| Summary: | Haematophagous, metazoan ectoparasites and protozoan blood parasites of intertidal fishes from Wales and South Africa formed the main focus of this study. The ectoparasites examined were largely juvenile isopods of the genus Gnathia, and specimens ofthe leech, Zeylanico~della arugamensis. These taxonomically distinct ectoparasites were suspected vectors of the blood protozoans, and the latter were mostly apicomple{(ans of the genus Haemogregarina and euglenozoans ofthe genus Trypanosoma. Knowledge of digestive tract anatomy in juvenile gnathiids is essential for assessing their role as potential vectors and this system was examined using histological, microscopical and computer-based techniques. Features that had been unreported, or insufficiently recorded, previously' included several structures within the dorsal afid ventral stomach chambers, a typhlosolelike formation in the anterior hindgut, haemozoin-like deposits in the digestive caeca of Gnathia africana and Gnathia pantherina, and an extensive gut flora. Connections between the stomach, anterior hindgut and digestive caeca were identified, salivary gland ducts were followed towards the mouthparts, and the digestive cycles of Gnathia maxillaris and G. africana juveniles were examined over a period of 30 days. The pathology associated with the attachment sites of G. africana and Gnathia sp A to teleosts, and with G. pantherina to an elasmobranch, was found to be considerable in the case of G. pantherina, while Gnathia sp A prompted the death ofits host unde: experimental conditions. Oocyst stages, presumed to be those ofHaemogregarina bigemina, were ,' . located in sections through the digestive caeca of G. africana, confirming the vector status ofthe isopod. Development stages of a new species, Haemogregarina curvata, were observed in the leech, Z. arugamensis, and within peripheral blood smears from several fishes. The life cycle oftrypanosomes found in Z. arugamensis and in fishes was also established, although interpreting the morphometries ofthese flagellates was challenging and speciation proved difficult. Identical molecular sequences oftrypanosomes were derived from separate leech samples and these were close to published sequences for marine fish trypanosomes. Thus, both gnathiids and leeches were likely vectors ofthe fish blood protozoans. Finally, probable dual transmission of a haemogregarine and trypanosomes by a leech was illustrated, a rare event, reported infrequently in the literature.
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