Population dynamics of giant barrel sponges on Florida coral reefs

- Populations of Xestospongia muta were surveyed on Florida coral reefs for 2000-2012.
- Sponge abundance, percentage cover, and biomass increased at all sites.
- Survival of recruits and of the largest sponges increased at all sites.
- Population growth accelerated on Conch Reef due to increased recruitment.
- Reef-building corals are declining, but populations of X. muta on Florida reefs are increasing.

The structure of coral reef benthic communities has changed significantly in recent decades and will likely continue to change with the increasing frequency and scale of disturbances. Reef-building corals and macroalgae have been regularly included in assessments of coral reef communities, however very little is known about the long-term dynamics of sponge populations. The giant barrel sponge Xestospongia muta is a dominant component of Caribbean coral reef communities, and it was previously reported that populations significantly increased in the Florida Keys over 2000-2006. Using surveys of sponges from the same permanent plots on reefs off Key Largo, Florida, USA, this study extended the investigation of the population dynamics of X. muta for 2000-2012 at three depths (15, 20, 30 m). Over 12 years, the density of X. muta significantly increased by a mean of 122% (range = 53-336%) on Conch Reef and by a mean of 44% on Pickles Reef. Both the cover and volume of X. muta concomitantly increased at all sites and increases in both metrics were greater over 2006-2012 relative to 2000-2006 due to decreased mortality of the largest sponges in the population over time. Population growth accelerated at all sites on Conch Reef due to a significant increase in recruitment and sponge survival, and was greater at deeper relative to shallow depths; on Pickles Reef, recruitment decreased, but survival increased and population growth remained constant over time. Despite mortality due to putative pathogenesis, and in contrast to the persistent decline of reef-building corals, these results suggest that conditions on Florida coral reefs have been increasingly favorable for the growth of barrel sponge populations. Given the long lifespan of X. muta, it remains to be seen whether these results represent a persistent change with broad geographic relevance, or stochastic variation in local demographics.