Colorful angelfish — beauties of coral reef habitats — have capitalized on a remarkable set of jaws to exploit niches inaccessible to other reef denizens, according to new research.
Fish that eat food floating free in the water column capture their prey either through suction feeding — sucking water, and their prey along with it, in through their gaping mouths — or through "ram" feeding, where they open their mouths wide and surge forward, capturing food in their paths.
But angelfish are able to scrape, clip and rip food that's tethered to undersea surfaces using a "grab and tear" mechanism that relies on a specialized jaw design.
"They can protrude their upper and lower jaw out away from their head and then bite really hard," said Peter Wainwright of the University of California, Davis, who was not a part of the study. "Imagine if you could just sit there at the table and extend your jaw out toward the plate. A big angelfish can extend its jaws a couple of inches. They can reach into nooks and crannies on the reef. They're powerful biters. They can yank."
In addition to a protrudable jaw, they have an extra joint in their lower jaw that allows them to bite while their jaws are extended. And they have an extra set of bristle-like teeth that gives them a firm grasp of their food.
"They're basically able to do something that other reef fish can't," thanks to their unusual jaw, by attacking food that's firmly attached to a reef surface, and pulling it out of crevices that other fish can't reach, said Nicolai Konow, now of Brown University in Providence, RI, who led the new study.
While in some species, such specialized structures allow the animal to monopolize a single food type, angelfish have used this evolutionary breakthrough to expand into many niches on the reef.
"These guys are sort of jacks of all trades," Konow said.
Different angelfish species have retained the same jaw structure, but have specialized, instead, by varying their size and their gut to access many types of food.
With a range of patterns and colors including blue, yellow, black and white, the 80 or so different species of angelfish span more than a tenfold size difference and consume everything from sponges and algae to anemones and plankton, Konow and colleague David Bellwood of Australia's James Cook University reported.
By evolving into a range of sizes and gut configurations, the fish could adapt to different habitats, while still using their unique jaw. (One group has adapted to feed on unattached food in the water column and that group has changed its jaw structure the most.)
As Darwin described, species of finches in the Galapagos Islands evolved different beak sizes and shapes to capitalize on different food sources, and the angelfish findings are similar, but with a twist.
"It would be as though you went to the Galapagos and found some finches are eating seeds and some are eating insects, but darn, they all look the same, except one is the size of a crow and the other is the size of a small sparrow," Wainwright said.
"This is an interesting pattern that you can get different specialists in different things using a system that's not getting heavily modified," Wainwright said. "That is pretty interesting. You don't see that very often."
Konow collected specimens of 10 different types of angelfish and examined their jaw motion and construction carefully. He carefully observed their feeding behavior to determine exactly what each species ate.
He also noted their size and how their guts were configured, whether they included a hind-gut chamber or a gizzard, for example. He found large variations in size and gut configuration. He and Bellwood published the results in PLoS ONE.