Tidal-current transport of thread-drifting postlarval juveniles of the bivalve Macoma balthica from the Wadden Sea to the North Sea

Abstract

Laboratory experiments were carried out on thread-bearing spat of the tellinid bivalve Macoma balthica, and in the field high numbers of these juveniles (of up to 1 cm shell length) were caught in winter in plankton nets suspended in tidal streams of the Wadden Sea (The Netherlands). These data are used to explain the distribution and dynamics of a population of the species in the North Sea off the West-Frisian island Terschelling. Evidence is presented that nearly all M. balthica arrive in this area in winter by secondary settlement after tidal current transport, facilitated by long hyaline threads. M. balthica were numerous only close to a tidal inlet connecting the area with the Wadden Sea. Further away (more than 10 to 15 km), young M. balthica arrived in low numbers only and were smaller. All or nearly all M. balthica in the North Sea will have originated from massive primary settlement of just-metamorphosed larvae more than half a year previously (in spring and summer) on high tidal flats in the Wadden Sea. Some functional aspects of dispersal by secondary settlement are discussed

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

“…Reliable estimates of mortahty during the first year of life of Macoma cannot be derived from density changes at any place, because young benthic stages show mass movements both during their first summer (Annonies, 1992) and during their first winter (Beukema & de Vlas, 1989}. After their first-winter migration, however, the then 1-yearold animals appeared to stay in (almost) the same place.…”

“…In moving water, they form long mucous threads This significantly promotes their drag and thereby their chance to be carried away by currents. Their suspended stay in the water column is also lengthened (Beukema & de Vlas, 1989), and consequently the distance covered before sinking again to the bottom. By thread drifting, young bottom stages (up to a size of about 9 mm] of Macoma are transported by tidal currents over distances up to some tens of km (Beukema & de Vlas, 1989).…”

“…Their suspended stay in the water column is also lengthened (Beukema & de Vlas, 1989), and consequently the distance covered before sinking again to the bottom. By thread drifting, young bottom stages (up to a size of about 9 mm] of Macoma are transported by tidal currents over distances up to some tens of km (Beukema & de Vlas, 1989). As many more are present in the water at ebb rather than at flood tides, the net transport in winter is in an offshore direction, resulting in a shift to lower intertidal and even subtidal levels.…”

“…Only low proportions of the originally high numbers of spat at the" higher intertidal levels remained there (Fig, 6). At the lower extreme of the intertidal depth range, the populations near MLW must have originated almost completely from the immigration of full-grown spat in winter (as is the case with subtidal areas: Beukema & de Vlas, 1989}. For the few Macoma remaining at the highest intertidal levels, the habitat (so favourable for them during their first summer} ist no longer optimal. Lower intertida] areas appear to be a better habitat for adult Macoma in the following respects:…”

“…1). During winter, numerical densities of juvenile Macoma rapidly decline on high (coastal) tidal flats, which at the same time increase on low (offshore) tidal flats (Wolff & de Wolf, 1977;Beukema et al, 1978) and in the subtidal (Kleef & Essink, 1982;Beukema & de Vlas, 1989). During winter, juveniles with a shell length between about 2 and 8 rnm are found in high numbers in the water, particularly at ebb tide (Beukema & de Vlas.…”