Size‐specific clearance, excretion, and respiration rates, and phytoplankton selectivity for the mussel Perna canaliculus at low levels of natural food

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Clearance, respiration, and ammonia excretion rates, and phytoplankton selectivity were measured for the Greenshell™ mussel Perna canal‐iculus (Gmelin) in Beatrix Bay, Marlborough Sounds, New Zealand. Functional equations were developed describing the relationship between body size (length and weight) and clearance and metabolic rates for this species feeding on natural food assemblages in New Zealand waters. Relationships with shell length were described by allometric power curves but the relationship with chlorophyll a concentrations in the water for a particular size of mussel suggested there was a threshold between 0.2 and 0.3 μg Chi. a litre−1 and an exponential decline in clearance rate from c. 0.5–0.6 μg Chi. a litre−1 with increasing chlorophyll concentrations. Under these field conditions ingestion rate would increase linearly with increasing chlorophyll concentrations up to c. 0.5 μg Chi. a litre−1 then remain constant at higher chlorophyll levels. Food levels, as measured by chlorophyll a or particulate carbon, were very low (generally <1 μg Chi. a litre−1 and 300 μg C litre−1) throughout the study and this was reflected in low ingestion rates, despite clearance rates of up to 8.6 litre mussel 1 h−1. P. canaliculus generally appears to be non‐selective, at least for phytoplankton in the size range 5–100 μm, but there was some evidence that larger diatoms and dinoflagellates were cleared more efficiently by larger mussels. Some of our observations on food levels and feeding rates help explain the low mussel growth and condition experienced in the Pelorus Sound during 1996–97.