DYNAMICS AND GROWTH OF A BIVALVE, ABRA-TENUIS, AT THE NORTHERN EDGE OF ITS DISTRIBUTION

The small bivalve Abra tenuis (Montagu) (Bivalvia: Semelidae) reaches the north-eastern edge of its range in the Wadden Sea near the Dutch/German border. In the Wadden Sea it is restricted to a narrow intertidal zone close to the high-water level. Data are reported from a population at Balgzand, in the south-westernmost part of the Wadden Sea, which has been monitored for two decades. This population showed a strongly fluctuating density and was seemingly absent for more than three years following three severe winters in succession. Over-winter survival in adults was greater during mild than cold winters, and close to zero during all winters that were colder than average. Recruitment was higher in warm than in cold summers. Growth was more rapid in warm than in cold spring-summer periods. In warm years growth rates were higher than in more southern locations, and values for maximum size and life span were relatively high. It is concluded that a single environmental factor, viz. temperature, is limiting the distribution of this species in the Wadden Sea.