NUCLEOIDS AND COATED VESICLES OF EPULOPISCIUM SPP

We describe here aspects of the anatomy of two ''Epulopiscium'' morpho types, unusually large bacteria that are not yet cultured and that rep roduce by the internal generation of two or more vegetative daughter c ells. Two morphotypes, A and B, which are enteric symbionts of several species of herbivorous surgeonfish (Acanthuridae), were collected aro und the Great Barrier Reef of Australia, preserved there, and later st ained for light microscopy. Some samples were examined by electron mic roscopy. In both morphotypes, countless discrete nucleoplasms or nucle oids were found to occupy a single shallow layer just beneath the surf ace all around these organisms. At each end of the morphotype B cells, a membrane-bound compartment containing dense cords of chromatin was observed. When these were found at each end of growing daughter cells, no polar compartments were then found in their mother organism. Elect ron micrographs of sections of morphotype A symbionts show that their outermost region is composed of tightly packed coated vesicles, each s urrounded by a thin, dense, spacious capsule. Near the surface of type A organisms the remains of broken vesicles, broken capsules, and a fi nely fibrous matrix fuse to form a fabric that serves as the cell wall . Morphotype B organisms, however, were observed to have a distinct, m orphologically continuous outer wall.