Двустворчатые моллюски Белого моря - Зоологический институт ...

294SUMMARYis not cut down to participation in rare events with such dramatic results, as it took place inspring 1990.It was shown during our investigation in Onega Bay that the biomass of fouling organismson hard sediment enlarges in biotopes where high number of big clams was observed(Naumov, Fedyakov, 1985, a). Possible explanations can be the following: fouling organismsand mollusks investigated need similar environmental conditions, fouling organismsare attracted by mollusks metabolites and shells of big clams can be used as an additionalsubstrate by fouling organisms.Results obtained show that both extensiveness and intensiveness of the shell foulingmostly 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 valvesof living clams resembles the primary succession on hard sediment (Naumov, Fedyakov,1985, б, в, 1993; Naumov et al., 1986, а; Naumov, 1990).Two types of such successions were described by the author and his colleagues. In thefirst case, species composition did not change during the life of substrate species. It wasobserved for living clams with vertically orientated valves, such as Mytilus edulis andModiolus modiolus. Similar development of fouling was noticed for living Elliptica elliptica(Naumov, Fedyakov, 1985, б, в, 1993; Naumov et al., 1986, а; Naumov, 1990). Thus,the succession process in this case is reduced to simple increasing the abundance of differentsessile organisms.The second type of succession was observed on horizontally orientated valves of livingChlamys islandica. Species composition of fouling organisms on different valves of scallopshells significantly differs. This allows dividing of the consortium of fouling organisms intotwo strata: upper and lower. Unlike of the first type, in this case dominant forms changemore 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, а; Naumov, 1990).The succession processes lead to climax association in both cases since fouling of oldliving 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 twotimes higher than in clams of other origin. This can be explained by lower phytoplanktonproduction caused by thick ice cover leading to weak illuminace in Arctic seas. One cansuggest that gathering deposit feeders feeding on decaying organic matter will have an advantageover filter feeders in such conditions (Naumov, Fedyakov, 1990, 1994). Comparisonof the proportion deposit feeders in various seas of different biogeographical regionsconfirms this hypothesis (Tabl. 0). After this suggestion, the White Sea can be defined as awaterbody intermediate between boreal and Arctic seas.The peculiarities of individual species with respect to main environmental factors areimportant for characterizing any regional fauna. Seawater temperature and salinity are mostsignificant for marine organisms. Since the White Sea waters are not stratificated by temperaturein winter, being cooled down close to 0°C or even below in the entire watercolumn,distribution of bottom animals depends on summer temperature. Original author’sdata stored in the “Benthos of the White Sea” database were used for investigation of clamdistribution concerning the temperature in July–August. The database contains results ofbenthic 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 bivalvemollusks prefer summer temperature diapason ranged between 0 and 10°C. Three groups ofthem can be defined according to their ability to survive in the White Sea conditions underimpact 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.