Mutualism And Coral Persistence: The Role Of Herbivore Resistance To Algal Chemical Defense - Statistical Data Included

Ecology, Sept, 1999 by John J. Stachowicz, Mark E. Hay

In the fouling experiment, performed from September 1994 to May 1995, we fastened a single coral (mean size = 229 g) to each of 20 stakes, then placed a single Mithrax forceps on 10 of the corals, and left the other 10 vacant. At the end of the experiment we removed all epibionts from the coral, sorted them into gross taxonomic categories, and dried them in an oven at 65 [degrees] C for 48 h, before weighing to the nearest milligram. After removal of epibionts, corals were reweighed to measure growth. Because corals are modular organisms, and growth involves the production of new modules (polyps), growth can be considered a direct measure of fitness (Buss 1985). We analyzed crab-occupied and unoccupied corals for differences in epibiont load and growth rate using an unpaired t test. Because some corals were lost due to storms, and some crabs disappeared from their corals during the experiment, our final sample size for analysis was six corals with crabs and eight without crabs.

In the encroachment experiment, which ran from August to November 1996, we transplanted 30 pairs of corals to Radio Island Jetty where they were grown on flat cement blocks (39 x 19 x 4 cm) for 102 d. Each block with a coral was affixed to a 39 x 19 x 19 cm cinderblock sunk 8 cm into the sand between rocks of the jetty. Blocks were arranged in pairs within 1 m of each other, and pairs of blocks were spaced by at least 2 m. Within a pair, a crab was added to one block but not the other. Half of the pairs were placed at 2.0 m depth (shallow site; N = 15 pairs) and half were placed at 6.0 m (deep site; N = 15 pairs). The blocks to which the corals were attached had been placed in the field at the shallow and deep sites for 10 wk prior to the start of the experiment to allow a natural fouling community to develop. At least once a week throughout the course of the experiment, each coral was checked for crabs; missing crabs were replaced and new colonizers were removed to maintain treatments and controls. Mean retention of crabs was 80% between monitoring intervals (see Results). We thus avoided using cages to maintain treatments, eliminating potential artifacts associated with reduced flow, light, and abundance of large consumers.

To monitor the effect of crabs on the benthic community, we recorded the percent cover of benthic organisms on each block every 4 wk using a metal frame the size and shape of the block that was fitted with a monofilament grid of 100 points. We placed the frame over a block and recorded the seaweed or invertebrate species beneath each point; unoccupied points were recorded as bare substrate. After 102 d in the field, corals were collected, cleaned, and reweighed, and the benthic community growing on the blocks was harvested. Prior to removal of the community, each block was divided into three equal-sized sections: the middle third of the block immediately adjacent to the coral, and the outer third on each side. The outer thirds were pooled and contrasted with the middle third to determine if the crabs' impact on community biomass was restricted to the area adjacent to the coral. Because we were not interested in interactions among times of sampling, we analyzed percent cover data separately for each sampling date using a two-factor ANOVA with depth and crab presence as fixed factors. We adjusted [Alpha] for these tests using the Bonferroni correction ([Alpha] = 0.01) to avoid inflating our Type I error rate due to nonindependence of cover measurements across time. Coral growth was also analyzed by two-way ANOVA with fixed factors. Community biomass (dry mass per square centimeter) was analyzed separately for shallow and deep sites using ANOVA with position on the block and crab presence as fixed factors. Post hoc comparisons for each ANOVA, where appropriate, were made using Ryan's Q test (Day and Quinn 1989).