Abstract:
There has been increasing interest in freshwater mussels (order Unionoida) in recent years because their
numbers are declining in many parts of the world and also because they have potential as monitors of
pollution. Most studies have been performed on the families Unionidae and Margaritiferidae from North
America and Europe, and comparatively little is known of the Hyriidae from Australasia. The present study
describes the microscopic structure of tissues in the mantle and palps of two hyriid mussels, namely
Velesunio ambiguus and Hvridella depressa, as viewed by light and electron microscopy. The two mussels
show similarities with the unionids and margaritiferids, particularly the presence of extracellular
mineralised granules. The mantle and palps of V ambiguus and H. depressa consist of flaps of tissue
bordered on the inner and outer surfaces by simple epithelia. The intervening tissue is dominated by
connective tissue containing vesicular cells, muscle, nerves and blood spaces with haemocytes.
Orange-yellow extracellular calcified granules are a prominent feature of the interstitial tissues. The
abundance of calcified granules in the mantle of H. depressa is greater than that in V ambiguus and there
are differences in the appearance of the apical vesicles in epithelial cells.
