Cheer and Sanderson combined computer models with careful observations of live fish to work out how fish filter feed -- a problem that had puzzled biologists for decades. In a study published in the journal Nature, they show that fish use crossflow filtration, a method widely used in industry, to concentrate food particles.
Many species of fish, including herring, mackerel, tilapia and goldfish, feed by swimming around with their mouths open or by pumping water into their mouths. Water is pushed out through gill slits in the side of the head. Food particles in the water are trapped and swallowed. Fish don't swallow much water with their food, so somehow food and water are separated.
Look in the mouth of one of these fish, and you'll see a series of arches lined with combs called gill rakers. They look like a sieve, and for a long time, biologists assumed that the rakers collected food particles as water flowed through them.
This sieving model raised a number of questions, such as why the filters don't clog with food, how food particles get from the rakers to the esophagus, and how fish can filter particles as small as a bacterium, said Sanderson.
Sanderson's team used fiber-optic endoscopes, like those cardiologists use to probe blocked blood vessels, to watch what actually happens when fish are feeding. They found that food particles were not hitting or sticking to the rakers at all. They were being carried straight past and collected at the roof or the back of the mouth.
"The theory is that they're sieving, but there's no food on the rakers," said Cheer.