COLLECTION OF GAMETES
Gametes were harvested by inducing spawning using standard thermal stress protocols (Pettersen et al. 2010). This involved cycling seawater temperature between 16-24 °C to induce spawning. Males and females were identified at the time of gamete release, rinsed with filtered seawater, and isolated into individual spawning chambers (120 × 175 × 70 mm). Eggs were rinsed through a 125μm mesh, and sperm through a 30μm mesh, to remove any debris released from the adult mussel during spawning. The concentration of sperm for each male was determined from three replicate counts using an improved Neubauer haemocytometer and sperm concentrations were initially standardized to 2 × 108sperm ml-1. A serial sperm dilution was then carried out to obtain the desired experimental concentrations of sperm for our experiments: 2 × 102; 2 × 103; 2 × 104; 2 × 105; 2 × 106; and 2 × 108 sperm ml-1. Egg concentrations were estimated from three replicate counts using a Beckman multisizer™ 3 Coulter counter and standardized to 2000 eggs ml-1 (stock egg solution).