Porpostoma guamensis n. sp., a philasterine scuticociliate associated with brown-band disease of corals.

Brown band disease of coral is caused by a ciliate that consumes the tissue of the corals in the genus Acropora. We describe the ciliate associated with this disease on Guam, based on: general morphology, division stages, and ciliature observed on live and protargol-stained specimens; modification of the oral structures between divisional stages, observed on protargol-stained specimens; and some aspects of behavior in field and laboratory studies. Porpostoma guamensis n. sp. is elongate and has ciliature typical for the genus; live cells are 70-500 × 20-75 μm; the macronucleus is sausage-like, elongate but often bent, positioned centrally along the main cell axis; the oral ciliature follows a basic pattern, being composed of three adoral polykinetidal regions, as described for other species in the genus, although there is variability in the organization, especially in large cells where the three regions are not easily distinguished. Ciliates fed on coral with their oral region adjacent to the tissue, which they engulfed, leaving the coral a bare skeleton. Both zooxanthellae and nematocysts from coral occurred in the ciliates. Zooxanthellae appeared to be ingested alive but deteriorated within 2-3 days. Ciliates formed thin-walled division cysts on the coral and divided up to 3 times. Cysts formed around daughter cells within cysts. We provide some observations on the complex division pattern of the ciliate (i.e. tomont-trophont-cyst) and propose a possible complete pattern that requires further validation.