Abstract
Summary Zoothamnium niveum (Hemprich & Ehrenberg, 1831) Ehrenberg, 1838, a colonial, sessile representative of the Peritrichida (Ciliophora, Oligohymenophora) from the mangrove channels of Twin Cays (Belize Barrier Reef, Caribbean Sea), is remarkable for an obligatory association with ectosymbiotic, chemoautotrophic bacteria. The ultrastructure of the host and the symbiont is investigated by means of low-temperature and conventional scanning electron microscopy, thin section and freeze-fracture transmission electron microscopy. The cortex, the main interface between host and symbiont, differs from the general pattern found in other peritrichs only in having special inclusions of unknown nature and function in the pellicular alveoli. The stationary microzooids and the swarming macrozooids have a fully developed oral ciliature, similar to other peritrichs. The cytopharynx in the macrozooids is highly reduced and no food vacuoles have been detected. Two morphotypes of bacteria, different in size and shape, are present on different parts of the colony and are assumed to represent only one species. The ultrastructural features resemble those of sulfur-oxidizing bacteria. The microzooids feed on bacteria whose size, shape, and ultrastructure is similar to their symbionts. This supports the assumption that Z. niveum gains at least part of its nutrition from chemoautotrophic bacteria.
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