A specimen of a population of large-sized Marginopora sp., collected at 26 m depth north of Wistari Reef, near Heron Island (Capricorn Bunker group of islands, southern Great Barrier Reef), reproduced overnight in an aquarium. The specimen and its offspring were photographed. The reproductive chamber growth was quite fragile, and much of the periphery of the disc was fragmented. Most of the released offspring formed two or three chambers; a few had formed four. The overall sizes of the offspring reflected considerable variance in the sizes of proloculi and flexostyles. This suggests possible uneven division of the parental cytoplasm and/or differences in the growth rates of the juveniles. All of the offspring were pigmented brown, and their tests were surrounded with zooxanthellae. Although it was clear that zooxanthellae were present in them when the juveniles were released, it was not clear whether they were acquired directly while the parental cytoplasm was being subdivided, or if they were acquired by the feeding juveniles. Because this organism has not been in continuous culture, which would allow routine observation of its life cycle, and because the reproductive stage is fragile and not likely to be preserved, and further because the stage has rarely been observed, this paper gives the reader a snapshot observation of the natural history of this giant protist.

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