ABSTRACT: Responses in feeding behaviour to wide variations in the amount and composition of natural seston were studied in the green-lipped (Greenshell(TM)) mussel Perna canaliculus of standard 58 ± 2 mm shell length. Nutrient acquisition wasprimarily regulated by the rate that mussels cleared water of particles (l h^-1), which varied in hyperbolic relation with the abundance of chlorophyll-rich organics, rather than with the remaining organic or inorganic components of availableseston. Maximal clearance rates of about 15 l h^-1 g^-1 consistently occurred when chlorophyll a was available at only 1 to 2 µg l^-1. Although clearance rates decreased exponentially, rates of filtration and ingestion(mg h^-1) continued to grow as the total dry particulate mass (TPM) of available seston increased up to about 1000 mg l^-1. The proportion of filtered matter rejected as pseudofaeces before ingestion remained at a constant fractionwhich averaged 0.87 at seston concentrations above about 40 mg TPM l^-1. Only above about 1000 mg TPM l^-1 did a decline in filtration rate suggest any physical overloading of feeding mechanisms. Within particles filtered on thectenidia, significant net enrichment of up to 0.23 ± 0.10 (mean ± 2 SE) times the organic fraction in available seston stemmed primarily from the preferential retention of particles that were rich in chlorophyll a. The efficiency of such retentionvaried in quadratic relation with seston composition, being maximal at an optimal concentration of chlorophyll a per unit total organic matter. Within ingested particles, further net enrichment of up to as much as 1.47 ± 0.26 times the organicfraction within filtered matter stemmed from the differential pre-ingestive rejection of particles within pseudofaeces. Efficiencies of both these selective mechanisms increased with filtration rates. The combined effect was that the organic content ofingested matter (OCI) was enhanced at faster filtration rates, to as much as 7 times the organic content of available seston (OCS). Associated benefits were amplified by a positive hyperbolic relation that was confirmed between OCI and the net absorptionefficiency from ingested organics. Findings showed that tissue wasting occurred in P. canaliculus of 1 g dry soft tissue feeding upon seston that contained less than 0.86 ± 0.19 µg chlorophyll a l^-1. Maximal net organic absorptionrate indicated potential growth of 6.5% d^-1 g^-1 dry soft tissue, associated with the peak in clearance rate. A high capacity for filtration and the ability to adjust clearance rate enabled P. canaliculus to optimise particleselection and absorption efficiencies at levels that maintained organic absorption rate independent of the reduction in OCS as TPM increased to at least 1000 mg l^-1. Collective findings help to explain why P. canaliculus is sowell-suited for cultivation, establish that reduced mussel growth within the New Zealand Greenshell(TM) industry from 1996 to 1998 was due at least in part to food limitation, and suggest a potential for significant expansion of farming away fromtraditional 'clear water' sites to more turbid areas.