Diagnostic Problems in Connection with Fish Diseases Caused by Monogenoideans
Most monogenoideans (monogenetic trematodes) are fish parasites. Some of them are serious pests in fish cultures, best known are those caused by certain species of the genera Gyrodactylus and Dactylogyrus Both these genera are represented by a large number of host specific species on feral fishes in Eurasia and North America. However, certain others of the monogenoidean genera can also damage their hosts, e.g. in tanks, fish ponds (chests) and aquaria. This should not be overlooked, for instance when introducing species as new aquariam fishes from other regions, such as the Indian or the African region. On the other hand not all the monogenoideans in a region, or those imported from a foreign region, are likely to be potential threats to fish culture. In this respect, however, our present knowledge is very limited. In fact, the monogenoidean faunas of both the Eurasian and the North American regions are still far from fully known, while those of the other biogeographical regions are relatively unknown. Each year, however, a large number of new monogenoidean species are described. Unfortunately, many of these descriptions are (in various respects) incomplete. Mostly they lack the necessary comparative drawings for separating the species from other similar species. This presents considerable problems in numerous genera, though probably not everywhere as serious as for the genus Gyrodactylus. This lack of good comparative drawings results, in turn, in inadequate keys to the monogenoidean faunas. The existing keys are mostly true compilations (including the drawings) of described species. We therefore badly need new keys, based upon comparative analysis of the included species and well illustrated with comparative drawings of the taxonomically important characteristics. This in turn will involve considerable economical investments, partly in terms of salaries for taxonomic specialists over a period of many years, but also because of the high printing costs for profusely illustrated publications.
The monogenoideans are very small creatures and consequently the species characteristics are often minute. The species diagnostics mostly call for special methods and optics (e.g. phase microscope). This means that even if we have very good keys, the true species diagnostics will remain a problem to be solved by the specialists. However, good keys will assuredly help to increase the number of good specialists.