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

Predation on crabs in the field

To determine if association with Oculina reduced predation on Mithrax, we tethered crabs at Radio Island Jetty either with or without access to a coral. Fifteen cm long tethers of monofilament fishing line were affixed to crab carapaces using super glue according to methods in Stachowicz and Hay (1996). The free end of the tether was tied to galvanized nail, which was driven into the substrate. Crabs with and without access to a coral were paired within 0.5 m of each other; separate pairs were spaced by at least 2 m. Each tether was checked after 1 and 24 h to see if crabs were still present. We analyzed these data using Fisher's exact test. Although these crabs are mobile, their typical defensive response is not to flee, but rather to remain motionless and hold tightly to the substrate (J. Stachowicz, unpublished data and observations); thus we believe that tethering is an acceptable method for testing the refuge hypothesis.

As an additional test of the refuge benefits of the coral at a different site, and to ensure that our results from the previous experiment were not artifacts of the tethering process (Peterson and Black 1994), we placed untethered crabs at the Liberty Ship either with or without access to a coral, and observed them over a 2-h period. Each pair of crabs (one with and one without access to a coral placed 1 m apart) was observed by a scuba diver from a distance of 2 m for [approximately]15 min or until the crabs had been consumed. We recorded the length of time from placement of a crab until its consumption, as well as the species of the predator. After 15 min, the location of surviving crabs was marked to allow them to be relocated; crabs were marked with a small drop of super glue to allow us to distinguish them from other crabs in the field. Two hours after initial placement, we rechecked any crabs that had survived the initial 15-min period. We analyzed for differences in the frequency of consumption of crabs with and without access to a coral using Fisher's exact test.

Nutritional benefits to the crab

To determine if crabs use factors other than protection from predators in selecting a habitat, we offered individual Mithrax a choice between associating with a live or a dead coral of similar size and structural complexity. Tissue was stripped from the "dead" coral using a high-pressure jet of fresh water; we then rinsed each coral in fresh water, and placed them in running seawater for 24 h. In a 10-L tub, we placed equal-mass heads of live and dead corals (means [ or -] 1 SE, live = 211.8 [ or -] 13.25 g, dead = 211.1 [ or -] 10.14 g; N = 20, P = 0.967, paired t test). Thus, the two choices were equivalent with the exception of the presence of live coral tissue. We placed a single crab in the center of the tub, equally distant from the live and dead coral, and monitored its habitat choice after 0.25, 12, 18, and 24 h. We analyzed crab distribution on live and dead corals (N = 20) at each time interval using a G test.