EXPERIMENTAL-INFECTION OF YELLOW PERCH (PERCA-FLAVESCENS) WITH APOPHALLUS-BREVIS (DIGENEA, HETEROPHYIDAE) - PARASITE INVASION, ENCYSTMENT, AND OSSICLE DEVELOPMENT

Yellow perch (Perca flavescens) were exposed to cercariae of Apophallu s brevis collected from naturally infected snails and maintained for u p to 63 d postinfection (PI). Histochemistry revealed that attachment, penetration, migration, and encystment were complete by 2 d PI. By 14 d PI, a precursor to a host ossicle capsule surrounded the encysted m etacercaria. Between 14 and 63 d PI a calcified matrix aggregated in a specific developmental pattern of islets, crescent, band, then ring, within an equatorial band at the cyst-capsule interface. The result wa s an ossicle ring of calcified tissue separating two uncalcified-matri x poles. Ossicle rings thickened and advanced over the cyst, but did n ot seal it off completely, forming an intermediate ossicle 49 d PI wit h two polarized canals. Pigmentation of ossicles through the appearanc e of melanocytes was not consistent between infections of the same age . Scanning electron microscopy revealed two distinct regions within fu lly developed ossicles. Randomly distributed interstitial fibres and o void and tubular lacunae surrounded by perilacunar collars formed the inner cellular region. The interstitial fibres within the outer region formed acellular/lamellar bands.