Amphistegina are the most common foraminifers with algal endosymbionts found on reefs and carbonate shelves worldwide. Like zooxanthellate corals and other reef organisms with algal symbionts, Amphistegina respond to photoxidative stress by bleaching. This paper documents ultrastructural changes that occur during bleaching under field and laboratory conditions. Nine chambers from the outer whorl of each of 22 normal-appearing and 11 partly bleached specimens of Amphistegina gibbosa, which were collected from Conch Reef, Florida, USA, were examined using transmission electron microscopy. The condition and numbers of algal symbionts, as well as the cell area occupied by 10 other intracellular structures of the host, were quantified. Normal-appearing specimens averaged three times more viable symbionts and less than a fourth as many deteriorating symbionts as partly bleached specimens. Foraminifers experimentally exposed to visible light intensities > or = 13 micromole photon m(-2) s(-1) for 35 d were statistically similar to partly bleached field specimens in the number and condition of symbionts, and in chamber area occupied by the evaluated host structures. Exposure to 32 degrees C water temperature at 6-8 micromole photon m(-2) s(-1) for 28 d induced symbiont loss but did not degrade host endoplasm.

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Specimens of a new species of the trochospiral genusFloresina Revets, 1990, were collected from depths of 10-30 m at Conch Reef in the Florida Reef Tract, USA. This small foraminifer is predatory on Amphistegina gibbosa. It attacks its much larger prey by climbing upon the A . gibbosa's test, drilling as many as 10 holes per attachment site and extracting cytoplasm from several chambers. Prey is killed in 3-7 days or longer, though individuals occasionally survive and fend off the predator. Asexual reproduction by microspheric adults is commonly cytotomous; 5-20 macrospheric young form, presumably by multiple fission, in a terminal brood chamber. That chamber is weakly calcified and is abandoned after reproduction.