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

RESULTS

Field surveys

Corals were common at a depth of 3 m on coastal rock jetties and at 17 m on nearshore reefs where water was turbid, but were rarer on offshore reefs at 27 m where water was clear [ILLUSTRATION FOR FIGURE 1 OMITTED]. Oculina was the only coral recorded in any of the quadrats. The seaweed [TABULAR DATA FOR TABLE 1 OMITTED] community at all sites was dominated by brown algae. At sites where corals were most abundant, Sargassum was the dominant seaweed, with Dictyota and Padina also being common [ILLUSTRATION FOR FIGURE 1 OMITTED]. Surveys of the abundance of crabs in Oculina heads from three offshore sites in North Carolina indicated that Mithrax forceps was the most common crab on these corals (Table 1). This crab occurred at densities between 0.32 and 1.42 crabs per coral. The other majid found in our surveys (Mithrax pleuracanthus) was far less common, occurring at a maximum density of one crab per 10 corals at the 10-km reef. Neither the lone porcellanid species found in our samples (Petrolisthes galathinius) nor the xanthids occurred as commonly as Mithrax forceps. Additionally, although the xanthid and porcellanid crabs were common at the two locations on the Liberty Ship, they occupied no corals on the three natural reefs that we sampled (Table 1).

Field manipulations

In the fouling experiment, corals occupied by Mithrax forceps grew 10 times faster than unoccupied colonies (P = 0.008, [ILLUSTRATION FOR FIGURE 2A OMITTED]) and accumulated only one-seventh of the fouling organisms (P = 0.0002, [ILLUSTRATION FOR FIGURE 2B OMITTED]). Seaweeds (primarily Codium and Ectocarpus) formed a small component of the overgrowth on corals without crabs during this fall-spring experiment; most overgrowth was due to invertebrates, particularly colonial and solitary ascidians (Botryllus, Styela), sea anemones (Aiptasia), and barnacles (Balanus spp.). Corals without crabs did not show a change in mass (i.e., growth) that was significantly different from zero (P = 0.409, one sample t test). Given the large difference in the abundance of fouling organisms on corals with and without crabs, we expected greater coral mortality among colonies without crabs, so we used a one-tailed Fisher's exact test to evaluate this hypothesis. Corals without crabs did exhibit greater mortality ([ILLUSTRATION FOR FIGURE 2C OMITTED], P = 0.051), as three of the eight coral colonies without a crab experienced total mortality and three sustained partial mortality (i.e., areas of the coral where living polyps had been killed), whereas only one of the six corals with crabs exhibited even partial mortality.

In shallow, well-lit habitats where algae dominate, Mithrax was especially important in defending corals from algal overgrowth [ILLUSTRATION FOR FIGURES 3-5 OMITTED]. In the encroachment experiment, Mithrax enhanced coral growth and decreased the biomass and cover of the benthic community surrounding corals on the shallow, but not the deeper blocks. After just 20 d, crabs had depressed total cover of organisms (community cover), relative to controls, on the shallow blocks, while there was never a difference between corals with and without crabs on the deep blocks ([Alpha] = 0.05, [ILLUSTRATION FOR FIGURE 3 OMITTED]). This was not a function of differential retention of crabs because site fidelity did not differ between the shallow and deep location, as 79.4 [ or -] 3.7 and 76.2 [ or -] 5.2% of crabs placed on corals were still present each time corals were checked at the shallow and deep sites, respectively (7-10 d intervals, N = 11 different determinations during this experiment; P = 0.609, paired t test). At every sampling period where data could be taken at both depths, there was an interaction between depth and treatment, reflecting the fact that crabs depressed total cover in shallow water (P [less than] 0.05) and slightly enhanced it in deeper water (nonsignificant trend, P [greater than] 0.05, [ILLUSTRATION FOR FIGURE 3 OMITTED]). The large decrease in percent cover on the shallow blocks both with and without crabs around day 30 was associated with Hurricane Fran. Rough seas during this period removed much of the benthic community from the substrate both on the blocks and on the surrounding reef, and destroyed one replicate at each depth. Additionally, immediately following the storm the water remained turbid for several weeks, probably slowing algal growth and interrupting the crab treatment effect for [approximately]1 mo. By early October (day 53) there was a nonsignificant trend toward a decrease in community cover on blocks with crabs at the shallow site, and by late October (day 76) this effect was statistically significant [ILLUSTRATION FOR FIGURE 3 OMITTED]). Because of the hurricane-generated disturbance, our data may underestimate the normal cumulative impact of these crabs at the shallow site.