When Nicolai Konow flew to Australia to begin his PhD with David Bellwood at James Cook University, he was planning to work on butterflyfish. But once he saw his first angelfish, there was no turning back. Swimming with these colourful reef fish, he realised that `they do things with their jaws that no other fish do!' Puzzled by their peculiar mouths, Konow set out to unravel angelfish's biomechanical secrets(p. 1421).
There is astounding variety in fish dining habits, but researchers have helpfully shunted feeding techniques into two large groups. The first group,ram-suction feeders, include fish that envelop their unsuspecting prey, or suck tasty floating morsels into their mouths. Fish in the second group, which includes angelfish, are biters. But how do bulky angelfish munch on sturdily attached prey like sponges and tunicates, which are normally tucked away and well-protected?
To find out how angelfish attack their prey, Konow decided to examine angelfish during mealtimes. Luckily, the sponge reefs that angelfish call home are right on his doorstep. The real problem was catching the feisty fish; he discovered that angelfish are remarkably agile escape artists. `You herd them like cows,' Konow says, `but then they bolt!' Eventually, he managed to catch six fine specimens. The next hurdle presented itself almost immediately. `Back in the lab, I discovered that angelfish are really fussy eaters,' Konow recalls, `and it took a while to find the juicy stuff they like to eat.'Fortunately, he noticed layers of untouched marine fauna growing on local harbour pylons, which he soon realised was a limitless supply of gourmet angelfish food.
Finally, he was ready to examine angelfish jaw movements as they chomped on their victims. Sticking small reflective dots on various joints and bones of angelfish jaws, he set up a high-speed camera and recorded the fish as they gorged on the feast he provided. With special software used to analyse high-speed motion during car crash testing, Konow tracked how the reflective dots moved relative to each other and the prey. He noticed that the tip of the angelfish's lower jaw rotates relative to the rest of the jaw. But what really surprised Konow was that this jaw rotation has very unusual timing. Angelfish snap their protruded jaws shut before pulling them back with the prey securely lodged between their teeth, making sure that they don't bite off more than they can chew. `We know of no other fish with such a mechanism dedicated to closing the jaws before retracting them,' Konow says.
How do angelfish manage this feeding feat? Taking a closer look at angelfish jaws, Konow saw that they have a unique feature that enables what he has dubbed the `grab and tear' technique. An extra joint in their lower jaw allows them to snap their jaws shut while they still jut out. Once their jaws are closed, angelfish pull them back with exceptional speed. Konow is amazed at the `terrific pulling power' of this novel feeding technique. Thanks to their lower jaw's extra joint and speedy retreat, angelfish can feast on prey in the reef's hard-to-reach places.