The ciliated protozoan Hyalophysa is a symbiont of a variety of crustaceans, including grass shrimp, hermit crabs, and blue crabs. At the molting of the host, the attached protozoans excyst and swim into the exoskeleton to feed by pinocytosis on exuvial fluid. The feeding trophonts expand to many times their original size, transforming into cells that are little more than large food vacuoles with a rim of cytoplasm stretched around the perimeter. After feeding for the only time during their life cycle, they settle on the substrate to encyst and divide into daughter tomites, and in doing so they divide the food vacuole. The tomites then leave the cyst and attach to a crustacean for their next opportunity to feed at molting.

We have followed endocytosis in Hyalophysa using two dyes, nigrosin and congo red, in order to understand the digestive process.