Abstract
This study aimed to elucidate the mechanism of glochidial release in Unio pictorum (L., 1758) and U. tumidus Philipsson, 1788, and thus interpret the confused reports of unionine reproduction in the literature. Both species release mature, hooked glochidia on mucous threads, resembling a pearl necklace. Individual threads can reach 15 cm in length and carry c. 500 glochidia. Under hypoxic conditions both species released conglutinate-like structures comprising c. 500 eggs and immature, unhooked glochidia. The conglutinate-like structures released by U. pictorum and U. tumidus are coloured yellow and white, respectively. Increased temperature did not induce the release of conglutinates. The conglutinate-like structures are unlikely to be involved in increasing reproductive success by attracting host fishes. Rather, the premature evacuation of the gill contents may relieve the respiratory burden on adult mussels under hypoxic stress. Ambient hypoxia during the reproductive season may help explain the irregular recruitment success of relatively long-lived mussels, such as U. pictorum and U. tumidus.
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