These sharks get help swallowing from their shoulders
Slurp in, and work those shoulders. It might not be glamorous, but that’s how one species of shark gets its dinner. A white-spotted bamboo shark doesn’t rip its prey apart. Instead, it opens its mouth very wide, very fast. The sudden movement creates suction, bringing water — and any food within it — into the shark’s hungry maw. But to move the food farther down toward the stomach, the mouth needs a little help. Some of that comes from a movement of the shark’s “shoulders,” a new study shows.
In people, shoulders are where the arms attach. Sharks have pectoral fins instead of arms, and instead of attaching to shoulders, those fins attach to a part of the skeleton called the pectoral girdle. This makes the pectoral girdle “kind of like the shoulders” of a shark, explains Ariel Camp. She studies biomechanics — or how living things move — at Brown University in Providence, R.I.
Because the pectoral fins are attached to the pectoral girdle, many scientists have studied this part of the shark skeleton to investigate how sharks swim. But no one had looked at a shark’s shoulders when it comes to feeding.
“Fish [including sharks] don’t have a tongue like we do,” Camp notes. Humans and other animals use a tongue to move food around in the mouth. Sharks instead probably “carefully control the flow of water in the mouth and use that to move food around,” she says. Camp wondered if the shark’s shoulders might help it control that water — and food — in their mouths.
Her team brought three white-spotted bamboo sharks into the lab. These reef sharks are long and narrow, growing to about 93 centimeters (37 inches) long. Using high-speed X-rays, they created a video of the hungry sharks’ skeletons as they were sucking up their food. Then they used the video to create a computer model of a what the skeleton does when a shark takes a bite.
As the shark’s mouth opens, water and any food rushes in. Then, the model showed, as the mouth begins to close, the pectoral girdle moves forward and back. “We found that they are swinging back a bit after the big suction event,” says Camp. So it seems, she says, that the shoulder shrugging is not helping as much to getting the food in the mouth as it is to moving food toward the stomach.
For two of the sharks, that movement was something like a reversed shrug. The pectoral girdle moved forward toward the shark’s head and then dropped back toward the stomach. Camp says the third shark, though, “did his own thing.” It just swung the shoulders backward. “I wouldn’t say [this motion is] replacing a swallow, but it is helping” move the food , she says. Camp and hercolleagues published their results July 26 in the journal Proceedings of the Royal Society B.
“This is very cool,” says Misty Paig-Tran. She’s a marine biologist who studies sharks at California State University in Fullerton. She was not involved in this study. “There’s still so much we don’t know about the basics of how organisms do what they do,” she says.
The new study shows that “similar anatomy can work in completely different ways,” Paig-Tran says. Sharks can use their shoulders for swimming. They can use them for feeding. Or they can work their shoulders both ways. The result, she explains, is that sharks use their belly muscles and shoulder muscles to gulp down some food.
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anatomy (adj. anatomical) The study of the organs and tissues of animals. Or the characterization of the body or parts of the body on the basis of its structure and tissues. Scientists who work in this field are known as anatomists.
biomechanics The study of how living things move, especially of the forces exerted by muscles and gravity on the skeletal structure.
cartilage A type of strong connective tissue often found in joints, the nose and ear. In certain primitive fishes, such as sharks and rays, cartilage provides an internal structure — or skeleton — for their bodies.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
locomotion The ability to move from place to place.
marine biologist A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
muscle A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
pectoral fins The fins that emerge from the side of a fish, just behind its head. They help direct the animal’s motion. They effectively correspond to the arms (or forelimbs) of a land animal.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
reef A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.
sharks A family of primitive fishes that rely on skeletons formed of cartilage, not bone. Like skates and rays, they belong to a group known as elasmobranchs. Then tend to grow and mature slowly and have few young. Some lay eggs, others give birth to live young.
X-ray A type of radiation analogous to gamma rays, but having somewhat lower energy.
Journal: A.L. Camp et al. Dual function of the pectoral girdle for feeding and locomotion in white-spotted bamboo sharks. Proceedings of the Royal Society B: Biological Sciences. Vol. 284, July 26, 2017. doi: 10.1098/rspb.2017.0847.