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Two bivalve molluscs, Tellina tenuis and Abra tenuis , apparently susceptible to low winter temperatures, were studied on the intertidal Balgzand in the western Wadden Sea during a 30-y monitoring programme. Both species live in the Wadden Sea on the northern edge of their intertidal distribution, and reproduce in mid-summer. By using a 1-mm mesh sieve, only the individuals of ∼1 y and older are sampled. Both the adult and the juvenile stages of Tellina were susceptible to cold winters. In Abra , in contrast, only adults suffered from cold winters. The abundance of Tellina in the Wadden Sea showed an episodic pattern, with periods of presence of several years alternating with relatively long periods of (near) absence. In the Wadden Sea, the numbers of Tellina were more variable than in areas with milder winters. Collapse of the Wadden Sea populations coincided with severe winters. Abra was present in nearly all summers, although in highly variable densities. In Abra , but not in Tellina , a positive correlation was found between recruitment (at the age of 1) and the summer temperature during reproduction. In both species growth was positively correlated with temperatures during the main growing season. Abra , with its direct larval development into benthic juveniles, has limited possibilities of dispersal, and is quite able to maintain more or less stable populations in isolated sites in the Wadden Sea. Tellina , in contrast, periodically entirely dies out in the Wadden Sea, and recovery of the populations originates from offshore North Sea populations. |
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