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. ДВУСТВОРЧАТЫЕ МОЛЛЮСКИ БЕЛОГО МОРЯ. Опыт эколого-фаунистического анализа

is not cut down to participation in rare events with such dramatic results, as it took place in spring 1990.

It was shown during our investigation in Onega Bay that the biomass of fouling organisms on hard sediment enlarges in biotopes where high number of big clams was observed (Naumov, Fedyakov, 1985, a). Possible explanations can be the following: fouling organisms and mollusks investigated need similar environmental conditions, fouling organisms are attracted by mollusks metabolites and shells of big clams can be used as an additional substrate by fouling organisms.

Results obtained show that both extensiveness and intensiveness of the shell fouling mostly depend on the shell length. It means that both are functions of substratum time existence, i. e., of a mollusk's individual age. The development of fouling community on valves of living clams resembles the primary succession on hard sediment (Naumov, Fedyakov, 1985, б, в, 1993; Naumov et al., 1986, a; Naumov, 1990).

Two types of such successions were described by the author and his colleagues. In the first case, species composition did not change during the life of substrate species. It was observed for living clams with vertically orientated valves, such as Mytilus edulis and Modiolus modiolus. Similar development of fouling was noticed for living Elliptica ellip-tica (Naumov, Fedyakov, 1985, б, в, 1993; Naumov et al., 1986, a; Naumov, 1990). Thus, the succession process in this case is reduced to simple increasing the abundance of different sessile organisms.

The second type of succession was observed on horizontally orientated valves of living Chlamys islandica. Species composition of fouling organisms on different valves of scallop shells significantly differs. This allows dividing of the consortium of fouling organisms into two strata: upper and lower. Unlike of the first type, in this case dominant forms change more than once, especially on the upper valve, i. e. the process affects the structure of consortium (Naumov, Fedyakov, 1985, б, в, 1993; Naumov et al., 1986, a; Naumov, 1990).

The succession processes lead to climax association in both cases since fouling of old living clams closely resembles the fouling of rocks in the same ecosystems (Naumov, Fedyakov, 1985, б).

The fraction of deposit feeders among Arctic bivalve endemics is approximately two times higher than in clams of other origin. This can be explained by lower phytoplankton production caused by thick ice cover leading to weak illuminace in Arctic seas. One can suggest that gathering deposit feeders feeding on decaying organic matter will have an advantage over filter feeders in such conditions (Naumov, Fedyakov, 1990, 1994). Comparison of the proportion deposit feeders in various seas of different biogeographical regions confirms this hypothesis (Tabl. 0). After this suggestion, the White Sea can be defined as a waterbody intermediate between boreal and Arctic seas.

The peculiarities of individual species with respect to main environmental factors are important for characterizing any regional fauna. Seawater temperature and salinity are most significant for marine organisms. Since the White Sea waters are not stratificated by temperature in winter, being cooled down close to 0°C or even below in the entire watercol-umn, distribution of bottom animals depends on summer temperature. Original author's data stored in the "Benthos of the White Sea" database were used for investigation of clam distribution concerning the temperature in July-August. The database contains results of benthic investigations carried out by the White Sea Biological Station (Zoological Institute) since 1981. Analysis of this material showed that the most part of the White Sea bivalve mollusks prefer summer temperature diapason ranged between 0 and 10°C. Three groups of them can be defined according to their ability to survive in the White Sea conditions under impact of high temperature:

1) Species which cannot withstand a short rise of temperature higher than 11°C.

2) Species which can withstand a short rise of temperature approximately up to 15°C.

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