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Linear growth of soft-shell clams, Mya arenaria L., was studied at six locations in the White Sea situated in the middle and low tidal zones. The main aim was to analyse the growth heterogeneity both within beds (with respect to individual growth) and between beds (with respect to group growth). Mya population was characterized by a slow growth and a long lifespan. Maximum shell size was 70-80 mm, and longevity was at least 25 years. In addition, growth rates were highly variable, statistically significant differences being present both within and between the beds. No differences were found in group growth between different tidal horizons of the same site. The slowest growth of soft-shell clams was observed at sites with the greatest content of fine particles (silt and clay) in sediments. Differences between beds in Mya group growth could reflect variability of feeding conditions, which were probably determined by some local features (e.g. hydrodynamic regime). However, the variation of the individual growth rate within beds was comparable to that of the group growth rate between studied locations. To a great extent it seemed to be determined by the characteristics of the initial period of soft-shell clam growth.

We studied distribution and growth of Serripes groenlandicus and Macoma calcarea in the southeastern part of the Pechora Sea. The hypothesis was tested that trends in the site-to-site variability of population characteristics of these two bivalve species were driven by their feeding types (suspension-feeder and deposit-feeder, respectively). However, such a trend was found only in the abundance distribution of these species and site-to-site variability in growth rates of S. groenlandicus. M. calcarea density on silty sediments was almost twice as high as on sandy sediments, while Serripes biomass was almost 1.5 times higher on sandy sediments than on silty sediments. The slowest-growing Serripes were found at the deepest stations and in habitats with the largest content of fine fractions (silt) in sediments. Differences in the growth of S. groenlandicus could reflect variability of feeding conditions of this suspension-feeder (e.g., hydrodynamic conditions). Thus, the growth rate of S. groenlandicus was sensitive to environmental conditions, which means it can be used as an indicator of their changes. In general, S. groenlandicus in the Pechora Sea is very slow-growing compared to other areas (maximum life span and shell length are 28 years and 70 mm, respectively). Their growth rate was closest to that in Arctic-influenced locations. On the contrary, the maximum life span and shell length of M. calcarea in the Pechora Sea (15 years and 30 mm, respectively) were similar to those in other parts of the distribution area. No significant differences were found in the group growth of M. calcarea from different studied localities.

The Mya arenaria generation in the White Sea was observed for almost the whole life cycle (around 25 years). Using the data on this generation dynamics, the cohort life table was built. The main purpose of the research is analysis of age-specific mortality in this softshell clam population. The mortality rate was found to change more than tenfold throughout the study period. No regular changes in its value were observed. Periods of low mortality alternated with periods of much higher mortality. Several reasons for the increase in M. arenaria mortality rate during its life cycle are proposed: (a) living in the surface sediment layer at early stages of life cycle (unstable environment, high mortality of non-viable clams, predator impact); (b) intense intraspecific relationships in dense aggregations of young clams; (c) increased intraspecific competition during periods of the rapid individual growth; (d) ageing (clam mortality increases with older ageachievement of an average and a maximum lifespan).

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