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Microscopic techniques were used to examine the role of gill cirri in particle capture by Dreissena polymorpha. The latero-frontal cirri, formed from two fused ciliary plates, consist of about 40 pairs of cilia. Each cilium in the plate contains a typical 9 + 2 axoneme in the fused region of the cirrus, but the structure of the axoneme in the long, free ciliary tips is reduced. The cilia in a cirrus are graded in length, with the shortest cilia positioned frontally. The cirral cilia move in unison, allowing the cirrus to move from a flexed position with its tip arched over the front of the gill filament, to an extended position with the cirrus projected in the plane of the latero-frontal cell and extending across the interfilament space. In the latter position, the free ciliary tips of opposing and neighboring cirri form a "trap" (net) with a spacing of about 0.5 μm. Observations with laser confocal microscopy indicated that these structures can physically trap particles &lt;1 μm in diameter. Particles captured by the extended cirri are moved to the frontal surface of the gill, where the cirri are swept by the lateralmost frontal cilia. During cirral movement the shift from extended to flexed position is, in part, achieved by the base of the cirrus pivoting at a hinge region. Morphologically, the hinge region shows axonemal specializations that consist of electron-dense plates and other structures of undefined function that may be important in the overall movement of the cirrus. In addition to trapping by cirri, we also observed particles moving in the water currents, particularly in the frontal current located over the apical surface of the filament, suggesting that some particles are captured in these water currents without being physically trapped. Probably, therefore, trapping by the cirri and establishment of water currents by the filaments both participate in the interception of particles by Dreissena polymorpha.

Particle Capture by the Gills of Dreissena polymorpha: Structure and Function of Latero-frontal Cirri