Studies on the heterophyid trematode Apophallus brevis, the "sand-grain grub" of yellow perch (Perca flavescens). II The metacercaria: position, structure, and composition of the cyst; hosts; geographical distribution and variation

Abstract

The metacercarial cyst of Apophallus brevis, the "sand-grain grub," is composed of fish bone within peripheral blood vessels of yellow perch (Perca flavescens) forming a tire-like structure; two escape canals are maintained opposite each other. In thin section, lines indicating interruption of growth apparently delimit annual incrementation as in scales and other bony structures of fish. Cysts are oriented with their long axes paralleling the long axis of a host's body with escape canals contiguous to walls of enclosing blood vessels. Cysts of A. brevis in situ at times appear partially or entirely pigmented but are actually transparent; pigmentation, when present, is a phenomenon of a cyst's position within certain types of blood vessels and is not an integral part of a cyst's construction. The organism as a metacercaria is almost exclusively a parasite of yellow perch (known deviations are noted) and is apparently confined to North America, having a known broad range from Saskatchewan to Cape Cod. Massachusetts. Distribution is extremely diffuse and appears dependent on patchy distribution of the organism's molluscan host, Amnicola limosa. Geographical variation in cyst site selection and clustering indicates some sort of intraspecies inhibition on the part of metacercariae of A. brevis.