DENSITY-DEPENDENT GROWTH IN COCKLES (CERASTODERMA-EDULE) - EVIDENCE FROM INTERANNUAL COMPARISONS

To examine density-dependent effects on growth in Cerastoderma edule (L.) (Bivalvia), growth rates of individual cockles established by chance at high and low densities on the same intertidal mudflat in two different years (1984 and 1989) were compared. Two-year-old cockles occurring at high densities (>2000 individuals m-2 in 1984) attained mean lengths from 16.1 to 18.8 mm on the lower shore at the end of their third growing season, while low-density cockles (<50 individuals m-2 in 1989) of the same age reached mean lengths in the range of 26.5 to 30.3 mm. In terms of individual weight the difference between cockles from the two periods was even more striking, as the flesh ash-free dry-weight of a cockle from 1984 constituted only 7% of that from a 1989 cockle. During both periods growth in cockles increased with the duration of tidal submersion, but the interannual growth differences exceeded by far the effect of differences in submersion time. The present growth data indicate the largest interannual growth difference hitherto reported from a given locality. It is argued that a density-dependent mechanism is the most likely process causing the observed growth differences. The present results support intraspecific competition as an important process influencing individual growth rates and life-history traits of natural populations of suspension feeders.