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, algae, anemones and plankton, Konow and colleague David Bellwood of Australia's James Cook University reported.