Abstract
The metabolic cost as measured by respiration and ammonia excretion rates associated with a
selective as compared to a non-selective feeding behaviour was determined for the blue mussel,
Mytilus trossulus. Mussels were challenged with four environmentally relevant seston matrices of
different quality and quantity, which were known to evoke either a sorting response Ži.e., selective
feeding. where organic-rich particles were selected over inorganic particles as compared to no
sorting Ži.e., non-selective. where either inorganic or organic particles were ingested by the
bivalve. Seston matrices were prepared by mixing known quantities of silt and algae such that the
following extent of feeding responses would occur; no pre-sorting of ingested material, Ž1. no
algaeq50 mg silt ly1, Ž2. 150=106 cells ly1 of algaeqno silt, and, where pre-sorting of
ingested material occurred, Ž3. 20=106 cells ly1 of algae and 20 mg ly1 of silt, and Ž4.
150=106 cells ly1 of algae and 50 mg l 1 of silt. A control, which represented basal metabolism
consisting of mussels exposed only to filtered seawater Ž0.45 mm., was included for a total of five
treatments. Mussel respiration and ammonia excretion rates were independent of whether mussels
were pre-sorting or not sorting ingested material. Of the four matrices, only rates determined for
conditions of maximum seston quality and quantity where pre-sorting of the seston occurred were
significantly different from control mussels Ž p-0.05, ANOVA.. Estimates of net energy used for
feeding, where net energy is total energy intake Žfood. minus energy expenditure Ženergy lost
through respiration and excreta as measured by ammonia excretion rates., indicated that feeding,
whether selective or non-selective required only 0.92% of net energy intake. Hence, mussels
appear to be highly adapted to a dynamic food environment with negligible costs associated with the feeding process, even when significant pre-selection of organic-rich particles occurs. q2001
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