The effect of tidal level on energy balance of the greenshell mussel, Perna canaliculus

This study investigated the energetics of the greenshell mussel, Perna canaliculus, one of New Zealand's most commercially important aquaculture species. Aerial and aquatic rates of O₂ uptake, filtration rate, assimilation efficiency and nitrogen excretion were measured for mussels collected from Taylors Mistake at two shore levels (0.0 (low shore) and 1.0 m (high shore) above mean low water). Measurements were made for winter (May to August) and summer (November to January) collected mussels. I also calculated the dry weight condition index for high and low shore level P.canaliculus from Taylors Mistake four or five times during the year. Cl was consistently higher for mussels found at the low shore level (yearly mean = 80.3 ± 7.6) than high shore mussels (yearly mean = 68.8 ± 1). Scope for growth (SFG) of Perna canaliculus was calculated for high and low shore mussels collected during winter and summer. Aerial and aquatic O₂ uptake increased with body weight with weight exponents between 0.57 and 0.91. Aerial and aquatic oxygen uptake was similar for high and low shore mussels. Aerial VO₂ was maximal at 10°C during winter and 15°C in summer. This energy conservation was interpreted as a mechanism to reduce desiccation. Clearance rates were calculated for mussels fed on a monoculture of Isochrysis galbana. They were higher for upper shore collected P.canaliculus than low shore mussels during both winter (high shore = 3.6 l hr -¹ g-¹; low shore = 2.5 l hr-¹ g-¹) and summer (high 2.1 l hr-¹ g-¹; low = 0.7 l hr-¹ g-¹). This was interpreted as an adaptation to compensate for reduced feeding time due to aerial exposure of high shore animals. Assimilation efficiency of P.canaliculus fed on a monoculture of I.galbana was constant over a broad range of algal concentrations with an average value of 83%. Assimilation efficiencies were similar for high and low shore mussels. Excretion rates of ammonia-nitrogen (NH₄-N) were higher in winter than summer. Winter rates of excretion were 59.7 and 20.3 μg NH₄ - N hr-¹ g-¹ for high and low shore mussels respectively. Summer rates were 14.6 for high shore and 13.7 for low shore mussels. Higher excretion rates during winter are thought to be due to utilisation of bodily reserves due to starvation. Scope for growth was greater for high shore collected mussels than low shore mussels due mainly to increased clearance rates. When SFG was adjusted to take into account reduced feeding time due to aerial exposure, high and low shore animals had a similar SFG during winter. However, high shore animals had increased SFG during summer. In contrast with findings of SFG calculations, natural growth rates of upper shore P.canaliculus appear to be slower than their low shore counterparts. This is reflected by a consistently lower Cl and a smaller average shell length (high shore = 57.6 ± 20.8 mm; low shore = 80.2 ± 20.8 mm). Although P.canaliculus has the ability to increase its filtration rate with reduced periods of tidal submersion the increased energy intake appears not to compensate for metabolic demand. During summer high shore mussels would experience high air temperatures when exposed causing increased anaerobic metabolic rates and risk of desiccation. It is concluded that the upper tidal limits of Perna canaliculus may be limited by physical factors such as temperature stress and desiccation before energetic constraints apply