Crossflow filtration in suspension-feeding fishes

Rows of comb-like or tufted gill rakers in the oral cavity of suspension-feeding fishes (for example, herring, anchovies and tilapia) have been thought to serve as (1) non-porous barriers that direct particle-laden water to the sticky oral roof, where particles are retained as water exits from the oral cavity, (2) conventional dead-end filters that sieve particles from water exiting between rakers, or (3) sticky filters that retain particles encountered by a hydrosol filtration mechanism. Here we present data from computational fluid dynamics and video endoscopy in suspension-feeding fish indicating that the rakers of three distantly related species function instead as a crossflow filter. Particles are concentrated inside the oral cavity as filtrate exits between the rakers, but particles are not retained on the rakers. Instead, the high-velocity crossflow along the rakers carries particles away from the raker surfaces and transports the particles towards the oesophagus. This crossflow prevents particles from clogging the gaps between the rakers, and solves the mystery of particle transport from the rakers to the oesophagus.