Marine and freshwater fish are hosts to a rich fauna of ectoparasites, living on their gills and skin feeding on blood, mucus and epithelial cells. Fish can easily be obtained and examined in large numbers. Fish ectoparasites represent a highly diverse group including monogeneans, crustaceans, isopods, mollusks and hirudineans. This makes them almost ideal objects for ecological studies. Such studies have been conducted by several researchers, using a range of host species and ecological techniques, with the aim of identifying patterns and processes in parasite communities. Studies have concentrated on different levels of community organization, i.e., those of infra-, component and compound communities, and examined questions of saturation vs. non-saturation of communities, degree of aggregation, temporal and spatial variability of organization, limiting similarity and niche segregation, host specificity, nestedness, and degree of structuring in communities as revealed by null model analyses. All these aspects are of significance in an evaluation of how common equilibrium and nonequilibrium conditions are in ecological communities, the main topic of this book. In this chapter, we provide an up-to-date account of relevant studies.
Parasite communities have been commonly studied at different levels, i.e., those of infracommunity, component community and compound community (Holmes & Price, 1986). A parasite infracommunity consists of all the infrapopulations (populations of all species) within a host individual. Infracommunities are incapable of self-perpetuation (because most parasites disperse their propagules into the free environment where they usually develop before reaching a host). A parasite component community consists of all infracommunities within a host population. The boundaries for the component community depend on spatial scale (Aho & Bush, 1993). For example, one can consider a component community as (1) all parasites in all individuals of a given host species from a specific collection site of a water body, or (2) all parasites of all individuals throughout the host’s geographical distribution or of a range of host distributions for which the data were obtained.