You might not ordinarily give your tongue much attention, but the solid piece of muscle is used for all sorts of things, not least swallowing food. So spare a thought for those animals that lack a tongue and yet still need to chow down to stay alive. It turns out that some sharks may instead shrug their shoulders to help them swallow.
Bamboo sharks are one of a few shark species, as well as quite a few bony fishes, that use suction to gobble up prey. When a tasty-looking creature has been identified, the sharks sidle up to it, before rapidly opening their mouths. This creates a vacuum in their mouths and the prey is slurped up as the water rushes in.
Ray-finned fish, which belong to the group that contains the vast majority of fishes alive today, have a pectoral girdle that is connected directly to the back of the skull. As these fish open their mouths to feed via suction, the joining of the bones enables the body's muscles to power the rapid expansion of the mouth, increasing the suction and thus chance that food will enter the mouth.
But in sharks, which evolved before the emergence of ray-finned fish, the pectoral girdle (also used to control the front fins) is not connected to the skull, meaning researchers did not think the muscles were involved with increasing suction when the animal opens its mouth.
However, by imaging bamboo sharks using CT scans and high-resolution X-ray movies as they fed, researchers found that despite the girdle not being attached to the skull, sharks can still use it to increase suction, simply by moving it backwards. Interestingly, they found that this occurred not when their mouth gape was widest as would be expected, but just afterwards, suggesting it is instead used to force the food down the throat and swallow.
“The girdle shows up [in the fossil record], around the time that jaws evolved,” explains Ariel Camp, co-author of the study published in the Proceedings of the Royal Society B, in a statement. “We aren't sure exactly what structures it evolved from or how that happened. Part of understanding that history is understanding what were the functions this structure had to carry out.”
As sharks are one of the earliest groups of animals to have evolved jaws, understanding how their mouths function can provide valuable insight into where they might have come from. It seems that in some cases, one of the functions may well have been feeding, in addition to that of movement.