‘Protozoans’

Free-living protozoans are ubiquitous and abundant in the aquatic environment, although they may go unnoticed because of their small size. According to Roberts & Janovy (2000), as many as 45,000 different kinds of protozoans have been identified to date and it is not surprising that contact between these organisms and fishes has produced a range of parasites with many different life styles. Some protozoans occur on the exposed surfaces (skin and gills) of fishes and include epizoic forms (using the host merely as a platform), like the trichodinid and scyphidiid ciliates, as well as forms with an obligatory ectoparasitic life style, such as the flagellate Ichthyobodo necator. Many of these organisms are so small that the resolution of the light microscope is not enough to distinguish their structural details and it is necessary to resort to the electron microscope to supplement our knowledge of their anatomy. On the other hand reproductive processes in many endoparasites may produce aggregations of individuals or spore-bearing nodules that are sufficiently large and conspicuous to be visible to the naked eye. Strictly speaking, these eye-catching endoparasites fall outside the remit of this book, but the ciliate Ichthyophthirius multifiliis (‘white spot’) is included because it spends its entire parasitic life inside the epidermis, and its interactions with the epidermis are relevant to our study of the biology of ectoparasites.