Ref ID : 4614
Robert W. Pennak; Ecological Affinities and Origins of Free-Living Acelomate Fresh-Water Invertebrates. In: The Lower Metazoa, Univ.Calif.Press, 1963, p.435-451
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The physiological and ecological peculiarities of acelomate fresh-water metazoa are summarized, with special reference to their marine counterparts. The difficulties confronting marine species in overcoming the brackish-water barrier and becoming further adapted to fresh-water habitats are of enormous importance, but the psammolittoral-phreatic pathway from salt to fresh waters offers a much less difficult ecological migration route than estuarine waters. Since the great majority of acelomate fresh-water metazoa have lost the motile larvae, are remarkably euryokous, are adapted to low concentrations of dissolved electrolytes, have developed effective osmoregulatory devices, and produce special resistant structures or disseminules, it must be concluded, on the basis of genetics, that the successful colonization of fresh waters by any taxonomic group is a rare evolutionary achievement.