Geographical differences in growth rates of Arctica islandica (Mollusca : Bivalvia) from the North Sea and adjacent waters

Geographical differences in the shell growth rate of several populations of the bivalve Arctica islandica (Mollusca: Bivalvia) were estimated by using the growth lines laid down during their first ten years of life. Attention was focused on populations from the North Sea, but for comparison small sa mples from adjacent waters were also analysed. A four-fold difference in th e average growth rate was found between the slowest and fastest growing she lls. Principal component analysis was used to summarize the inter-relationships between environmental variables and growth rates. Shell growth correlated p ositively with primary production and temperature and inversely with depth and the silt content of the sediment. The North Sea specimens were found to have a strong positive correlation with grain size. Since sediment charact eristics also depend on bottom currents, it is suggested that these increas ed rates reflect lateral seston flux as additional food supply. In a multiple regression model, applied to all available data, average annu al temperature, primary production and the interaction between production a nd water depth explained 50% of the variance. The derived standard coeffici ents for temperature, primary production and the interaction between depth and primary production were 0.90, 0.47 and -0.92 respectively. The results of this study suggest that the temperature effects on in situ shell growth are easily overruled by other environmental factors. If a similar model was calculated with North Sea data only 75% of the varia nce was explained by temperature, primary production and depth x primary pr oduction. The standard coefficient for primary production was 1.26. The rol e of temperature in explaining the observed growth differences is negligibl e since the standard coefficient is -0.098. The interaction term, depth x p rimary production had a standard coefficient of -0.95.