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Zoographical distribution of metazoan fish parasites in herring, Clupea harengus, from the Baltic Sea was analysed in order to use them as potential biological indicators. A total of 210 herring from six different sampling sites were investigated, harbouring 12 different parasite species [five digeneans (D), one cestode (C), three nematodes (N) and three acanthocephalans (A)]. The distribution of the parasite species differed according to region, with a distinct gradient of decreasing species richness towards the east of the Baltic Sea. The western localities at Kiel Bay, Rügen and Poland had the highest parasite diversity, including the marine parasite species Anisakis simplex (s.s.) (N), Brachyphallus crenatus and Hemiurus luehei (both D). The eastern localities had low parasite species richness, predominated by the freshwater digenean Diplostomum spathaceum. We could identify three different Baltic herring stocks, the spring-spawning herring of the western Baltic reaching from the Kattegat to the German and Polish coast, the stock of the central Baltic proper and the northern stock of C. harengus var. membras of the Gulf of Finland. The limited distribution of the herring parasites within the Baltic Sea enables their use as biological indicators for migration patterns and stock separation. The acanthocephalan Pomphorhynchus laevis that has already been used as an accumulation bioindicator for heavy metals was only recorded for the western herring stocks. However, the presence of mainly generalistic parasites and their uneven distribution patterns make their use as indicators for regional environmental and global change more difficult.
A total of 38 Coryphaenoides mediterraneus from the Charlie-Gibbs Fracture Zone (CGFZ), a part of the Mid-Atlantic Ridge (MAR), was studied for parasites and feeding ecology. Sixteen different parasite species were found, most of them belonging to the Digenea (6 species) and Nematoda (6). Twelve new host and 11 new locality records were established, and 8 deep-sea generalists and 5 deep-sea specialists were found. Twelve adult and 3 larval parasites occurred, with Allopodocotyle margolisi (Digenea), Tetraphyllidea indet. (Scolex pleuronectis, Cestoda) and Ascarophis longiovata (Nematoda) being the predominant species. These parasites reached a prevalence of 50.0%, 86.8% and 68.4% with an intensity of infection of 1–10, 1–91 and 1–74, respectively. The food consisted of crustaceans and cephalopods; no further prey items such as fish could be identified. Coryphaenoides mediterraneus demonstrates a parasite fauna similar to that of other deep-sea macrourids. No MAR-specific parasite species were found, and the collected helminths are common fish parasites of the North Atlantic deep-sea basin and the adjacent continental shelf regions. Only few larval epipelagic generalists such as ascaridoid nematodes were found, due to the deep origin of the studied fish between 1700–3500 m. The recorded species rich parasite fauna reflects the wide depth range and opportunistic feeding behaviour of C. mediterraneus on benthopelagic food. The recorded parasite species composition around the MAR appears to be similar to other deep-sea locations in the Atlantic Ocean, demonstrating the wide zoogeographical distribution of these deep-sea metazoans. Consequences of the MAR, the CGFZ and the homogeneous deep-sea environmental conditions for the parasite species distribution are discussed.
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