Many of the world’s fishes feed by filtering water for tiny animals called zooplankton. A filter-feeding fish will open its mouth and let the water pass through gill slits on either side of its head while the plankton is collected and swallowed. It was previously thought that filter-like structures in the oral cavity functioned like a sieve, trapping food particles on a porous surface while letting water through. This type of “dead-end” filter can eventually become clogged, however, as particles accumulate on the filter surface.
In contrast, the filtering structures in fishes don’t seem to clog up, and furthermore, are able to retain food particles that are smaller than the pores in the filter. Two discoveries about the filtering mechanism used by several different fishes explain how they do this. First, the filtering structures in these fishes function like crossflow filters. When the fish opens its mouth, plankton-laden water flows in and instead of mainly flowing straight into the filter, it flows parallel to the filter. The fish’s filters are composed of hard, rib-like structures called gill arches, which bear rows of smaller protruding gill rakers. Together, these structures form a series of grooves with porous bottoms on either side of the fish’s oral cavity. The grooves are like alleys coming off the main path that water takes as it flows back toward the fish’s digestive tract. The water that interacts with the grooves gets redirected into them and exits the oral cavity through the porous gill rakers. Meanwhile, the mainstream flow lifts and transports most of the food particles toward the throat.
Even crossflow filters can clog over time, however, as some of the food particles make their way into the grooves. In some fishes, like the paddlefish (Polyodon spathula) and basking shark (Cetorhinus maximus), a second mechanism appears to prevent clogging further. The gill arches and gill rakers in these unrelated fishes form especially deep grooves. As water flows into one of these grooves, it interacts with the edge of the gill arch and rolls up into a region of swirling water called a vortex. This vortex stays within the groove and helps prevent clogging by forcing food particles to aggregate at the corners of the groove or stay suspended in the swirling water. This keeps particles out of the way of the porous gill raker surface where water exits. Additional structures, like the gill cover and muscle bands associated with the gills, appear to enable the fish to manipulate water flow further and capture food that collects in the grooves.
Check out this video to see a paddlefish filter feeding.Edit Summary
“Our cross-step filtration model based on paddlefish and basking shark morphology takes advantage of vortical flow in porous slots to reduce clogging by concentrating particles along the slot margins.” (Sanderson et al. 2016:3)
“As the crossflow travels posteriorly in the conical oral cavity, the interaction of the crossflow with the branchial arch that is directly upstream of each slot generates a spanwise vortex in that slot. The use of d-type ribs with a groove aspect ratio (groove width w divided by rib height h) of <3 to 4 (refs 15,16) causes the vortex to affect flow across the entire width of the slot.” (Sanderson et al. 2016:6)
“Cross-step filtration is a unique fluid dynamic process that concentrates and transports particles by integrating all four major components of the 3D architecture in fish oral cavities: (i) branchial arches that are backward-facing steps forming d-type ribs and slots attached to the (ii) porous gill raker surfaces of (iii) the conical oral cavity covered by (iv) an operculum or elasmobranch gill flap that directs the axial travel of the vortices within the slots.” (Sanderson et al. 2016:6)