Description of M. sphaericum from B. belone

Plasmodia were spherical to irregular (Fig. 1a, b), attached to gall bladder epithelium or free, often with brushy region representing the zone of epitheliar adhesion. Parts of plasmodia with distinct ectoplasm when unsporulated (Fig. 1a), while ectoplasm often not apparent in fully sporulated ones (Fig. 1a, b). Plasmodia contained refractive granules, scattered or in aggregations, increasing in number during sporogony. Mature plasmodia were usually markedly vacuolate (Fig. 1b). Plasmodia were generally disporic, rarely tetrasporic. Sporogony was disporic. Occasionally, plasmodia releasing spores were seen to contain a second sporoblast at an early stage in sporogony. Plasmodia without spores measured up to 36 μm in average diameter (N = 16), plasmodia with immature spores 20–30 μm (N =4), and plasmodia with two mature spores 23–37 μm (mean 28 μm, N =11).

Spore main outline was ellipsoid (Fig. 1c), but valvular protrusions were associated with the tip of the polar capsules (PC), giving a sigmoid appearance in sutural view and spindle shape in valvular view (Fig. 2). Immature spores were occasionally crescent shaped. Valves were smooth, thicker along the ellipsoid outline of the main spore body and thin surrounding the protruding part of the PCs (Fig. 1c, d). Suture was weakly sigmoid, faint and symmetrical (Fig. 2). PCs were terminal, equal, elongate pyriform, PC axis in medial plane in valvular view, but with an angle reaching c. 47° to spore axis in sutural view. Polar filament coils are perpendicular to PC axis, with filament coil diameter averaging 70% of PC diameter. Coils are not present in the protruding part of the PCs (Fig. 1c–e). Polar capsule length to spore length was 1:2.1–2.8 (mean 2.3± 0.2; N =21). Fully extruded polar filaments reach 134– 153 μm in length. Sporoplasm was binucleate, filling the barrel-shaped spore cavity between the PC cells. Spore measurements are given in Table 1.