The phenomenon of "switching" of the early ontogeny type (pelagic versus nonpelagic) is considered in the context of the presence of alternative modes of early ontogeny in recent and fossil gastropod mollusks. Possible environmental inducing mechanisms (decrease in salinity and/or water temperature), as well as the role of this phenomenon in the evolution of Gastropoda, are discussed. The concept of a "mesopoikilohaline" zone is introduced; it is interpreted as a biologically important barrier of salinity (presumably about 13-15 per hundred) which plays the key role in suppression of the free-living larval stage during the process of gradual water desalination. The change in strategy of early ontogeny is interpreted as a regulator of the adaptation process and, to some extent, as a speciation mode in Mollusca.
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