Most sharks have five gill slits on either side. But Hexanchus griseus, a giant and mysterious shark species, has an even six gill slits. These fish, appropriately called the sixgill shark, live in both tropical and temperate waters around the world and can reach up to 14-feet-long. They’ve existe d since before the dinosaurs, and yet marine biologists still don’t know very much about them.  One of the problems—for researchers, anyway—is that sixgills usually live in deep oceanic waters, at depths of up to 9,800 feet. It also doesn’t help that they usually favor extremely low-light environments. Among other reasons, these aspects make sixgills difficult to study. Sixgill sharks (Hexanchus griseus) are older than dinosaurs and are typically found in the deeper parts of the ocean. Image: Seattle Aquarium. However, these ancient giants have been spotted in Washington State’s Puget Sound year-round, and in water as shallow as 20 feet. Scientists at Seattle Aquarium believe that female sixgills are giving birth in these waters, and new research by the aquarium demonstrates that they have birthing site fidelity. According to the aquarium, they appear to come back to the Salish Sea to give birth numerous times.  Once the baby sharks—or pups—come into this world, Puget Sound turns into their nursery for some time, though researchers don’t know for how long. The young sixgills spend the summer and fall in more southern locations of the Salish Sea, and migrate more north in the winter and spring. They usually travel less than two miles a day, and frequently come up to shallow waters at dusk before going down into deeper waters at dawn, probably looking for prey.  “We think these patterns repeat until they eventually depart for the open ocean. This consistency of movement and behavior reinforces the strength of our opportunity to study sixgill sharks in Puget Sound,” according to a statement from Seattle Aquarium. “Through our research, we hope to answer questions about the life history and ecology of sixgill sharks—including migration, growth rates and prey preferences.”  The aquarium also aims to study previously unexamined physiological aspects of sixgills, and understand human influence.  The team created a custom “cradle” to safely hold a shark while they work quickly to examine it. Image: Seattle Aquarium. From May to September, Seattle Aquarium researchers and veterinarians will try to study the elusive species at three different locations in Puget Sound, going to each one once a month. There, the team will lift sharks to the surface, and either bring them onto the boat or keep them at the side of the vessel and flip them upside down. This position triggers a trance-like state in several shark species. Either way, the team will make sure that the sharks can breathe through all of those gills. Once the sharks are secured, the team will examine them. They should be able to collect measurements, obtain tissue samples, take photos, and deploy wearable tags in only five to 10 minutes. The tags that will then supply information about movement, habitat use, and feeding ecology. The scientists will then return the sharks to the open water.  “Our goal is to answer as many questions as possible,” Dani Escontrela, a researcher at the Seattle Aquarium, said in the statement. “We’re collaborating with agencies like the Washington Department of Fish Wildlife, the Big Fish Lab at Oregon State University, Point Defiance Zoo Aquarium and other researchers to fill gaps in expertise, all while keeping animal health and well-being our top priority.” The post Mysterious giant sharks that outlived the dinosaurs lurking in Puget Sound appeared first on Popular Science. ...read more read less