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Comparative studies on reproductive and developmental features in brackish water nereidid polychaetes of the genus Hediste (especially two Asian species, a small- and a large-egg form) are reviewed. General characteristics of the gamete ultrastructures of the two forms are very similar, though some minor differences were detected between them. The mode of reproduction, body size of mature adults, egg size, fecundity and mode of development are markedly different between the two forms. Not surprisingly, the two forms have contrasting life histories: the small-egg form with planktonic development migrates between brackish waters and the sea, while the whole life history of the large-egg form is usually completed in brackish waters within a river. The different life histories are reflected in different levels of genetic differentiation between geographically separated populations of the two forms. The life history characteristics of the above two Asian forms are compared with those of three other Hediste species with special reference to a relationship between developmental pattern and speciation.