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Not-so-simple sieving by ascidians: re-examining particle capture at the mesh and organismal scales

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Abstract

The particle capture mechanisms of biological filters determine the particle spectrum that is ingested by filter-feeding animals. Although ascidian feeding has been extensively investigated, the organismal-scale fluid dynamics and mesh-scale particle-filter interactions are not fully characterized. Fluorescein dye visualization of flow through the branchial sac of the ascidian Ciona intestinalis showed organismal-scale flow was laminar and moved both parallel and perpendicular to the mucous mesh. Endoscopic investigations of Herdmania momus revealed the mesh-scale filtration process, including the pre-capture velocities, particle approach angles, and mesh behavior. The mesh speed was variable (range 0–0.4 mm s−1). To determine how particle shape affects hydrosol capture, Styela plicata was fed differently shaped polystyrene particles (ellipsoids and spheres); sampling the inhaled and exhaled water revealed that microellipsoids (0.3 × 0.7 µm) were captured at significantly lower efficiency (32%) than 1 µm microspheres (86%). The capture efficiency of microellipsoids resembled that of microspheres with a diameter similar to the microellipsoids’ minor axis (0.3 µm, 31%) suggesting that the minimum diameter of ellipsoidal particles determines the capture efficiency. Flow near the filter was parallel to the mesh even ~ 10 s of micrometers away, implicating a “crossflow” component to ascidian filtration, where the fluid being filtered is directed along the surface of the filter rather than exclusively perpendicular to it. Collectively, these results suggest that ascidian filtration acts as a hybrid-flow filtration system rather than a classical direct sieve.

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Acknowledgements

The investigation was supported in part by Grants from the US-Israel Binational Science Foundation (Grant #2012089 to K.R.S. and G.Y.), the Alfred P. Sloan Foundation (K.R.S.), Israel Science Foundation 1280/13 to G.Y., a Prof. Rahamimoff Travel Grant for Young Scientists (to K.R.C.), an Oregon Sea Grant 2014 Robert E. Malouf Marine Studies Scholarship (to K.R.C.), and a Julie and Rocky Dixon Graduate Innovation Award (to K.R.C.). We thank Raz Moskovich for flow cytometry assistance, and Peter Beninger, Evan Ward, Misty Paig-Tran, Hans Ulrik Riisgard, and Poul S. Larsen for useful comments and discussions.

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Correspondence to Keats R. Conley.

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Ethical approval was not required for this research because ascidians are non-vertebrate chordates and as such are exempt from regulation by the Institutional Animal Care and Use Committee.

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The authors declare no conflicts of interests.

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Responsible Editor: S. Shumway.

Reviewed by P. Beninger and E. Ward.

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Conley, K.R., Ben-Tal, A., Jacobi, Y. et al. Not-so-simple sieving by ascidians: re-examining particle capture at the mesh and organismal scales. Mar Biol 165, 45 (2018). https://doi.org/10.1007/s00227-018-3300-8

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  • DOI: https://doi.org/10.1007/s00227-018-3300-8

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