Bleaching of the giant barrel sponge Xestospongia muta in the Florida Keys

The giant barrel sponge Xestospongia muta is now the dominant habitat-forming organism on many Caribbean coral reefs and has been observed to undergo cycles of bleaching similar to those seen for reef-building corals. We examined bleaching of X. muta at 15-m, 20-m, and 30-m depths over 2000-2005 on Conch Reef, Key Largo, Florida, to determine the spatial and temporal patterns of bleaching, whether bleaching affected sponge mortality, and whether there was a relationship between bleaching and seawater temperatures. Bleaching increased significantly with depth and was greater in autumn than spring, but the response was not uniform across the sponge population. Bleaching of X. muta did not result in sponge mortality, corroborating the conclusion that cyanobacterial symbionts of the sponge provide little or no benefit to the host. There was greater sponge bleaching with higher minimum daily seawater temperature anomalies within 30 d of surveys in the spring. For both the spring and autumn, there was greater bleaching with a higher number of positive 0.5 degrees C daily anomalies within 14 d before surveys. Anomalously warm seawater temperatures fail to completely explain the variability of the observed bleaching response, however, and additional work would be required to conclusively determine whether a causal relationship between bleaching and temperature exists. As has been described for corals, bleaching of X. muta is likely a complex response that may be affected by multiple factors, including light availability and differences in bleaching susceptibility among Synechococcus symbionts.