Ecology.
Authors: Rohde K. 
Published: November 2017

This chapter includes discussions of ecological aspects of marine parasites that have been studied in some detail, and which are attracting much attention and can therefore be called 'hot topics'. These aspects include transmission of parasites to hosts; specificity of parasites to hosts and microhabitats; parasites as hosts for other parasites; adaptation of parasites to an extreme habitat, brackish water; metapopulation biology; the structure of marine parasite communities; and parasite communities as non-equilibrium systems. All parasites depend for survival on hosts, at least during part of their life cycles. It is therefore essential that transmission to hosts is assured, as discussed in this chapter. There is no 'universal' parasite that infects all available microhabitats on all available host species. In other words, each parasite species occupies a particular niche - it is microhabitat and host specific. But how is specificity measured? The second section gives an account of such measures. It also discusses proximate and ultimate causes of niche restriction (i.e. the immediate chemical and physical causes that direct a parasite to its niche and are necessary for its survival there), and the biological function of niche restriction. Parasites not only parasites their hosts, but also may be hosts to parasites (hyperparasites) themselves (e.g. Udonella, Epicaridea, Microsporidia). Marine hyperparasites include crustaceans, monogeneans, nematodes, myxozoans and many protistans. Brackish water systems are intermediate between freshwater and genuine marine ones. Parasites have various degrees of adaptations to these brackish water habitats, which are discussed in the relevant section. The next 3 sections deal with aspects of population and community ecology including metapopulation biology. The concept of metapopulation is not old. It was introduced to emphasize that populations are not homogeneous collections of individuals but are often composed of subpopulations that are, to a large degree, spatially separated, with limited exchange of individuals between them. This has important consequences for population biology, affecting the survival of species, as discussed in the metapopulation biology section. Much effort has gone into the study of marine parasite communities. A concise and up-to-date discussion of such communities is given in the relevant section, which includes not only a discussion of community patterns but also of processes leading to the patterns. Ecological systems have long been interpreted as ones in equilibrium, in which habitats are saturated with species and individuals and in which interspecific competition is of overwhelming importance. However, it has become increasingly evident that this interpretation is not correct or is of limited value only. Ecological systems, rather than being in equilibrium, are almost always in non-equilibrium, and much evidence for this view comes from the study of marine parasites, reviewed here. The chapter concludes with a detailed discussion of an ecological system of marine parasites that has been studied over many years in different localities, that of larval trematodes in mollusc hosts. Because of the thoroughness of studies, it can be referred to as a model system and the section discusses aspects of both population and community biology.