© Joseph Dougherty. All rights reserved.
The Giant Barrel Sponge (Xestospongia muta) is a conspicuous and abundant member of coral reef communities at depths greater than 10 m throughout the Caribbean, with populations of this species occupying greater than 9% of the available substrate on some reefs. It is sometimes call the "redwood of the reef" because, like a redwood tree, they are often very large, very old, and home to many other creatures that use them as a refuge. The largest barrel sponges on Caribbean reefs are over 2000 years old.
Populations of this sponge provide essential habitat for numerous fish and invertebrate species, and the biomass and seawater-filtering capacity of this species exceeds that of any other benthic invertebrate. Individuals are often very large, with heights and diameters in excess of 1 meter.
Sponges support several key ecological processes on coral reefs. They filter large volumes of seawater and thereby influence the coupling of water-column and benthic processes. Sponge filtration can enhance water clarity and may indirectly affect coral and algal populations that are dependent on light availability. Sponges also serve as habitat to numerous reef organisms, are often the dominant competitors within the benthic community, and may harbor a diverse assemblage of bacteria that can take part in nitrification and carbon fixation (Diaz and Rützler 2001). In many parts of the Caribbean, the diversity and abundance of coral reef sponges exceeds that of reef-building corals.
The characteristic reddish-brown coloration of X. muta is due to cyanobacterial symbionts that live within the peripheral tissues of the sponge. While the majority of sponges that harbor symbionts appear to be unaffected during coral reef bleaching events, giant barrel sponges have commonly been reported to bleach. Long-term monitoring has revealed that barrel sponges undergo cycles of bleaching and recovery of pigmentation. Bleaching can be moderate, affecting only localized areas of the sponge, or it can be severe, but rarely results in sponge mortality. This form of “cyclic” bleaching is distinct from the bleaching observed from diseases, such as Sponge Orange Band (SOB) disease.
Scientists have found that sponges with symbiotic cyanobacteria appear to fall into one of two general groups– one group includes sponges that rely on their symbionts for much of their nutrition (known as a mutualism, as both the sponge and symbiont benefit from the relationship), and another group includes sponges that do not receive any benefit from their symbionts (known as a commensalism, as the sponge receives no benefit from the relationship while the symbiont does benefit) (Erwin and Thacker 2008). To determine if bleaching is a stressful condition for barrel sponges, researchers used molecular genetic techniques to monitor the production of heat shock proteins (HSPs) – proteins common to all animals, that are produced under stressful conditions – in the tissues of bleached and unbleached barrel sponges. Bleached sponges did not produce HSPs. Therefore, unlike corals, bleaching of barrel sponges is not a sign of stress, but rather a response by cyanobacterial symbionts that has no negative effect on their host sponge (López- Legentil et al. 2008).