Effect of Food Concentration Inside Eelgrass Beds on the Energy Balance of the Invasive Mussel Musculista Senhousia

Abstract

Samples of Musculista senhousia, 20 mm in length, were gathered from San Diego Bay and fed on three different food concentrations: 0.2, 0.4 and 0.8 mg AFDW L m 1, and their energy balance studied. Food concentration ranges were selected to reflect those observed in the field where the invasive mussel M. senhousia co-occurs with the angiosperm, Zostera marina. The lowest and intermediate concentrations correspond to concentrations described in and over large eelgrass patches, respectively. The upper concentration is equivalent to that occurring during the spring phytoplankton blooms. Ingestion rates varied between 81.6 µg AFDW h m 1 for the mussels fed on the lowest concentration, 191.2 µg AFDW h m 1 when the concentration rose to 0.4 mg AFDW L m 1, and 164.0 µg AFDW h m 1 for the highest concentration. Food concentration had a significant effect on absorption efficiency, which was approximately 70% at the intermediate and upper concentrations, but rose to over 80% at the lowest concentration. The food absorption rate varied with food concentration in a manner similar to that of the ingestion rate, i.e., with a maximum value of 142.34 µg AFDW h m 1 for the mussels fed on the intermediate concentration, this being higher than the values for both the highest concentration (114.72 µg AFDW h m 1) and the lowest (65.89 µg AFDW h m 1). Respiration rates were 54 µg O 2 h m 1 for the mussels fed on the lowest concentration and 74 µg O 2 h m 1 for those fed on the intermediate concentration. These results show a close relationship between respiration and ingestion rates, which can be fitted to the following equation: RR = e (m 0.42 + 0.10 IR) (r = 0.5703, R 2 = 32.53, p <0.01). Scope for growth (SFG) estimations were higher for those mussels fed at the intermediate and upper concentrations (1.95 J h m 1 and 1.50 J h m 1, respectively), between which no significant difference was observed (p > 0.05), whilst SFG for the lowest food concentration was only 0.69 J h m 1. The physiological rate with the greatest effect on growth was the absorption rate. Its relation to SFG is described by the equation SFG = 0.67 + 0.90AR (r = 0.977, R 2 = 95.56%, p <0.01). The discussion of these results is based on the growth rates that have been described for this mussel in its natural environment where habitat structure, such as the canopy provided by the eelgrass, can affect the success of the invasive mussel.