When a thresher shark attacks, it doesn’t lunge in teeth-first. Instead, it approaches a school of fish, lowers its head, flexes its body and thwaps its whiplike tail over its head. The smack of the thresher’s tail stuns a few unlucky fish, which the shark then gobbles up.
This piqued the interest of Marianne Porter, a biologist at Florida Atlantic University.
“You’ve got this shark doing extreme yoga,” Dr. Porter said. “What does its backbone look like to make that happen?”
She and her colleagues tried to answer this question in a paper published this month in the journal Royal Society Open Science.
Dr. Porter’s team could tell quite a few tales about what they had to do to obtain thresher shark vertebrae for their study. Threshers are rare and tend to keep to the open ocean — Dr. Porter has never seen one in the wild. And like many sharks, they’re vulnerable to extinction and thus highly protected. The research team collaborated with the National Oceanic and Atmospheric Administration to gain access to thresher specimens that had stranded onshore or been salvaged from fishing competitions. All in all, the researchers examined the vertebrae of 10 threshers, ranging from an embryo to full-grown adults over 13 feet long.
The team examined mineralized structures within the sharks’ cartilage skeletons. With CT scanning, the researchers essentially created countless X-rays and compiled them into digital 3-D structures.
Dr. Porter said that “the ability to CT scan and look at all this 3-D morphology and 3-D anatomy” made the current moment a “great time in anatomical exploration and discovery.”
Inside the sharks’ vertebrae, the researchers could see a “really beautiful arrangement of these mineralized plates that kind of spread out, if you imagine, like spokes on a bike wheel,” said Jamie Knaub, a doctoral candidate at Florida Atlantic University and an author of the study.
The scientists counted these plates and examined their physical structures, in hopes of finding features that might explain the different forms of movement performed by the different sections of a thresher shark’s backbone, from the standard side-to-side oscillations of the torso in swimming to the catapult-like thrashing of the tail.
“We saw a lot of differences that we think will biomechanically act more stable in the main body of the shark, and then towards the tail, it’s a lot more flexible,” Ms. Knaub said. Notably, the structure of the mineralized plates in the body vertebrae promotes greater stability in the shark’s trunk, akin to struts supporting a bridge. The vertebrae themselves also varied along the body’s length, with longer individual vertebrae in the trunk and shorter vertebrae in the tail to provide additional flexibility there.
John H. Long Jr., a biologist at Vassar College who was not involved in the study, said that the project was noteworthy for exploring the hidden physical structures that contribute to the sharks’ “totally crazy” tail-whipping behavior. “You don’t know until you look under the hood how this thing is working,” Dr. Long said. “It’s really beautiful exploratory biomechanics.”
And while thresher sharks, with their unique appearance and behavior, make for flashy research subjects, Ms. Knaub notes, their exaggerated features help shed light on how more “normal” sharks’ bodies work, too.
“Sometimes studying the weirdos allows us to get a greater understanding of form-function relationships,” she said.
