Abstract
The predominant reproductive mode for the Unionoida is hypothesised to be dioecy (gonochorism), with hermaphroditism sporadically distributed within the order. However, since the reproductive modes of unionoidean bivalves have been relatively well documented in comparison to the situation within the Etherioidea, additional data on the reproductive patterns of the latter are necessary to test this generality. A summary of the historic works dealing with hermaphroditism in North American and European Unionidae was presented by Van der Schalie. Within the North American fauna, a relatively large number of dioecious species exhibit a low incidence of hermaphroditic individuals, while a smaller number of species are typically hermaphroditic. Of the ca 300 unionoid species in North America, only seven species have been Figure 4. Daily population size estimates based on the modified Lincoln index. Vertical bars give standard errors of population estimates. The dashed horizontal line shows the total number of individuals found (Fig. 3). Estimates could not be calculated for surveys 9 and 10, because no new individuals were found. Figure 3. Cumulative number of individual sea slugs recorded over our ten-day survey. Open circles indicate observations at high tide, filled ones observations at low tide. ACKNOWLEDGEMENTS
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