Двустворчатые моллюски Белого моря - Зоологический институт ...

296SUMMARYin the Gorlo Strait can play the role of such an obstacle (Derjugin, 1928). The barrier iseasily overpassed by species without planktonic larvae. Statistical methods used confirmDerjugin’s idea of the species number reduction in the White Sea and estimate it quantitatively.Twenty of thirty-nine known White Sea bivalve species, a half of the entire species list,were not found in the Gorlo Strait. They were encountered only near its northern or southernboundary. There is no possibility for accumulation of fine-particle sediment fractions inthis strait because of strong currents; therefore no soft bottom is present there. Soft depositis necessary for nutrition of detritophagous species, hence it is not surprising that only oneof eight White Sea deposit feeding clams was found in the Gorlo Strait. Nevertheless, ininternal parts of the sea deposit feeders are very common. It allows suggesting that in thegeological past hydrodynamic conditions in this strait could be different.Burrowing filter feeders, which need fine-particle mud, were not found in the GorloStrait as well. Some non-burrowing filter feeders, mainly Musculus species, also were notencountered there. One can assume that the absence of such forms is somehow connectedwith their peculiar reproduction properties. To all appearances, they became extinct in theGorlo Strait after the contemporary hydrodynamic regime was formed. It occurred at theend of Atlantic climatic phase about 4–5 thousand years ago. This tide can be regarded asan isolation term in the White Sea of the species spoken about.In general, only species connected with hard bottom and strong currents or species lessspecialized and widely distributed were encountered in the Gorlo Strait.One can consider that the distribution pattern of bivalve mollusks in the Gorlo may byexplained by the geological history of this strait and by biological features of the White Seaclams.A related problem should be mentioned. There are several very interesting inlets withridges in their mouths in the White Sea. Such ridges prevent the summer water exchangebetween deeper part of an inlet and the main White Sea water area. As a result cold water ofwinter origin remains during the whole year in depressions of such small waterbodies (geologicalstructure and hydrological features are described in Chapter 7). The inlets, as a rule,have a little bit wasted fauna of the Central White Sea Depression (Naumov, 1979, б;Naumov, Oshurkov, 1982; Naumov et al., 1986, б; Naumov, Fedyakov, 2000, a, b). In thecase of two depressions within one inlet, the fauna of mouth-part one normally is richer.The vertical distribution of benthos in such inlets closely resembles those in open parts ofthe White Sea. The inlets mentioned can be considered as natural models of the White Seaitself. Number of bivalve mollusks became isolated in them, and the term of isolation canbe calculated with a good accuracy. Explorations of such inlets can lighten the study ofcolonization of the White Sea by marine bottom fauna in Holocene.Only process of colonizing the White Sea in Holocene by bivalve mollusks can by reconstructedrelatively truthfully, for other taxa are just poorly represented as subfossils.As for clams, 27 species were found in subfossil state, which makes about 70% of theircontemporary fauna. Thus the colonization of the White Sea by clams can be considered asan approximate model of other present-day taxa invasion into this waterbasin.Colonization of the White Sea by bivalve mollusks was studied by many authors(Govberg, 1968, 1970, 1973, 1975; Nevessky et al., 1977). There were almost no attemptsas at the present to reconstruct hydrological regime in this waterbody during Holocene usingactualism principle.Portlandia aestuariorum was the first species which subfossil shells were found in theWhite Sea deposits of the Young Dryas climatic phase. This allows suggesting a low salinityrange (about 10–12‰) in this waterbasin during that times. The true marine speciesPortlandia arctica and Mytilus edulis were found already in the deposits of Preboreal climaticphase however. Probably, the first of them being an Arctic endemic species pene-