Gallagher, A. J., De Silva, C., Delaney, D., Harris, S. D., Phillips, B. T., Shipley, O. N., … & Giddens, J. (2024). Novel behavioral observations and body scarring for the bluntnose sixgill shark (Hexanchus griseus) offer clues to reproductive patterns and potential mating events. Frontiers in Marine Science, 11, 1305487.
Human beings have come far in their conquest to explore the earth, but are still facing the final frontiers on this planet. After all, water covers roughly 71 percent of the earth’s surface (most of it within the oceans) but only 5 percent of the global seafloor has ever been mapped with high enough resolution to observe any detail.
Even less is known about the living beings that inhabit the watery depths. When scientists surmise that only one third of the estimated millions of creatures and microorganisms found in the oceans have been documented, it’s less surprising that humans are in the dark. So when researchers get the opportunity to study a new species from the deep, they can often be left to with more questions than answers.
Giants of the Deep
This is the case for the bluntnose sixgill shark (Hexanchus griseus,) one of the ocean’s largest deep-sea shark species discovered. A recent study published in Frontiers in Marine Science sheds light on some cryptic mating characteristics of this elasmobranch, thanks to some unexpected deep sea observations.
The bluntnose sixgill shark boasts some baffling biological qualities. It has been found living in many oceans around the globe at depths of 180–1,100 m (590–3,610 ft) to 2,500 m (0–8,202 ft) and have room to grow up to 6 meters (20 feet) in length. What’s impressive are their staggeringly large litters of 50 to 100 pups, but they reach maturation at quite a late age of up to 26 years old. While information about their habitat and feeding behavior has been documented, their mating habits and breeding grounds have remained a mystery until recently.
Romancing a Predator
In November of 2021 in the Exuma Sound of The Bahamas, researchers deployed deep baited remote underwater video systems or “dBRUVs.” Their cameras captured rare footage of three individual sharks at depths up to 1110 meters, including two females in different life stages. But what intrigued scientists most were the distinctive markings on their bodies that became the first clues to potential mating behavior in this elusive species.
One female, estimated to be around 3.5 meters long, exhibited prominent scars across her body, particularly around the gills, pectoral fins, and pelvic fins. These scars bore a strong resemblance to puncture wounds and lacerations due to bites left by another sixgill shark’s distinctive teeth. While it is possible that these kinds of markings could also come from predation or competition, researchers believe they are likely the result of recent coercive mating attempts, based on established criteria used to identify similar marks in other shark species. Why these particular predators display this behavior is still relatively unknown, but the presence of these mating scars is still quite significant.
These marks suggest the Bahamas’ Exuma Sound could be a potential mating ground for these deep-sea giants. While the exact location of such grounds has long been a subject of speculation, scientists previously relied on capturing sharks at different life stages to make educated guesses. This study highlights the valuable role that new deep-sea camera technology can play in directly observing the behaviors of deep sea creatures in the future.
Beyond the Scars
The study also offers valuable insights into the challenges of studying deep-sea populations. An extensive review by the researchers revealed a not-so-surprising lack of scientific studies focused on the reproductive behavior of sixgill sharks, with most past research concentrated in the Pacific Ocean. This underscores the critical need for further exploration using more advanced tools like deep-sea camera systems.
Understanding the reproductive behavior of these apex predators will be absolutely crucial for conservation. Sixgill sharks are particularly vulnerable due to their late age of maturity, and even with large litter sizes their reproduction rates are slow. It is clear more information on their mating behavior and the whereabouts of their mating grounds could inform targeted conservation efforts to better protect these sharks.
While deep sea discovery is only in its genesis, every piece of new information feels like a rare gem of revelation, a glimpse through the dark waters of the wildy unknown on humanity’s own planet. It is a reminder that the world is still a vast place for discovery, and there is more work to be done to protect and conserve these final frontiers.
I’m a California native with a lifelong curiosity for all things related to the ocean. I got my bachelors in Marine Biology from the University of California Santa Cruz, and I’m currently pursuing a masters degree in Animal Science at the University of Idaho where my main focus of study is fish nutrition in aquaculture. My favorite subject to study outside of school is the deep sea. I enjoy learning about new mind boggling species, the latest discoveries of the deep, and the history of deep sea pioneers, research and technology. If I’m not studying the mysteries of the ocean, I’m probably roller skating or watching scary movies.
