a continental shelf, the island has great depths near the coast, which exceed 1000 m of depth 29 . The average sea surface temperature is 18 °C with ranges between 16 and 20 °C, although maxima of up to 25 °C have been reported 29,30 . The island measures 32 km long and 6.5-9.5 km wide, although the white shark cage diving activity takes place in the northeast of the island in a bay known as Rada Norte (Fig. 2) mainly during the months of August-November 31 .
A set of approximately 2500 underwater pictures of white sharks were obtained during several expeditions onboard the tourist boats that visited the northern area of Guadalupe Island (Fig. 2) throughout August-December (2008-2019). The procedures for estimating total length, sex and photoidentification of white sharks were followed as indicated by Becerril-García et al. 31 , while the determination of sexual maturity was made according to Bruce and Bradford 32 . Obtained underwater photographs and videos were analysed, in which scars and www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.