Identification of larval Contracaecum osculatum s.l. and Phocascaris sp. (Nematoda: Ascaridoidea) from marine fishes by allozyme electrophoresis and discriminant function analysis of morphometric data

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Abstract
Biochemical markers (allozymes) were used to identify third-stage larvae of nematodes from the Contracaecum osculatum s.l. – Phocascaris sp. complex found in five marine fish species (Hippoglossoides platessoides, Gadus morhua, Mallotus villosus, Myoxocephalus scorpius, Pleuronectes americanus) collected off Newfoundland and Labrador. Three taxonomic groups, genetically identical to adult C. osculatum A, C. osculatum B, and Phocascaris sp. previously reported from North Atlantic seals, were found. Among the 859 larvae examined, C. osculatum B was most common (74.2%), particularly in H. platessoides (79.4% of 501 larvae), G. morhua (88.8% of 160), M. scorpius (87.5% of 32), and P. americanus (100.0% of 20), but Phocascaris sp. was more numerous in M. villosus (66.4% of 146). Contracaecum osculatum A was found only in H. platessoides (15% of 501). Discriminant function analysis revealed differences in shape among the nematode species. Larvae of Phocascaris sp. and C. osculatum B can generally be distinguished using three measurements and classified according to coefficients for quadratic discriminant functions, but larvae of C. osculatum A could not be reliably distinguished from the other two species. A simpler, more rapid method of screening biochemical markers, employing a cellulose acetate support medium, is also described.

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