Naturally low seston concentration and the net energy balance of the greenshell mussel (Perna canaliculus) at Island Bay, Cook Strait, New Zealand

This paper describes an investigation of the physiological energetics of th e New Zealand greenshell mussel, Perna canaliculus (Gmelin, 1791), which is aimed at determining if mussels are absent from many shores of Cook Strait as a consequence of a negative energy balance resulting from ambient low s eston quantity and quality. Seston characteristics and mussel physiological functions were measured under ambient summer conditions at Island Bay, a s ite in Cook Strait. Estimates of total particulate matter (TPM in mg litre( -1)), particulate organic matter (POM in mg litre(-1)), and particulate org anic carbon (POC in mug litre(-1)) were low but of a similar magnitude to v alues reported for many comparable temperate regions. However, seston % org anic matter (% OM) values were consistently low (less than or equal to 25%) and resulted in negative net absorption efficiency (AE) values and concomi tant negative scope for growth (SFG = net energy balance) values. We sugges t that the ongoing costs of energy loss associated with extra- and intracel lular digestion (termed metabolic faecal. loss) are greater than the energy derived from seston of such low organic matter. Our data indicate that amb ient levels of seston % OM of less than or equal to 25% are insufficient to promote a positive net energy balance in P. canaliculus, and as such proba bly play a major role in explaining the almost complete absence of mussels from many Cook Strait shores.