The ecology of deep-water coral reef sponges
Principal Investigator: Dr. Joe Pawlik (University of North Carolina Wilmington)
Sponges play an important role in the structure and function of Caribbean coral reef communities. One species, the giant barrel sponge, Xestospongia muta, is particularly prominent on deep-water reefs (>10 m). Like reef-building corals, the tissues of X. muta contain photosynthetic algal symbionts. Also like corals, specimens of X. muta have recently been observed to “bleach” during summer months, sometimes resulting in sponge mortality. Questions addressed by this project include:
- How old are these sponges? How fast do they grow?
- How often do new sponges (called recruits or juveniles) appear on the reef, and how fast do they grow?
- How common is sponge bleaching? Is bleaching always fatal, or do some sponges recover?
- How does bleaching affect sponge chemical defenses against predatory fishes and microbial pathogens?
- Is bleaching in X. muta a response to seasonal temperature maxima, or a result of post-reproductive trauma?
The purpose of this Aquarius mission is to continue monitoring populations of X. muta on Conch Reef that were mapped and tagged last year. The project compliments similar surveys of shallow-water populations of X. muta from Conch and Pickles Reefs, also in the Florida Keys National Marine Sanctuary. The specific objectives of our Aquarius missions that are designed to answer the questions above, are as follows:
- Monitor permanent transects at each of two depths, 18 and 34 m (60' and 110') and tag all X. muta found within the transects (to compliment 3 transects at 14 m we sample using conventional scuba diving). Transects at these three depths match up with a temperature monitoring program maintained by NURC/UNCW at similar depths (21 and 32 m).
- Photograph all X. muta within the transects to monitor sponge growth, bleaching, and wound recovery.
- Sample tissue from selected sponges in the transects to monitor bioactivity of crude extracts in assays of antimicrobial activity and fish feeding deterrency. This work addresses the chemical ecology of sponges (see Dr. Pawlik’s web site for more information).
Surveys of X. muta will provide basic demographic information on this important sponge species, and specifically addresses NURC/UNCW research priorities regarding recruitment processes, natural system variability, and the possible impact of global environmental change (the last of these because bleaching is thought to be linked to summer temperature maxima).
One ancillary graduate student thesis project will also be conducted on this Aquarius mission, entitled, Host choice of sponge-dwelling brittle stars. Interestingly, some brittle stars live within sponges, but little is understood about this phenomenon. Projects conducted during this mission address the following questions:
- Do suspension-feeding ophiuroids (brittle stars) that inhabit certain sponge species ever leave their sponge hosts?
- If so, do they return to the same sponge (thus exhibiting homing behavior), or to sponges of the same species but different individuals?
- Do brittle stars compete for hosts?
To answer these questions (and others), the brittle star Ophiothrix suensonii, which is found in tube sponges of the genera Callyspongia and Niphates, will be tagged and distributed among sponges at different densities. Daily surveys will be conducted to assess movements, competition, and host specificity.
More information about Dr. Pawlik’s research programs is available at his web site: http://people.uncw.edu/pawlikj/index.html