Bleaching in a Reef-dwelling Foraminifer, emAmphistegina gibbosa/em

In field collections from reefs off Key Largo, Florida, in September 199 1, 80% of living adolescent and adult Amphistegina gibbosa, a larger foraminifer that hosts diatom endosymbionts, ex- hibited either mottling or extensive symbiont-color loss. Many Cnidaria in the vicinity also were bleached. Comparison of September and Novem- ber abundance and size-frequency data indicated high mortality, particularly in bleached size classes. Although the cause of bleaching in A. gibbosa has yet to be determined, laboratory observations in- dicate that this species may be vulnerable to ele- vated ultraviolet radiation. In 199 1, volcanic aero- sols globally reduced visible and infrared radiation and promoted stratospheric ozone depletion, thereby increasing UV radiation reaching the sea surface.