ABSTRACT
Parasites and pathogens are a ubiquitous part of the environment of every organism. Hereafter called parasites, these invasive species themselves represent a full range of taxa, including viruses, bacteria, protists and several metazoan groups including cestodes and crustaceans. Some parasites develop directly on or in a host, others have extremely complex life cycles, requiring several sequential obligate life cycle hosts in order to complete their development. The infobox for this chapter provides defi nitions of the important parasitological terms used in this chapter. Ecologically speaking, host organisms can be seen as the environment for the parasite providing a number of distinct habitats for parasites to colonize (Bush et al. 1997). Most parasite species occupy specifi c tissues or organs on or within their host, and there may often be a whole community of different parasite species colonizing a single host individual. However, the parasite community of one host species may differ between localities and geographical regions because in addition to the immediate surroundings of the host’s body, parasites are also infl uenced by abiotic parameters of the wider environment, such as the temperature, salinity or pH of the wider environment in which the host lives. Furthermore, many parasite species have free-living stages or depend on the occurrence of specifi c intermediate hosts or vectors (see infobox) that may vary in abundance or occurrence from place to place.
