The study of large marine organisms is limited due to the difficulty of making field observations throughout the expansive, deep and relatively opaque ocean environment. While significant advances in electronic tagging and molecular techniques have broadened the scope of questions that can be effectively addressed, major aspects of life history remain unknown. White sharks (Carcharodon carcharias) in central California, for instance, are one of the most studied elasmobranch species with over 21,000 instrumented tracking days (Andrzejaczek et al., 2022), documented regional population trends (Kanive et al., 2021), and a sequenced genome (Marra et al., 2019). Despite decades of dedicated research, targeted tourism, and natural history observation (Jorgensen et al., 2022), significant gaps remain in our basic knowledge of their natural history – for example, no one knows the gestation period of a white shark, where they give birth, when and where they mate, and how they interact with prey, predators and each other (Jorgensen et al., 2022). Over the course of these efforts, a proliferation of photographic devices with increasing miniaturization and ever-improving picture resolution has resulted in vast image libraries documenting individual white sharks repeatedly photographed over decades (Anderson et al., 2011). Our observations of white sharks off Central California, documented in an image library, reveal a large number of injuries, wounds, and scars that occur and heal at regular intervals. The unique patterns and the timing of these marks may provide novel insights into the life history of white sharks that have remained undescribed. Additionally, similar libraries from other regional populations (Towner et al., 2012) can provide insights into region specific as well as broad comparisons of life history patterns. However, before we can use these scars to better understand white shark life history, the source of the wounds must first be determined and systematically classified.

To date, a modest number of studies focused on wounds or scars on sharks and marine mammals have made progress in identifying their causes. Injuries resulting from boat strikes and subsequent healing rates have been described on white sharks (Towner et al., 2012) as well as whale sharks (Speed et al., 2008) and manta rays (McGregor et al., 2019). Diagnostic wounds made by propeller blades are reflected by sets of evenly spaced parallel cuts approximately perpendicular to the direction of vessel travel (Rommel et al., 2007; Towner et al., 2012). Scarring on white sharks consistent with interaction with large squid have also been reported (Becerril-García et al., 2020). Squid marks bear a distinctive pattern with rows of circular scars tapering in diameter consistent with suckers ringed with sharp teeth, that flank the arms and tentacles of larger squid species. Small bites extracted by cookiecutter sharks, common on open ocean cetaceans, have also been identified on white sharks (Hoyos-Padilla et al., 2013). Killer whales are a known white shark predator (Jorgensen et al., 2019), and a failed predatory attempt can leave the signature impression of their tightly spaced pointed teeth (De Maddalena, 2023).

In this paper we propose a systematic classification system to describe the different types of wounds and scars commonly observed on white sharks in the northeastern Pacific associated with prey handling, parasites, conspecific aggression and anthropogenic impacts. The systematic approach described in this study includes a dichotomous key for more efficient and consistent scar classification which may be applicable to numerous other marine organisms where long-term image documentation is obtainable.

From 1987 to 2024, while conducting seasonal research on white sharks (Carcharodon carcharias) around seal rookeries in central California (Año Nuevo Island, Southeast Farallon Island, Point Reyes, Tomales Point, and Monterey Bay), we archived hundreds of hours of underwater video recordings of individual sharks (Anderson et al., 2011; Chapple et al., 2011; Kanive et al., 2015, 2019). White sharks were attracted toward our research boat using a seal decoy made from outdoor carpet and deployed with a standard medium tackle rod and reel. In most cases a small (~2kg) piece of marine mammal blubber (use permitted by National Marine Fisheries Service) was tethered to the boat to provide a localized olfactory attractant to increase the shark’s proximity to the boat once they approached. In Monterey Bay, no decoy or attractant was used and sharks were visually spotted at the surface and approached. With the shark near the boat, we recorded video images using a ‘dip camera’ attached to a 2–4 m pole (camera specifications varied over this period but should not impact the results of this study). Whenever possible, both sides of the shark were imaged, with special attention given to the dorsal fin profile, for individual identification, and the rear ventral area, to determine sex (Anderson et al., 2011).

While individual ID and sex were initially the primary reason for video documentation, many different types of wounds and scars on the white sharks were recorded. To systematically classify the various scars and wounds, we reviewed a selection of clear scar images and matched the wound impressions with candidate causes based on known interactions classes that could result in injury, these included, 1) Conspecific - e.g. these were aggressive bites or mating hold bites, 2) Prey handling - e.g. scratches or bites from their preferred pinniped prey, 3) Environmental – e.g. scrapes from contact with rocky reef, 4) Parasitic - e.g. cookiecutter shark bites or copepod attachment, and 5) Anthropogenic - e.g. gear entanglement or boat strikes. To facilitate standardizing scar classification, we developed a dichotomous key based on a series of yes/no questions about the size, depth and pattern of an observed wound or scar.

Between 1987 and 2022 we recorded over 2500 video clips documenting more than 500 unique individual white sharks in central California. Reviewing every scar was beyond the scope of this study, however from a representative collection of scar observations across a range of age, sex, and locations our sorting by similar scar and wound shapes resulted in 12 observed injury sources, each falling under one of the following five broad interaction classes (Table 1).

We identified five interaction classes of scars/wounds on white sharks in Central California from underwater video (Conspecific, Prey Handling, Environmental, Parasitic, and Anthropogenic), which each described the interaction attributed to one or more of the distinctive scar types. To more efficiently and consistently classify subsequent wounds and scars, our dichotomous key uses a series of yes/no questions about the size, depth and pattern of an observed wound or scar ( Figure 2 ). Some scars will inevitably be less clear because of poor imaging or due to an unusual shape and cannot be classified. In addition, our scheme does not identify all injury types. Still, this work provides a simple and standardized classification system, identifying the most common and recognizable scars/wounds found in these sharks. We further identify their likely causes, either through direct observation or induction, or building on previous work to create a foundation for future studies utilizing scars/wounds to inform our understanding of white shark life history.

The categories of this simplified classification system for scars/wounds on white sharks can also be applied to numerous other species where long-term photographic records can be obtained. For example, many large surface-feeding sharks incur boat-strike injuries (Womersley et al., 2022; Chapple et al., 2024) and interact with static fishing gear and most species are subject to parasite and mating scars. Additional categories may also be added in order to further refine species or population specific scar/wound patterns.

This work sets the foundation for continued quantitative analyses of scars/wounds on white sharks with larger datasets and for other populations globally. White sharks heal at generally predictable rates (Jewell et al., 2011; Towner et al., 2012; Chapple et al., 2015), including animals that have experienced significant injury. All wounds that break the skin undergo a visually distinctive healing progression reflected in the coloration ranging from red or pink when fresh, to white and then black as they heal. The black areas fade over many years to the original skin pigmentation. However, if the wound is not obscured by other newer scars, they may remain visible for decades. The white underside of the shark will eventually heal to white accordingly. As such, future studies can capitalize on both the rate of healing and our categorization scheme to identify when, where, and how various scars/wounds occurred thus informing further understanding of their life history. For example, it is uncertain where northeast Pacific white sharks mate (Jorgensen et al., 2010, 2012). Combining known movement data with data on the occurrence and freshness of mating scars and other scar classifications, researchers can infer the timing and location mating is likely to occur (Gallagher et al., 2024). For instance, if mating marks and cookiecutter bites are healed to a similar degree it could indicate those interactions occur at a similar time and place in the migratory cycle - in the open ocean environment. Conversely, if fresh wounds inflicted by coastal pinniped prey coincide with observed fresh mating bites on females then those interactions might be likely to co-occur in space and time. Similarly, identifying prey handling scars/wounds in smaller sharks can indicate the timing of ontogenetic diet shifts. In summary, this work provides a foundational framework for future studies utilizing available data on scars/wounds to inform our understanding of the cryptic life of white sharks and other elasmobranchs more widely.