Big-eyed thresher shark provides students rare study opportunity

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A male thresher shark has washed up on Crystal Crescent Beach—35 minutes from central Halifax. The shark was discovered by beachgoers Tuesday night and brought to the attention of Dalhousie veterinarian, Chris Harvey-Clark by the Department of Natural Resources.

Harvey-Clark was thrilled about the discovery saying, “It’s quite exciting… [there are]not too many records of this type of shark.”

credit: nikki beauchamp

Thresher sharks, known for their long tails—which can be as long as the shark itself—like warm water and are most commonly found in tropical zones globally. The particular species discovered this week is a big-eyed thresher shark (Alopias superciliosus), characterized by large eyes that can rotate upwards and are adapted to hunt in low-light, pelagic zones. They are very uncommon in Canadian waters. Big-eyed thresher sharks are listed as Vulnerable by the International Union for the Conservation of Nature (IUCN) due to their limited capacity to recover from moderate exploitation.

“Some threshers are visitors—they’re not common by any means. There are records, but they’re not something you see an awful lot of. Most people would never see one in their lives outside of fishermen,” says Harvey-Clark.

The specimen provided a unique opportunity for Dalhousie biology students to learn about thresher sharks. With the shark’s arrival to the Dalhousie Biology Department, Harvey-Clark jumped at the chance to perform a dissection for students of the Dalhousie shark class. “Anything beachcast is usually rotten, but this guy is in lovely shape to study its anatomy. Common threshers are not that common,” he says. “But this one, it’s a rare species, it’s terribly exciting for students to see something rare and so intact. All the structures are there; we’re able to tell a full story.”

Thresher sharks, also known as long-tailed sharks, use their caudal (tail) fin to slap and stun smaller prey fish. “The anatomy of how they do that isn’t well described,” says Harvey-Clark. “It’s fascinating to be able to figure out the biomechanics of this shark using its strong dorsal muscles like a whip.”

The big-eyed thresher shark’s eyeball as examined during the necropsy performed by Dr. Chris Harvey-Clark

One interesting discovery from the necropsy was the shark’s orbital rete—a small spongey structure in the shark’s eyes that directs ambient heat to the retina. Warming of the shark’s retina helps processes visual information, like detecting prey, in the colder, darker pelagic ocean zones.

The shark was likely hooked by an offshore pelagic longliner vessel catching tuna or other large open-sea commercial species. “Sharks are here, they’re offshore not inshore for the most part.” Deep-sea longlines are the second most common cause of shark mortality next to gillnet fishing, and represent a significant risk to many overexploited shark species globally. Sharks are long-lived and have slow reproduction rates. “There used to be so many other shark species around here,” says Harvey-Clark. “These animals are becoming scarce for a number of reasons, fishing becoming one of them.”

“Their conservation is an extremely important research question, and needs a research answer.”

 

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