Fear the tails, not the jaws, of these 'weird' sharks
When a thresher shark attacks, it doesn't jump in with its teeth first. Instead, it approaches a school of fish, lowers its head, bends its body and whips its whip-like tail over its head. The blow of the thresher's tail stuns a few unlucky fish, which the shark then swallows.
This piqued the interest of Marianne Porter, a biologist at Florida Atlantic University.
“You have a shark doing extreme yoga,” said Dr. Porter. “What does the backbone look like to make that possible?”
She and her colleagues tried to answer this question in an article published this month in the journal Royal Society Open Science.
The team of Dr. Porter could tell many stories about what they had to do to obtain thresher shark vertebrae for their research. Threshers are rare and usually keep in the open ocean. Dr. Porter has never seen one in the wild. And like many sharks, they are vulnerable to extinction and therefore highly protected. The research team worked with the National Oceanic and Atmospheric Administration to access threshing samples that had been stranded on land or recovered during fishing competitions. In total, the researchers examined the vertebrae of ten threshing machines, ranging from an embryo to mature adults measuring more than 4 meters in length.
The team examined mineralized structures in the sharks' cartilaginous skeletons. Using CT scans, the researchers essentially created numerous x-ray images and compiled them into digital 3D structures.
Dr. Porter said that “the ability to do CT scans and look at all this 3D morphology and 3D anatomy” made the current moment a “great time in anatomical exploration and discovery.”
In the sharks' vertebrae, the researchers could see a “really beautiful arrangement of these mineralized plates that, if you imagine, spread out like spokes on a bicycle wheel,” says 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, hoping to find characteristics that could explain the different forms of movement performed by the different parts of a thresher shark's spine, from the standard side-to-side swings from the torso while swimming to the catapult-like thrashing of the tail.
“We've seen a lot of differences that we think will be more biomechanically stable in the main body of the shark, and it will be a lot more flexible towards the tail,” Ms Knaub said. In particular, the structure of the mineralized plates in the body vertebrae promotes greater stability in the shark's trunk, similar to struts that support a bridge. The vertebrae themselves also varied along the length of the body, with longer individual vertebrae in the torso and shorter vertebrae in the tail to provide additional flexibility.
John H. Long Jr., a biologist at Vassar College who was not involved in the research, said the project was notable for examining the hidden physical structures that contribute to the sharks' “totally crazy” tail-whipping behavior. “You don't know how this thing works until you look under the hood,” said Dr. Lung. “It's really beautiful exploratory biomechanics.”
And while thresher sharks, with their unique appearance and behavior, make flashy research subjects, Ms. Knaub notes, their exaggerated features help shed light on the way the bodies of more “normal” sharks work.
“Sometimes by studying the crazies we can gain a better understanding of the relationships between form and function,” she said.