Site preference of fish myxosporeans in the gill.

In addition to the morphological and size characteristics of the spores, indicating the exact location and tissue specificity is also essential for differentiation of the large number of species belonging to the group of gill-parasitic fish, the myxosporeans. According to the observations of the present author, Myxobolus, Henneguya and Thelohanellus species are characterised by strict tissue specificity, and species showing affinity to the epithelium, connective tissue, cartilage or vascular tissue usually occur in a strictly defined location within the gill apparatus. Some of the species typically form plasmodia in the lamellae of the gill and others in the gill filaments. Yet other species develop their plasmodia at the base of the gill filament or in the gill arch. Instead of the generally accepted but misleading terms 'intra-' and 'interlamellar', the present author distinguishes interlamellar-epithelial and intralamellar-vascular types in the case of plasmodia developing in the gill lamellae, and intrafilamental-epithelial, intrafilamental-vascular and intrafilamental-chondroidal types in the case of plasmodia developing in the gill filaments. Regarding site of development within the gill, the location of basifilamental plasmodia and that of plasmodia developing in the cartilaginous matrix, connective tissue or blood vessels of the gill arch are well distinguishable from the above types. The different types and their variations are shown in histological illustrations.