Successive changes in distribution patterns as an adaptive strategy in the bivalveMacoma balthica (L.) in the Wadden sea

Abstract

During their first year in the Wadden Sea, high proportions of the tidal-flat populations of the telhnid bivalve Macoma balthica (L.) redistribute twice: immediately after their settlement in late spring, the postlarvae show a net transportation in a shoreward direction -whereas in the subsequent winter, the grown spat move in the reverse direction. As a consequence of these two periods of high mobihty, distribution patterns shift twice: though initial settlement takes place mainly in the lower half of the intertidal, most spat grow to a size of ~-0.5 cm in the upper half of it, whereas most adults live in the middle and lower zones. The successive distribution patterns of spat and older Macoma are described in detail for Balgzand, an extensive tidal-flat area in the westernmost part of the Wadden Sea. Long-term observations in this area and pubhshed evidence from other areas are used to evaluate the suitability of different tidal zones as a habitat for successive life stages of Macoma. For spat, the upper zone is a more favourable habitat than the lower, because predation pressure (mainly from shrimp) and disturbance are less and growth is more rapid. For adults, the upper parts are no longer a favourable habitat, because the few animals that stay suffer from high parasite load, low survival, slow growth and low reproductive output, tt is concluded that in their successive life stages the majority of M. balthica live at the intertidal level most favourable to them. The strategy of timely shifts tO areas more suitable to the next life stage contributes to the success of the species: it is the most widespread and common (and one of the most stable) macrozoobenthic species in the Wadden Sea.

Turnover of postlarval bivalves in sediments of tidal flats in Königshafen (German Wadden sea)

“…In the clam M. balthica there are two periods of migration, during their first summer and first winter. From site-specific growth and survival rates, Beukema (1993) concludes that timely migration to areas more suitable for the next hfe stage can be regarded as an adaptive strategy explaining the relatively low interannual variability of recruitment success noted in this species (Beukema, 1982}. Postlarval migrations may thus increase the temporal stability of populations.…”

Changes in distribution and decrease in numbers during migration of the bivalve Macoma balthica

“…A nursery is an area inhabited by many juveniles but few adults of a species. This is typical, for example, for the lugworm Arenicola marina (Farke et al 1979), the shore crab Carcinus maenas (Klein Breteler 1976, Beukema 1991, the shrimp Crangon crangon (Kuipers & Dapper 1984) and the bivalve Macoma balthica (Beukema 1993).…”

“…Many intertidal benthic species in the Wadden Sea, a shallow coastal sea in The Netherlands, Germany and Denmark, use nurseries in the high intertidal (Beukema 1993, Flach & Beukema 1994. A nursery is an area inhabited by many juveniles but few adults of a species.…”

“…It is assumed that these nurseries provide juveniles of these benthic species with protection against predation by marine organisms such as shrimps, crabs and small fish (Beukema 1993), that physical disturbance is less (Flach 1992) and that perhaps feeding conditions are better (Armonies & Hellwig-Armonies 1992). However, when nurseries are spatially separated from the locations inhabited by adults, the juveniles must migrate to and from the nurseries.…”

“…The fact that there is a large variation in growth and mortality rates between locations for many benthic species may be an indication that many individuals fail to migrate to the optimal location (Beukema 1993).…”