THE ADAPTIVE VALUE OF OFFSPRING SIZE AMONG FRESH-WATER MUSSELS (BIVALVIA, UNIONOIDEA)

1. 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. The number and type of marsupia (four marsupia, two marsupia, two marsupia on the posterior part of demibranchs) also does not seem to be related to glochidial size. 4. The morphological factor 'hook on the glochidial shell' and the ecological factor 'host range' (number of host fish families) are of importance. Glochidia with a hook are larger than unhooked ones. They frequently attach to tough host tissues and therefore have to be powerful. Glochidial size also increases as the host range increases. This relationship probably evolved because of an interplay of the duration of the parasitic stage and the host's immunological defences. 5. The analyses suggest that glochidial size is a crucial component in the evolution of a continuum of reproductive strategies among the Unionoidea. Representatives of one end of the continuum are the Margaritiferidae with small glochidia, high fertility, long parasitic stage, protracted growth on the host and narrow host range. They are highly specialized with respect to the habitat. Some Anodonta species represent the opposite end with large glochidia, low fertility, short parasitic stage with no growth being involved and large host range. They live in a variety of habitats.