| Summary: | viii, 77 leaves : ill. (some col.) 28 cm. Includes abstract. Includes bibliographical references (leaves 32-35). An Ergasilus sp. and Thersitina gasterostei infected Gasterosteus aculeatus (threespine stickleback), Apeltes quadracus (fourspine stickleback), Gasterosteus wheatlandi (black-spotted stickleback), and Pungitius pungitius (ninespine sticklebacks) along coastal Atlantic Canada. The taxonomic position of Ergasilus sp. was under question due to its resembling both T. gasterostei and Ergasilus spp. While the greatly inflated cephalothorax of Ergasilus sp. was similar to that of T. gasterostei , morphology of the antennule and antennae, mouthparts and legs were more similar with that of representatives of Ergasilus. Ergasilus sp. infected A. quadracus and G. wheatlandi , both endemic to the area, at significantly higher prevalences than on G. aculeatus . In contrast, T. gasterostei occurred primarily on G. aculeatus , which shares the parasite's circumpolar distribution. Mixed species infections were recorded most often on G. aculeatus , supporting host specificity displayed by T. gasterostei for this stickleback host. On G. aculeatus, T. gasterostei attached most frequently to the inner surface of the operculum. At low intensities on A. quadracus, Ergasilus sp. also attached to this site. However, when intensity was higher, the prevalence of Ergasilus sp. increased on the gills. Comparison of attachment of Ergasilus sp. on the inner surface of the opercula to that on the gills of A. quadracus resulted in a slight, but significant, difference in individual egg size but a marked difference in both egg sac length and the number of eggs. Histological sections of infection associated with Ergasilus sp. showed tissue proliferation and hyperplasia at sites of attachment and feeding. Molecular sequences of 28S rDNA of Ergasilus sp., T. gasterostei and Ergasilus manicatus supported the classification of this undescribed copepod as Ergasilus sp. By investigating the taxonomy, ecology, and molecular taxonomy ...
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