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Sponges are becoming the dominant habitat-forming organism on Caribbean reefs as corals continue to decline because of environmental stressors like ocean acidification and warming. While factors adversely impacting coral abundance have been well researched and understood, the same couldn’t be said for sponges in the Caribbean. Some scientists believed that sponge populations are controlled through bottom-up processes, such as the availability of food resources (picoplankton ranging in size from 0.2 to 2 μm). Others believed that sponge populations are controlled through top-down processes, like predation by fish.

During two saturation missions aboard Aquarius in 2010 and 2011 and subsequent dayboat missions, Joseph Pawlik and his team from University of North Carolina Wilmington tested both hypotheses at Conch Reef. They used a simple predator-exclusion cage test to determine which factor affected sponge abundance most – food availability or predation. They placed sponges inside and outside predator-excluding cages at sites with differing picoplankton availabilities in depths of 15m and 30m.

There was no evidence that sponge populations were controlled by food resource limitations in the five species that were tested. Benthic surveys of Conch Reef revealed that sponges were found in higher numbers along the shallow reef transects versus deep reef transects even though picoplankton availability was lower on the shallow reefs. Additionally, the populations of two species even increased on the shallow reefs throughout the duration of the study.

Another important factor that was found to influence sponge community structure at Conch Reef is whether or not the sponges utilize chemical defense strategies. Chemically defended sponges experience less predation – but grow slower – than their non-chemically defended counterparts that experience increased predation – but grow faster. This indicates that there is a resource trade-off in which sponges that invest energy into creating chemical defenses that would otherwise be used for growth, development or reproduction.

This study tells us that the structure of sponge communities on Caribbean coral reefs is primarily controlled through top-down predation effects. This means that overfishing of spongivorous fishes can impact the overall health of reef communities by giving fast-growing, non-chemically-defended sponge species an advantage for space over reef-building corals that are already threatened by coral bleaching, disease and other stressors.

Without the gift of time that Aquarius Reef Base provides to researchers this study would not have been possible. Having the ability to dive for hours on end at depths of 45-90 feet increases the productivity of the research team ten-fold. This gives the team the unparalleled opportunity to conduct months worth of research during short week-long missions.

Learn more about the research here.

2011 Aquanaut Team:
Scientists: Steve McMurry, Tse-Lynn Loh, John Hanmer, Lindsey Deignan
Habitat Technicians: Roger Garcia, Saul Rosser

2010 Aquanaut Team:
Scientists: Chris Finelli, Tiffany Lewis, Tse-Lynn Loh, Steve McMurry
Habitat Technicians: Mark Hulsbeck, Ryan LaPete