Crossflow filtration in suspension-feeding fishes

Rows of comb-like or tufted gill rakers in the oral cavity of suspension-fe eding fishes (for example, herring, anchovies and tilapia) have been though t to serve as (1) non-porous barriers that direct particle-laden water to t he sticky oral roof, where particles are retained as water exits from the o ral cavity, (2) conventional dead-end filters that sieve particles from wat er exiting between rakers, or (3) sticky filters that retain particles enco untered by a hydrosol filtration mechanism(1-6). 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 ins tead as a crossflow filter(7,8.) Particles are concentrated inside the oral cavity as filtrate exits between the rakers, but particles are not retaine d on the rakers. Instead, the high-velocity crossflow along the rakers carr ies particles away from the raker surfaces and transports the particles tow ards the oesophagus. This crossflow prevents particles from clogging the ga ps between the rakers, and solves the mystery of particle transport from th e rakers to the oesophagus.