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

To determine if crabs might be feeding on mucus or other materials from the surface of the corals, we dyed live Oculina (N = 7) by holding them for 10 min in baths of filtered seawater containing 0.57 g/L of neutral red. Seven control corals (unstained) were held in equivalent baths of filtered seawater without neutral red. Corals were rinsed briefly in fresh seawater to remove unbound dye and placed individually into flow-through containers, each with a single crab. Crabs were allowed to feed on stained or unstained corals for 24 h, and we then analyzed crab feces for the presence of the stain. Crab feces were collected from each container, weighed, and extracted with 2.5 mL acetone. Extracts were filtered through a plug of glass wool to remove solids, then solvents were removed using a slow stream of [N.sub.2] gas. The dry extract was then dissolved in 2.5 mL distilled water and absorbance measured at 530 nm (the wavelength of maximum absorption for neutral red) in a Spectronic 21D spectrophotometer (Spectronic Instruments, Incorporated, Rochester, New York). The relative absorbance of fecal extract from crabs on stained vs. unstained corals was compared using an unpaired t test. We also made direct observations of crab behavior on live corals at night with video cameras under red light, which did not interfere with their nocturnal behavior (white light caused them to hide).

To assess whether access to live corals affected crab growth, we raised individual crabs in 1500-L outdoor mesocosms for 6 wk on both live and dead corals. Unfiltered seawater from nearby Bogue Sound entered the mesocosms through a wave generator that produced water movement conditions similar to those encountered in the field. Because these tanks were located outside and used unfiltered seawater, physical conditions in the mesocosms were similar to, and varied with, conditions in the field. Within each mesocosm, individual crabs were raised in 39 x 19 x 24 cm rectangular vexar cages (0.5 cm mesh) each of which contained a flat concrete slab on which was placed either a live or a dead coral (N = 13 for live and dead corals). Using a LICOR LI-193SA light meter attached to a LI1000 data logger (LICOR, Lincoln, Nebraska), we determined that the vexar used in making the cages screens out 50% of ambient light. We further reduced the level of incident light to 25% of surface irradiance by placing an additional sheet of vexar across the entire mesocosm. This reduced light levels inside the cages below the maximum levels that may naturally be encountered by corals and crabs under typical field conditions. Prior to the experiment, crabs were weighed (blotted wet mass), measured (carapace width), and sex was determined to allow us to pair the crabs on the live and dead corals within a replicate by initial size and sex. Cage sides and tops were cleaned weekly to prevent excessive fouling and sediment accumulation, but corals and cage floors were not scrubbed; rather, they were allowed to develop an epibiotic community on which the crabs could feed. Thus, crabs on dead corals were not starved, and this experiment tested whether crabs derive a greater nutritional benefit from the coral than they obtain from the natural fouling community. We compared changes in wet mass and carapace width for crabs grown on live vs. dead corals using a paired t test, with separate analyses for males and females due to the large difference we found in growth rate between sexes.