Host-parasite interactions: Ecology and evolution Of malacosporean-bryozoan interactions

Abstract

The discovery that freshwater bryozoans are hosts of Tetracapsuloides bryosalmonae, the malacosporean causative agent of proliferative kidney disease (PKD) of salmonids prompted a flurry of research on the ecology of malacosporeans and their interactions with bryozoan hosts. This chapter provides an overview of malacosporean-bryozoan interactions by discussing effects of parasitism on host fitness and strategies of host exploitation through virulent overt and avirulent covert infections. Evidence for host-condition dependent development and substantial levels of vertical transmission in bryozoan propagules (statoblasts) and colony fragments suggests virulence strategies that are closely tied to clonal reproduction in bryozoan hosts. Although there is evidence for T. bryosalmonae strain variation and host specificity, there is much scope for examining patterns of infection in relation to host clonality and local adaptation. The development of overt infections in bryozoans in good condition suggests that environmental change (e.g. warmer temperatures, eutrophication) will entail changes in malacosporean-bryozoan interactions which may, in turn, relate to disease emergence in fish.