Article: McMurray SE, Finelli CM, Pawlik JR (2015) Population dynamics of giant barrel sponges on Florida coral reefs. J.Exp Biol 473;73-80. doi:10.1016/j.jembe.2015.08.007
Photo credit: Zea, S., Henkel, T.P., and Pawlik, J.R. 2014. The Sponge Guide: a picture guide to Caribbean sponges. 3rd Edition. Available online at www.spongeguide.org. Accessed on September 10,2015.
You might think of a sponge as a saturated ball of water and soap sitting in your sink ready to clean dishes, but there are also sponges that are alive and are important citizens of coral reefs. Much like the sponge sitting in your sink, these marine sponges have several large pores that allow water to pass through them. As the water passes through their pores, they filter out the food and nutrients they need, which in turn allows them to mediate the cycling of carbon and nutrients on coral reefs. If being part of the carbon and nutrient cycle isn’t important enough, these sponges also provide habitat for several fish species, and are dominant competitors within the coral reef community.
Sponges are usually less abundant than other benthic animals found on coral reefs, and have therefore been left out of the majority of benthic reef sampling and monitoring projects. However, if you consider a coral reef as a three dimensional platform, because of their size, sponges actually dominate the total biomass of coral reefs. Because of this, and their important functional roles, there is a need to better understand the demographics of sponges on coral reefs.
The giant barrel sponge (Fig 1) is a dominant species in the sponge community of the Florida Keys, comprising of about 65% of the total sponge community. McMurray et al. set out to survey and monitor the giant barrel sponge population in the Florida Keys in order to understand the demographic trends of the Florida Keys reefs.
Methods and Results
For twelve years, researchers analyzed the size and growth of giant barrel sponges on two separate reefs (Pickles and Conch Reefs) in Key Largo, Florida. Each giant barrel sponge found was mapped, photographed, and given an individual tag. These tags were drilled into the substrate next to the giant barrel sponge, not only to distinguish the individual, but also to monitor its growth. The data collected was then compared to previous data collections on the same reefs and the major components investigated were density, percent cover, and volume of the sponges.
From 2000-2012, the density of the giant barrel sponge population increased by 44% on Pickles Reef, while on Conch Reef it more than doubled (fig 2)! Of the 239 sponges tagged in 2000, 66% survived to 2012. This means that the increase in giant sponge density was in part due to the sponges growing and expanding, but also in part due to new recruits. The population structure of the giant barrel sponges changed over time and recruitment varied between the years, but overall there was a 76% survival rate of new recruits.
Discussion and Significance
The coral reefs of Florida have been through a lot from 2000 to 2012. Stressors such as storms, ocean acidification, and climate change have hit the hard coral population of Key Largo, FL pretty hard, decreasing the overall coral population significantly. In contrast, however, the giant barrel sponge population has increased in this same time frame! If the present conditions continue as is, it is expected that the giant barrel sponge population will continue to grow, which means sponges are going to soon dominate the benthic community of coral reefs. It remains to be seen whether the increased cover of sponges is consistent with all sponges, or is limited to the giant barrel sponge.
Reefs off the coast of Brazil have seen mass mortalities of many benthic species due to the thermal stress of the 1997 El-Nino, but sponge assemblages were largely unaffected and actually increased in abundance. More research is needed to better understand the threshold of resilience sponges have, and while right now the increased giant barrel sponge populations seems like a good thing, the long term implications are unclear and coral reef ecosystems may change functional roles.
Do you think coral reefs will benefit from an increase in sponge density and a decrease in coral density or do you think the coral reefs are in for a rough role reversal? Comment below and let us know!