Introduction: Parasitology is an important field in aquatic science. Because of its close linkeage to other fields in marine sciences such as fisheries, mariculture, fish ecology and environmental monitoring, marine parasitology should be seen in the context of other marine science disciplines (see Marine Science Special Training Course-MST). Fish parasites play a major role in marine biodiversity, infecting hosts at all different trophic levels. The growth of marine aquaculture, concerns about the effects of pollution on fish health, and the possible use of parasites as biological indicator organisms has led to a steady increase in interest in this topic.
Fish and fisheries products are important sources of protein and contribute a great deal to available food resources worldwide. Over-fishing and environmental degradation are already threatening most of the larger fish stocks, and a further increase in fisheries production seems to be dependent on the cultivation of aquatic organisms within semi-extensive and intensive mariculture. An intensive culture leads to an increasing risk of infection by disease causing agents, such as fungi, viruses, bacteria and parasites.
Parasites are an integral part of every ecosystem, representing a major factor in global biodiversity. Host-parasite checklists suggest that on average, there are at least 3-4 metazoan parasites per studied marine fish species within a specific environment. This led to a conservative estimate, by Klimpel, Palm, Seehagen & Rosenthal (2001), of 20,250 to 43,200 marine metazoan fish parasites, calculated on the basis of the 13.500 currently known fish species that inhabit brackish or marine waters. Fish parasites clearly constitute a major part of the living animal species within the world’s oceans.
Parasites in marine fish are of public concern if they are found dead or alive within food products that are intended for human consumption. Besides infection from living parasites, pathogens that are already dead or their remains within the fish tissue might harm the consumer by causing allergic reactions. The disgusting appearance of heavily parasitized fish can prevent them from being sold on local and international markets as their presence in the musculature can offend potential consumers. Thus, parasites can significantly contribute to financial losses for the fisheries industry.
Worms in the fish musculature
Parasites as disease causing agents can have various effects on the fish’s health. They can infest all organs, and depending on the site and intensity of infestation, they are more or less harmful. A high number of negative consequences does not only influence the fish itself but is assumed to also have a larger impact on whole ecosystems. Interestingly, disease causing agents and parasites have not yet been included within models to monitor the population dynamics of fish stocks in the field of fisheries biology, the basis of the actual management of commercially important marine fish stocks worldwide.
Worms in the fish liver
Fish parasites are important biological indicators to describe migration patterns of fish stocks, trophic and phylogenetic interactions as well as pollution and eutrophication. Within fisheries biology, fish parasites were successfully used to separate different fish stocks. Long-living species such as endoparasitic helminths can give information on the seasonal migrations of their hosts and migration habits of different age groups, while short-living species, combining a direct life cycle and high reproduction rates such as protozoan ectoparasites and monogenean trematodes can give information on environmental conditions. It can be expected that with an increasing knowledge of the species diversity, life cycle biology and ecological needs of the various marine fish parasites, further information becomes available for their potential use as biological indicators. This tool will surely provide a better ecological understanding of the whole marine ecosystem.
Marine fish parasitology contributes to various scientific disciplines. Zoological studies, such as those dealing with the taxonomy, classification, morphology and phylogeny of marine fish parasites are still needed, if our attempts to provide a natural animal system including the fish parasites are to be successful. Besides utilizing classical metho-dologies such as morphometrical studies or using scanning and transmission electron microscopy, modern techniques in biochemistry and molecular biology offer a wide range of new applications to better understand the phylogenetical relationships as well as the morphological and physiological characteristics of the different species. In addition, topics connecting the fish hosts with their parasites, such as host-parasite co-evolution or host parasite interactions (host and site specificity, immunology, pathognicity) have not yet been fully explored. Even the coelacanth Latimeria chalummnae Smith, 1939 is known to harbor several different marine fish parasites. Therefore, modern marine fish parasitology is firmly anchored within the different biological disciplines, and can offer various topics for integrated research activities.