Novel behavioral observations and body scarring for the bluntnose sixgill shark (Hexanchus griseus) offer clues to reproductive patterns and potential mating events

Year: 2024

Abstract

There are significant calls to scale efforts to advance our understanding of fundamental biological, ecological, and taxonomic information in the deep-sea to inform conservation and decision-making (Howell et al., 2020). There is growing interest into ocean depths between 200-1,000 m, building on recent research which suggested this depth band may contain up to 10 times the global fish biomass in all other marine habitats combined (Irigoien et al., 2014). A rich diversity of large marine predators is commonly found throughout this portion of the vertical water column, inspiring questions about the functional and ecological significance of this habitat (Braun et al., 2022).

The bluntnose sixgill shark (Hexchanchus griseus) is one of the largest species of deep-sea elasmobranchs (Barnett et al., 2012), reaching a confirmed total length of 482 cm (Bolivar, 1907), but with an unconfirmed report of 550 cm total length (Ebert and Compagno, 2012). The species is found circumglobally within tropical, temperate, and boreal latitudes (Finucci et al., 2020). While the species is known to have extremely large litters (47-108 pups, Ebert and Compagno, 2012), research also suggests a late age-at-maturity (females = 26.5 years; COSEWIC, 2007) and size at maturity (females at or near 400 cm TL; Ebert, 2002). The combination of these life history traits limits their population rebound potential, thus reducing their ability to sustain high levels of exploitation (Finucci et al., 2020). Due to its size and catchability, H. griseus is one of the better-studied deep-sea elasmobranchs, with a growing body of knowledge into its habitat use (Brooks et al., 2015Comfort and Weng, 2015Coffey et al., 2020) and trophic ecology (Reum et al., 2020). However, many aspects of H. griseus biology remain cryptic. For example, there is limited information on mating events, and thus the potential locations of reproductive grounds remain mostly speculative across its range (e.g., Ebert, 2002Amor et al., 2017). The recent emergence of compact, innovative deep-sea observation platforms, such as video-equipped landers, is now rapidly improving our ability to detect rare deep-sea shark species’ biodiversity and new record behaviors performed in the deep ocean (e.g., Phillips et al., 2019Gallagher et al., 2023).

Here we present rare observations of H. griseus from The Bahamas, including reports of body scarring, which provides some of the first clues to the spatial and temporal basis for mating behavior in a deep-sea elasmobranch. Given the challenges of determining where potential reproductive behaviors take place for deep-sea vertebrates, these observations also highlight how non-invasive remote lander video systems can provide timely information on critical habitat for poorly known species of deep-sea elasmobranchs.