Feeding, particle selection and carbon absorption inMytilus edulisin different mixtures of algae and resuspended bottom material

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Abstract
M. edulis (average length 3.7 cm) were fed mixtures of algae (10,000 Phaeodactylum tricornutum cells .cntdot. ml-1) and natural silt (0-55 mg .cntdot. l-1) and the effects of silt concentration on filtration behavior, food uptake and C budget were studied. Particle clearance averaged 66 ml .cntdot. min-1 at 3.5 mg silt .cntdot. l-1 and decreased at higher and lower concentrations. Increasing amounts of material were retained by the gills with increasing silt concentration but an increasing proportion of this was rejected as pseudofeces. Pseudofeces was produced at silt concentrations > 1 mg .cntdot. l-1 and the amount produced increased linearly with the amount of material retained (regression line: pseudofeces produced (mg .cntdot. min-1) = 0.98 .times. material retained (mg .cntdot. min-1) -0.057; r = 0.98). Dry matter ingestion increased with increasing concentration of silt. (Regression line: ingestion rate (.mu.g .cntdot. min-1) = 4.36 .times. ln concentration (mg .cntdot. l-1) .+-. 31.98; r = 0.52). The mussels were able to select between algae and silt particles by clearing the material rejected as pseudofeces of about two-thirds of the algae. At a low silt concentration (2 mg .cntdot. l-1) the algae were concentrated in the material ingested by a factor of .apprx. 3 and at a high concentration (55 mg .cntdot. l-1) by a factor of .apprx. 30. Specimens from the Wadden Sea, having larger labial palps, selected between algae and silt with a significantly higher efficiency. The C ingestion rate increased considerably at a low concentration of silt (6.6 .mu.g C .cntdot. min-1) compared to suspension of pure algae (4.4 .mu.g C .cntdot. min-1) and decreased at higher concentrations (to 4.5 .mu.g C .cntdot. min-1 at 55 mg .cntdot. l-1). C absorption efficiencies were high (between 59-65%) up to a silt concentration of 25 mg .cntdot. l-1 and decreased slightly at higher concentrations (to 52% at 55 mg .cntdot. l-1). The amount of C absorbed consequently increased by the addition of silt, from 2.9 .mu.g C .cntdot. min-1 in a suspension of pure algae to a maximum of 4.0 .mu.g C .cntdot. min-1 at a silt concentration of 2 mg .cntdot. l-1 and decreasing to 2.4 .mu.g C .cntdot. min-1 at 55 mg .cntdot. l-1. Mytilus is well adapted to silt concentrations up to 55 mg .cntdot. l-1. The potential food diluting effect of silt in suspension is effectively counteracted, especially by particle selection and increased ingestion, and Mytilus even benefits from concentrations up to 25 mg .cntdot. l-1.