MEPS 191:217-232 (1999)  -  doi:10.3354/meps191217

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 was primarily 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 available seston. 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 fraction which 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 the ctenidia, 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 retention varied 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 organic fraction 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 of ingested 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 absorption efficiency 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 absorption rate 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 particle selection 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 so well-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 from traditional 'clear water' sites to more turbid areas.

KEY WORDS: Perna canaliculus · Greenshell(TM) mussel · Suspension-feeding behaviour · Seston composition · Particle selection · Chlorophyll · Growth