Juvenile-adult distribution of the bivalve Mya arenaria on intertidal flats in the Wadden Sea: why are there so few year classes?

Abstract

Patchy distribution is frequently observed in benthic marine invertebrates. In order to indentify factors causing spatial patterns in the bivalve Mya arenaria, abundances of juveniles and adults, as well as death assemblages, were recorded on a 20-km scale in the intertidal zone of the Sylt-Rømø Bight. Both adults and juveniles exhibited pronounced patchiness. Shell length of juveniles rarely exceeded 2 mm in 1995, which was most likely a consequence of epibenthic predators truncating the size spectrum. Only a few yearclasses dominated the adult population. While the northern part of the Bight was colonized mainly by a 1993-cohort, most M. arenaria in the southern part were from the mid-1980s. It is hypothesized that epibenthic predation is a major cause of the lack of dense M. arenaria beds from other years. However, examination of the length-frequency distribution of death assemblages revealed that other unidentified causes of mortality exist. High abundances of adults were found in the mid and lower intertidal zone but not in the high intertidal zone. There was no indication that dispersal of M. arenaria spat in a landward or seaward direction contributed significantly to the observed distribution pattern, since spat occurred abundantly at all tidal levels except in the high intertidal zone. There was no evidence of negative adult-juvenile interaction. M. arenaria was not attracted by seagrass or projecting shell beds -the latter indicating erosion of the sediment -as abundances of adults and juveniles were generally low in these habitats. The effects of sediment type and of the bioturbating lugworm Arenicola marina were inconsistent. While adults were more abundant on muddy sand than on sand, recruitment was independent of sediment type. At all high density sites of adults (>50 ind m -2 ), lugworm densities were below 5 ind m -2 , which may indicate a negative interaction. However, lugworm densities >30 ind m -2 did not prevent relatively high M. arenaria recruitment of >500 ind m -2 .

A field experiment on the reproductive success of the invasive clam <i>Mya arenaria</i> (Bivalvia) in the Tagus estuary: coexistence with the native clam <i>Scrobicularia plana</i>

“…The maximum density of juveniles recorded in the study site is within the range reported by other authors in northern European populations (van der Veer et al, 1998;Strasser et al, 1999;Strasser and Gunter, 2001). …”

“…A plausible explanation is that settlement depends on local hydrodynamics, as pointed out by other authors (Armonies, 1996;Hunt and Mullineaux, 2002). Therefore, the aggregation of juveniles may be observed in different levels of the intertidal region (Emerson and Grant, 1991;Strasser et al, 1999) depending on the dominant current regime. However, the distribution process of Mya arenaria juveniles may also depend to some extent on the active role of the species, which is suggested by the juveniles of the invasive clam avoiding the excavated plots.…”

Life history characteristics of the clam Mya arenaria in the White Sea

“…Predominance of a single generation in local M. arenaria populations was also reported from the Wadden Sea (Strasser et al 1999). A number of studies demonstrated drastic (Smidt 1944;Goshima 1982) as well as cyclic (Glude 1954;Commito 1982;Sadykhova 1982) changes in the population structure of this species in different parts of its geographical range.…”

“…However, to assume that successful recruitment of soft-shell clams is only possible when the number of older specimens is low would probably oversimplify the mechanisms of population dynamics. There is a lot of data available about a lack of correlation between the number of spat and number of adult specimens in M. arenaria local populations (Kühl 1955;Brousseau and Baglivo 1988;Günter 1992;Strasser et al 1999). The obvious synchrony of M. arenaria bed dynamics in both investigated habitats suggests the existence of a common mechanism regulating recruitment rate and periodicity in these areas.…”

Withdrawn: “A health status survey of clams, Mya arenaria and Ensis siliqua, in the Irish Sea”

“…To date, little or no disease or pathogens have been reported in Mya arenaria on European shores (Strasser et al, 1999), in contrast to the number of diseases and parasites being present in the more intensively cultured M. arenaria populations of North America. No parasites, pathogenic states or disease were found in any of the soft shell clams sampled in either of the two sites in the current study, despite a heavy prevalence of trematodes, neoplasia and Nematopsis spp.…”