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Kathryn Knight

Some sharks get a bad press from humans while the gentler filter-feeding members of the family rarely get a mention. Whale sharks, megamouth sharks and basking sharks peacefully cruise the oceans and the public barely gives them a thought. However, ecologists and conservationists are keen to find out more about these creatures. Understanding the size of food particles consumed by these elusive animals is key to predicting their migration patterns and population levels; however, measuring how these fish filter food is almost impossible. Which is why Misty Paig-Tran and her colleagues at Friday Harbor Laboratories and the University of California, Irvine, USA, began cutting the bottoms off plastic 1 litre drinks bottles. Figuring that the cut body and neck of the bottle was a reasonable approximation to a small whale shark's gaping mouth, Paig-Tran began using the model mouths to look at how sharks filter-feed (p. 1643).

According to Paig-Tran and her colleagues, water entering a filter-feeding shark's mouth is filtered by gill rakers before passing out through the animal's gills. Adding 20–2000 μm particles to a flow tank, the team varied the number of gill slits and the permeability of the gill rakers (by altering the gauge of the mesh in the gill slits) in their model mouths while changing the water speed to find out how these factors affected the size and distribution of the particles captured by the filter-feeding animals.

Analysing the particle distribution in the mouth models after 3 min of simulated filter feeding, the team realised that some particles were filtered directly by the gill rakers, others were caught in the gill rakers after falling out of the flow and the remainder lodged in the back of the fish's mouths. The team also saw that changing the number of gill slits affected the size of the filtered particles: the model trapped two classes of particle – small (51–100 μm) and very large (>1000 μm) – when it had one pair of gill slits, but as the team increased the number of gill slits, they only found medium-sized particles (101–1000 μm) lodged in the model fish's head. Finally, changing the model's swimming speed (by increasing the flow tank rate) and the porosity of the gill rakers also affected the size distribution of trapped food particles. The fastest swimming models with the smallest gill pores trapped the most particles, and by simply speeding up the fish could switch to selectively filtering smaller plankton-sized particles.