Definitions, and adaptations to a parasitic way of life

Biologists approach the study of parasites from different angles: some are interested in their physiology, others in their ecology, or medical and economic aspects, to mention only a few. Consequently, definitions of parasitism vary greatly, reflecting the research interests and biases pf particular research workers. Baer (1952), Esch and Fernfindez (1993) and Rohde (1993, 2001) have discussed definitions and adaptations of parasites. In most sections of this book, parasitism is understood to be a close association of two organisms - which one - the parasite - depends on the other - the host - deriving some benefit from it. This benefit often is food. In some sections, however, the term parasite is used in a wider sense. For example, peritrich ciliates leading to severe problems in shrimp aquaculture are really fouling organisms that become pathogenic and fatal in heavy infections. Many bacteria, viruses and fungi are parasitic. These organisms have traditionally been studied by microbiologists, and only protistan and metazoan parasites are considered to be objects of study by parasitologists sensu strictu, although the border between protistans and fungi is somewhat blurred. In this book, only protistan and metazoan parasites are discussed.