Modelling of suspension-feeding and growth in the green-lipped mussel Perna canaliculus exposed to natural and experimental variations of seston availability in the Marlborough Sounds, New Zealand

Responses in feeding behaviour to wide variations in the amount and composi tion of natural seston were studied in the green-lipped (Greenshell(TM)) mu ssel Perna canaliculus of standard 58 +/- 2 mm shell length. Nutrient acqui sition was primarily regulated by the rate that mussels cleared water of pa rticles (1 h(-1)), which varied in hyperbolic relation with the abundance o f chlorophyll-rich organics, rather than with the remaining organic or inor ganic 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 mu g 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 inge stion remained at a constant fraction which averaged 0.87 at seston concent rations 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 m echanisms. Within particles filtered on the ctenidia, significant net enric hment of up to 0.23 +/- 0.10 (mean +/- 2 SE) times the organic fraction in available seston stemmed primarily from the preferential retention of parti cles that were rich in chlorophyll a. The efficiency of such retention vari ed in quadratic relation with seston composition, being maximal at an optim al concentration of chlorophyll a per unit total organic matter. Within ing ested particles, further net enrichment of up to as much as 1.47 +/- 0.26 t imes the organic fraction within filtered matter stemmed from the different ial pre-ingestive rejection of particles within pseudofaeces. Efficiencies of both these selective mechanisms increased with filtration rates. The com bined effect was that the organic content of ingested matter (OCI) was enha nced 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 occu rred in P. canaliculus of 1 g dry soft tissue feeding upon seston that cont ained less than 0.86 +/- 0.19 mu g chlorophyll a l(-1). Maximal net organic absorption rate indicated potential growth of 6.5 % d(-1) g(-1) dry soft t issue, associated with the peak in clearance rate. A high capacity for filt ration and the ability to adjust clearance rate enabled P, canaliculus to o ptimise particle selection and absorption efficiencies at levels that maint ained organic absorption rate independent of the reduction in OCS as TPM in creased to at least 1000 mg l(-1). Collective findings help to explain why P. canaliculus is so well-suited for cultivation, establish that reduced mu ssel growth within the New Zealand Greenshell(TM) industry from 1996 to 199 8 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.