| Summary: | The structure of the metazoan parasite faunas and communities in Atlantic cod, Gadus morhua L., from six NE Atlantic regions [Baltic, Celtic, Irish and North seas, Icelandic waters and Trondheimsfjord (Norway) and two fish farms in Iceland and Scotland] has been studied. 1,254 fish were sampled in the NE Atlantic during 2002-2003. Altogether 57 parasite taxa were found. The predominant groups in regional parasite faunas were the trematodes (19 species) and the nematodes (13 species). Nine parasite species were found for the first time in cod (Diclidophora merlangi, Rhipidocotyle sp., Fellodistomum sp., Steringotrema sp., Schistocephalus gasterostei, Cucullanus sp., Spinitectus sp., Acanthochondria soleae and Chondracanthus ornatus). Contracaecum osculatum, Hysterothylacium aduncum and Echinorhynchus gadi were the only parasite species found to infect farmed fish. Regional parasite faunas showed lower richness with respect to the total list (c. 65%) with a notable decrease in the Baltic Sea and Trondheimsfjord (21 and 32%, respectively). Eleven species were present in all regions: Lepidapedon elongatum, Anisakis simplex, C. osculatum, H. aduncum, H. rigidum, Pseudoterranova decipiens, Ascarophis crassicollis, Capillaria gracilis, Corynosoma semerme, C. strumosum and E. gadi. Despite of the small size of the monogenean D. merlangi recovered in the present study, the Analysis of Principal Components showed that morphologically they are more similar to D. merlangi from whiting (Merlangius merlangus, type-host), than to other congeneric species from the North Atlantic, thus supporting their assignment to D. merlangi. The aspect and smaller size of the oöcytes suggested that D. merlangi on cod could not produce viable ova, although a specimen exhibited an egg with normally sized and shaped shell. The most species rich and abundant parasite communities were observed in cod from the open water regions (Celtic, North and Irish seas and Icelandic waters). Overall, parasite infracommunities exhibited lower predictability ...
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