- Sixgill sharks have been sighted in Puget Sound and scientists didn't understand why the sharks were there.
- Genetic tagging shows that most of the sharks are actually brother and sister.
- The siblings appear to stick together in the area where they were born.
For years, scientists have been baffled by sightings of the sixgill shark in Puget Sound, off the coast of Seattle. The heavy-bodied sharks, which reach up to 13 feet in length, are found worldwide at the bottom of the ocean. But scientists couldn't understand why the sharks inhabited the waters of Elliot Bay and Puget Sound.
"The presence of the shark here in Elliot Bay and Puget Sound was kind of a little-known secret among scientists, commercial fishermen, and maybe local scuba divers, and the reason it's here is part of the big mystery," said Jeff Chrstiansen, a senior biologist with the Seattle Aquarium.
One reason, it turns out, is that the prehistoric-looking shark knows something about family.
By setting up a genetic tagging station in the Sound, researchers with the aquarium, along with help from NOAA and the Washington Dept. of Fish and Wildlife, learned that most of the sharks in the area were actually brothers and sisters.
Mother sharks return to Puget Sound to give birth to pups, and then these siblings make a point of sticking together.
"The sweeping majority of the sharks that you're seeing on the video monitors that come into our research events are brothers and sisters out of the same litter," said Chrstiansen. "And it appears that these animals are staying together as a family, not with mom, but as brothers and sisters, hanging out and growing up in loosely associated groups."
Christiansen points out that this is an area of shark research that has gotten little attention.
"What we're interested in is if we can flip this observation around: Are the relationships we're seeing between brothers and sisters traveling together and staying together as juveniles transferable to other shark species as well?" said Christiansen. "We don't know; we'd like to see some effort going into other shark species to see if that's present."
Sixgill sharks are so named since they have six gill slits, unlike most other sharks which have only five. It's also known as the cow shark or mud shark. This primitive species is one of the few surviving members of the Hexanchidae family and is related to today's dogfish and Greenland shark, although it's more closely related to species found only in fossils. Some of the sixgill shark's relatives date back over 200 million years. Females are usually larger than males, reaching lengths of up to 18 feet. Despite their large size and sluggish movement, they are capable of bursts of speed to catch prey.
The sixgill shark family groups in Puget Sound seem to come in waves. Some years there are a lot of them, while in other years, they're almost nonexistent. That's why the aquarium built a research station where the sharks literally come to them. Christiansen explained the aquarium's Sixgill Shark exhibit doubles as the actual research station.
"We switch these cameras from rolling video tape to live mode, and we'll put down a bait attractant to see if we can attract any of the sharks in the local area to come close enough to the very end of our pier -- 60 feet down -- where we can get a chance to put visual marker tags on them and get genetic biopsy tissue samples," explained Christiansen.
A shark cage mounted on the bottom of their station allows them to dive down and examine the animals up close.
Among the questions the team is hoping to answer is how many of the sharks are in the area, what they're doing and what their movement patterns are.
The Seattle Aquarium researchers also want to take their studies to other parts of the world to find out if their findings on family travel among the Puget Sound sharks applies to sixgills in other parts of the world.
"Sixgill Sharks live pretty much wherever the ocean bottom is," said Shawn Larson, a curator at the Seattle Aquarium. "They're known into water hundreds (of feet) to thousands of feet deep; the maximum depth range of the shark really is unknown because we really haven't sampled that much in extremely deep water."