ABSTRACT. From October 1987 to September 1988, l778 eels were sampled at monthly intervals in the Elbe estuary, Germany, and examined for parasites in the swimbladder and muscle. Juveniles and adults of the nematode Anguillicola crassus were the only parasites found in the swimbladder and L-111 stages of the nematode Pseudoterranova decjpiens were the only parasites found in the muscle. Averaged over all samples, 57.7 % of the eel were infected with A. crassus and 3.7 % with P. decipiens. Prevalence decreased for A. crassus with increasing fish length, but increased for P. decipiens. No clear seasonal fluctuations in parasite frequency were detected. Infection with A. crassus could not be related to any change in condition factor or liversomatic index. Some P. decipiens from the muscle of smelt after experimental oral transfer settled in the body cavity and muscle of eel. The majority of these nematodes, however, penetrated through the stomach wall, muscle and skin and left the eel.

In April 1990, 488 marine fish, belonging to 30 species from central Philippine waters, were investigated macroscopically for the occurrence of parasites in their flesh and for anisakid nematodes in their body cavity. Twenty-four fish were found to be infected by 1 of 4 different types of parasites. Unidentified Microspora were found in 4 host species from different families. Plerocercoids of the trypanorhynchid cestode Otobothrium penetrans occurred in the flesh of hemirhamphids and belonids only. Adult nematodes of the genus Philometra were found in the garfish Tylosurus crocodilus. The only parasite found which might be transferable to warm-blooded animals was the L-I11 stage of Anjsakis sp. from the body cavity and the muscle of Muraenesox cinereus. The risk of human infections by parasites through consumption of raw marine fish in the central Philippines therefore is considered to be low.

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The beaches of the North Atlantic and North Pacific are home to kelp flies of the Coelopa frigida/nebularum complex, which consists of one to three different species depending on whether the two nominal species are accepted and a cryptic species proposed by Remmert is counted. The morphological differences between two populations of C. frigida (Fabricius, 1805) from the North Sea and the Baltic Region and two populations of C. nebularum Aldrich, 1929 from Alaska and Japan are described and discussed for small, medium, and large specimens. Crossing experiments are used to demonstrate that, under laboratory conditions, no isolation mechanisms between either population exist. Coelopa frigida and C. nebularum are therefore regarded as a single biological species, a conclusion that is congruent with the observation that the genetic distances based on Ef 1‐α and 16S rDNA indicate lower levels of differentiation within C. frigida/nebularum than between undisputed Coelopa species. The substantial morphological, breeding and genetic information on the C. frigida/nebularum species complex is then applied to six different species concepts popular in the modern systematic literature. According to the Biological, Hennigian and Recognition Species Concepts, only a single species would be recognized. The Evolutionary Species Concept is too vague to be applicable and under two variants of the Phylogenetic Species Concept, C. frigida and C. nebularum would constitute separate species. This result confirms that Phylogenetic Species Concepts lead to a higher species number than concepts based on reproductive isolation. Practical and theoretical problems with the various species concepts are briefly discussed.

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