Cost of selective feeding by the blue mussel (Mytilus trossulus) as measured by respiration and ammonia excretion rates.

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 (i.e., selective feeding) where organic-rich particles were selected over inorganic particles as compared to no sorting (i.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, (1) no algae+50 mg silt l(-1), (2) 150x10(6) cells l(-1) of algae+no silt, and, where pre-sorting of ingested material occurred, (3) 20x10(6) cells l(-1) of algae and 20 mg l(-1) of silt, and (4) 150x10(6) cells l(-1) of algae and 50 mg l (1) of silt. A control, which represented basal metabolism consisting of mussels exposed only to filtered seawater (0.45 &mgr;m), 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 (p<0.05, ANOVA). Estimates of net energy used for feeding, where net energy is total energy intake (food) minus energy expenditure (energy 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.