SANDERSON, S.L.*; CHEER, A.Y.; GOODRICH, J.S.; GRAZIANO, J.D.; CALLAN, W.T.: How do gill rakers function in suspension-feeding fishes?

Gill rakers have been assumed to serve as either (1) a sieve that accumulates particles larger than the mesh size or (2) a sticky filter that retains particles encountered via a hydrosol filtration mechanism. However, our video endoscopy (30 – 250 Hz) in the oral cavities of freely-swimming fish during pump suspension feeding demonstrates that the rakers of species belonging to three families (gizzard shad, Dorosoma cepedianum; goldfish, Carassius auratus; ngege tilapia, Oreochromis esculentus) do not function as a sieve or as a sticky filter. The gill rakers of pump suspension-feeding Sacramento blackfish (Orthodon microlepidotus) have been hypothesized to serve as a barrier to fluid flow, directing particle-laden water to the sticky oral roof where particles are trapped (Science 251: 1346-1348, 1991). However, our use of computational fluid dynamics to simulate flow in the vicinity of rakers at intermediate Reynolds numbers (250 – 500) shows that the rakers of pump suspension-feeding fishes are leaky, never serving as a barrier to flow. Based on these results, we propose a new model of filtration in suspension-feeding fishes. Supported by NSF grants to SLS and AYC.