Light and electron microscopical data on the spores of Thelohanellus rhabdalestus n. sp. (Myxozoa: Myxosporea), a parasite of a freshwater fish from the Kwanza River,Angola

The myxosporean Thelohanellus rhabdalestus n. sp. (Myxozoa: Bivalvulida), a parasite of the freshwater fish Rhabdalestes maunensis (Fower) collected from the Kwanza River, Angola, is described based on light and electron microscopical studies. The parasite occurs in irregular, milky-whitish, cyst-like plasmodia (up to 0.8 mm in diameter) in close contact with the liver and heart. The spores are pyriform, with slight tapering anterior and round posterior ends, and measure 16.8 ± 0.5 μm (n = 50) long, 10.2 ± 0.6 μm (n = 50) wide and 5.6 ± 0.8 μm (n = 25) thick. The spore wall is partly surrounded by a discontinuous, closely adhered, external coat of electron-dense material of variable thickness (up to c.35 nm). A single flask-shaped polar capsule [7.2 ± 0.3 μm (n = 50) long and 4.0 ± 0.4 μm (n = 50) in diameter] lies close to the apex of the spores and contains a polar filament with six or seven (rarely eight) coils oblique to its longitudinal axis. Based on morphological and ultrastructural differences, compared with other members of Thelohanellus Kudo, 1933, and judging from the host-specificity of previously described species, we consider this species new to science. This is the first reported myxosporean from the Angolan fauna.