Some reef fish have the unexpected ability to move their jaws from side to side, biologists at the University of California, Davis have discovered. This ability – which is rare among vertebrate animals – allows these fish to feed rapidly and efficiently on algae growing on rocks. The work is published May 5 in Proceedings of the National Academy of Sciences .
Being able to move your jaw from side to side might not seem surprising from a human point of view, but if you look across all vertebrate species – half of which are fish – it is almost unique, said Peter Wainwright , professor in the Department of Evolution and Ecology at UC Davis and senior author on the paper.
For mammals, being able to move our lower jaw from side to side as well as up and down allows us to chew our food before swallowing it. That has helped make mammal species the dominant herbivores on land.
"It's extremely unusual among vertebrates and has interesting ecological and evolutionary consequences when it does occur," Wainwright said.
Most fish species feed on prey floating freely in the water by sucking in a mouthful and trapping anything inside it. But on reefs, about half of fish species feed by grazing on algae or other organisms growing on the reef.
Captured on high speed video
Michalis Mihalitsis, at the time a postdoctoral researcher in Wainwright's lab, used high-speed video to observe feeding in Zanclus cornutus or "Moorish Idol," a spectacular reef fish with an elongated snout. He discovered that Zanclus can move both its upper and lower jaws from side to side, allowing it to tug food items off a surface or work its mouth into cracks.
"I thought, holy cow, that's not something we expected," Wainwright said. CT scans and dissections confirm that Zanclus' upper and lower jaws are capable of sideways movement. The movements are much too rapid to see with the unassisted eye.
Mihalitsis and Wainwright then looked at the closely related surgeonfish, a large family of fishes that are the dominant herbivores on reefs. Surgeonfish feed by browsing on the "turf" of algae growing on rocks and dead coral.
The researchers found that surgeonfish also can move their upper jaw sideways, allowing them to bite food off a surface.
"They take a mouthful of algae and move their jaws to pull it off the surface," Wainwright said. They can also slice through strands of algae caught in their teeth.
"For Zanclus, this adaptation seems to relate to where they feed (reef cracks and crevices), whereas for the surgeonfish, how efficiently they feed," Mihalitsis said.
The researchers are now looking at other benthic-feeding fish that could have adaptations of the jaw for more effective feeding.
Mihalitsis is now an assistant professor at the University of Guam. Additional coauthors are: Denise Yamhure-Ramirez, Maelan Beil, Nathan Cole, Nicole Shum and Bryson Zheng, UC Davis; HoWan Chan, Rice University; Isabella Paglione, Hallee Petri and Dylan Wainwright, Purdue University.
