Characterization of the Unionoida (= Naiads)

This chapter will be a short introduction for a non-specialist to the order Unionoida (= naiads). These large mussels live exclusively in freshwaters all over the earth (Chaps. 2, 10, 15, this Vol.). A problem for the people dealing with this group is its systematic classification (Chaps. 3, 4, 14, this Vol.). This problem is due to the low number of adult characters which may be used for systematics (Chaps. 4, 14, 15, this Vol.). Though there are some 1000 species, naiads have radiated very little in basic structure. Furthermore, a high degree of plasticity (Chaps. 5, 7, this Vol.) renders characters like shell morphology or size unreliable for classification. New biochemical methods (e.g. immunoelectrophoretic or DNA sequence analyses) may be a way out of this dilemma, and most contributions in this book (e.g. Chaps. 5, 13, 15, this Vol.) therefore follow the classification proposed in Chapter 14 (this Vol.). The order Unionoida comprises two superfamilies: Etherioidea (with the families Mycetopodidae and Iridinidae) and Unionoidea (with the families Hyriidae, Unionidae and Margaritiferidae). Very little is known about the Etherioidea, which are confined to the tropics of America and Africa (Chap. 6, this Vol.). However, the data so far suggest that their life history is quite similar to that of the Unionoidea.