Feeding, particle selection and carbon absorption in Mytilus edulis in different mixtures of algae and resuspended bottom material

Abstract

Mytilus edulis (average length 3.7 em) were fed mixtures of algae (10000 Phaeodactylum tricornutum cells · ml^-1) and natural silt (0 to 55 mg · l^-1), and the effects of silt concentration on filtration behaviour, food uptake and carbon budget were studied. Particle clearance averaged 66 ml · min^-1 at 3.5 mg silt · 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 pseudofaeces. Pseudofaeces was produced at silt concentrations above 1 mg · l^-1, and the amount produced increased lineally with the amount of material retained (regression line: pseudofaeces produced (mg · min^-1) = 0.98 x material retained (mg · min^-1) - 0.057; r = 0.98).

Dry matter ingestion increased with increasing concentration of silt. (Regression line: ingestion rate (μg · min^-1) = 4.36 x In concentration (mg · l^-1) + 31.98; r = 0.52). The mussels were able to select between algae and silt particles by clearing the material rejected as pseudofaeces of about 2/3 of the algae. At a low silt concentration (2 mg · m^-1) the algae were concentrated in the material ingested by a factor ca. 3, and at a high concentration (55 mg · m^-1) by a factor of about 30. Specimens from the Wadden Sea, having larger labial palps, selected between algae and silt with a significantly higher efficiency.

The carbon ingestion tate increased considerably at a low concentration of silt (6.6 μg C · min^-1) compared to a suspension of pure algae (4.4 μg C · min^-1), and decreased at higher concentrations (to 4.5 μg C · min^-1 at 55 rng · m^-1).

Carbon absorption efficiencies were high (between 59 and 65 %) up to a silt concentration of 25 mg · l^-1, and decreased slightly at higher concentrations (to 52 % at 55 mg · l^-1). The amount of carbon absorbed consequently increased by the addition of silt, from 2.9 μg C · min^-1 in a suspension of pure algae to a maximum of 4.0 μg C · min^-1 at a silt concentration of 2 mg · l^-1 and decreasing to 2.4μg C · min^-1 at 55 mg · m^-1.

It is concluded that Mytilus is well adapted to silt concentrations up to 55 mg · 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 · l^-1.