Mussels (n = 180; 3.5–4.5 cm shell length), M. ga, were collected at spring tide from the lowesttidemark level(0–1 m) in Plentzia (Biscay Bay, N 43◦25 17; W 2◦5651) in July 2007. After routine histological examination of the gonad they were determined to be at post-spawning gametogenic stage. The month range of variation in surface seawater temperature at regional scale (Basque Coast) was 16–20 ◦C