Glochidial Mortality in Freshwater Mussels

Abstract

A crucial factor in the reproduction of parasites is the rate of successful host infection. In most parasitic species subtle mechanisms for the effective transmission of infectious stages have evolved. These include vector transmission, highly mobile propagules that are able to locate and infect new hosts actively, attraction of hosts, extremely resistant propagules that may spend an extended dormant period “waiting” for a suitable host, or parasite transmission by direct contact between host individuals (Anderson 1981; Begon et al. 1988). The precise mode of host infection must be considered an important evolutionary constraint (Calow 1983). Except for the African and South American families Mutelidae and Mycepodidae, which produce parasitic larvae with a specific morphology (Wächtler et al., Chap. 6, this Vol.; Hoeh et al., Chap. 14, this Vol.), in the Unionoidea, infection is achieved via a unique larval stage, the glochidium. Glochidial anatomy is well adapted for efficient attachment to epithelia of aquatic vertebrates (Lefevre and Curtis 1910; Arey 1924a). Three morphological types of glochidia can be distinguished based on the number and shape of hooks on their ventral apex. The most common (e.g. genera Margaritifera, Quadrula, Elliptio) form of glochidia is hookless, referring to the absence of a spined hook projecting inward from the ventral edge of each valve present in hooked glochidia (e.g. genera Anodonta, Unio, Lasmigona). A third form of glochidia (axe-head) with distinctly flared valve margins carrying double hooks is known from the North American genus Proptera (Lefevre and Curtis 1910).