Morphology of spiroplasmas in the Chinese mitten crab Eriocheir sinensis associated with tremor disease

The structure of spiroplasmas in the Chinese mitten crab associated with tremor disease was studied by transmission electron microscopy techniques, including both negative staining and ultrasectioning. A spiral structure, which is a typical form of spiroplasmas, could be detected and the observations showed for the first time the presence of spiroplasmas in the cells of the crab. Propagation of spiroplasmas within the cells presented various forms that could be sorted into three morphological types: rounded, regular helical, and pleiomorphic or intermediate forms. The spiroplasmas appeared as round bodies during the fallow stage of development or under poor conditions. They showed various shapes such as helices, saccate, branched, tadpole-like and tortoise-like structures while growing, and became long and congregated in late stages of development. When spiroplasmas were isolated from chicken eggs and cultured in M1D medium they appeared to undergo similar morphological changes to those in the crab. The spiroplasmas contained chromatin filaments and peripheral ribosome-like granules and were delimited by distinct unitary membranes. Average diameters were calculated at 0.1 to 0.35 microm for rounded forms and 0.1-0.2 microm for helical or long forms, and they varied in length from 3-12 microm.