The Adaptive Value of Offspring Size among Freshwater Mussels (Bivalvia; Unionoidea)

The glochidia (larvae) of the freshwater mussels Unionoidea must attach to a host fish to pass through their parasitic stage. Glochidial size is species-specific: varying from c. 0.05 mm to c. 0.45 mm shell length. 2. There is a trade-off between size and number such that fertility is high in species with small glochidia and vice versa. 3. There appear to be no allometric constraints on glochidial size. Phylogenetic constraints seem to be of minor importance (perhaps with the exception of the Margaritiferidae). Furthermore, no influence can be shown for the breeding type (long- or short-term breeder) or the habitat type with respect to the water current