Freshwater mussels in the order Unionida are a highly endangered and globally distributed taxon of parasitic bivalves. Consequently, many conservation and reintroduction projects have emerged to protect this valuable order. Adult mussels release parasitic larvae (glochidia) that affix themselves to their hosts, typically the gills of fish, where they become encysted in the tissue and metamorphose into juveniles; later, excysting as juveniles to develop into free-living adults in the sediment, completing the lifecycle. As Unionids are well-known ecosystem engineers, reintroduction efforts are generally met with support. However, members of the public unfamiliar with the scientific literature often have reservations when learning that the restoration project aims to introduce parasites to their local waterways. Little is known about the impacts of glochidiosis on host fishes; however, most effects are negative. With the growth of mussel conservation projects, more fish will be exposed to these negative impacts. 

In this dissertation, I explore the parasitic interactions of two of Europe’s most endangered mussels with some of their respective host fishes. I initially produced a general literature review to summarize all previous work on the topic and better understand which aspects needed the most attention (Paper I). From this, I surmised that the behavioural and molecular impacts of glochidiosis were the least studied and that there was a general over-representation of the interaction between the freshwater pearl mussel (Margaritifera margaritifera) and its salmonid hosts, a highly specialized interaction not well-suited for cross-species generalizations. I followed Paper I with an investigation into the impacts of infestation by the thick-shelled river mussel (Unio crassus) on the schooling behaviour of its host minnow (Phoxinus phoxinus; Paper II), and the impacts of both adult and larval life stages of U. crassus on the predator-prey interactions of its host bullhead (Cottus gobio) with a prey gammarid (Paper III). The results of Papers II and III suggested that unionid mussels may manipulate host behaviour; to test this hypothesis, a year-long field study was conducted on the movement patterns and habitat choice of brown trout (Salmo trutta) when infested with glochidia from M. margaritifera, the species most likely to express an extended phenotype (Paper IV), as speculated in Paper I. This investigation provided evidence in support of an expressed extended phenotype by a unionid mussel on its host fish.

Unionid mussels are a unique order of bivalves characterised by the presence of a parasitic larval life stage, known as a glochidium. In this dissertation, I explore the behavioural effects of glochidiosis (the condition of bearing glochidia) on common hosts of Europe’s most endangered unionids. The dissertation includes a general review of the effects of glochidiosis, two laboratory studies, and a year-long field study. The results presented here demonstrate that glochidia induce effects on their host fish similar to those caused by other parasites, including reduced growth, decreased swimming performance, and a general preference for calmer water and more structurally complex habitats. Together, the behavioural alterations induced by glochidiosis may benefit unionid recruitment, as infested host fish may transport excysting juvenile mussels to habitats favourable for later development. This provides evidence for the presence of an extended phenotype in unionids.