The exhalant jet of mussels, Mytilus edulis

Riisgård H.U., Jørgensen B.H., Lundgreen K., Storti F., Walther J.H., Meyer K.E. and Larsen P.S., Marine Ecology Progress Series

Literature values of the exhalant jet velocity of mussels vary considerably, and the detailed fluid mechanics of the mussel-near flow generated by the exhalant jet has hitherto been uncertain although this flow in conjunction with currents and/or other mussels may strongly influence their grazing impact. Computational modelling of this phenomenon depends on knowledge of the velocity distribution near the exhalant siphon aperture of mussels to provide appropriate boundary conditions for numerical flow models, and to be useful such information should be available for a range of mussel shell lengths. Here, we present results of a detailed study of fully open mussels (Mytilus edulis) in terms of filtration rate, exhalant siphon-aperture area and jet velocity, gill area, dry body weight, all as function of shell length over the range L = 16.0 ± 0.4 to 82.6 ± 2.9 mm. Scaling laws for these parameters in terms of size by shell length are presented. The exhalant jet velocity was determined by three methods: 1) measured clearance rate divided by exhalant aperture area, 2) manual particle tracking velocimetry (PTV) using video-microscope recordings, and 3) particle image velocimetry (PIV). The latter provides detailed two-component velocity distributions near the exhalant siphon in 5 planes parallel to the axis of the jet and the major axis of the oval aperture, hence estimates of momentum and kinetic energy flows in addition to mean velocity. Here, data obtained on particles inside the exhalant jet of filtered water was ensured by the use of TiO2 seeding particles which were de-agglomerated by ultrasound to size-range 0.7 to 2 µm prior to addition to avoid retention by the gill-filter of the mussels. Notably it was found that the exhalant jet velocity is essentially constant, about 8 cm s-1, and independent of shell length. Based on geometric similarity and scaling of pump-system characteristics of the mussel it was found that these characteristics coincide approximately for all sizes when expressed as pressure head versus volume flow divided by shell length squared.

doi: 10.3354/meps09268 (pdf)