Microscopic structure of the mantle and palps in the freshwater mussels Velesunio ambiguus and Hyridella depressa (Bivalvia : Hyriidae)

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 Hyridella 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.