Experimentally induced effects of water temperature and immersion time on reproductive output of bivalves in the Wadden Sea

Results of experiments are reported on the effects of water temperature and immersion time in winter on egg size and egg numbers in three intertidally living bivalves in the Dutch Wadden Sea, the Baltic tellin Macoma balthica, the common cockle Cerastoderma edule and the common mussel Mytilus edulis. Macoma (14-17 mm shell length) produced large eggs (diameter of 107 mu m) in relatively small numbers (20000-70000) in early spring. Later in spring, Cerastoderma (28-33 mm shell length) produced smaller eggs (77 mu m, excluding the surrounding jelly layer) in tenfold larger numbers (200000-700000). Mytilus (45-55 mm shell length) spawned even smaller eggs (72 mu m) in high (but not easily assessed) numbers over a more extended period. In Macoma egg size was not affected by winter temperatures or immersion time. Effects of winter-spring temperatures and immersion time on egg size could be demonstrated in Cerastoderma. Smaller eggs were produced at the higher temperatures. Effects of immersion time were non-consistent: at lower water temperatures larger, but at higher temperatures smaller eggs were produced by animals kept at longer immersion times. In Mytilus, no temperature effects were observed. However, a longer immersion time resulted in larger eggs. In Macoma as well as in Cerastoderma significantly more eggs were produced at the lower temperature. Immersion time effects were most pronounced at the lower temperature, where more eggs were produced at the subtidal level than at the tidal level. At the higher water temperature differences between egg numbers produced at the two tidal levels were small. Just prior to spawning, egg numbers were strongly positively related to body mass at a certain shell length. (C) 1998 Elsevier Science B.V.