ANTI-PREDATORY DEFENSES OF THE BRITTLE STAR OPHIOCOMA ECHINATA

Abstract

Brittle stars are included in the diets of crabs, shrimps, and fish, including at least 33 species of Caribbean reef fish. The abundance of echinoderms in coral reef ecosystems suggests that they are able to resist predators through effective defense mechanisms. The most apparent characteristic of echinoderms is their tough and spiny outer body wall composed of embedded calcium and magnesium carbonate ossicles. Although numerous studies have been conducted on calcium carbonate (CaCOɜ) as a feeding deterrent in plants, few experiments have looked at CaCOɜ as a feeding deterrent in benthic marine invertebrates. Research was conducted off of Dump Reef on San Salvador Island, Bahamas and in a climate-controlled laboratory at Hood College in Frederick, MD. In a tethering experiment, I observed predation by reef fish on tethered brittle stars immediately after dusk and dawn. I then determined whether or not the percentage of CaCOɜ found in a typical Ophiocoma echinata individual is a deterrent to fish predation through a combination of field and laboratory experiments. In the field, I presented artificial foods containing CaCOɜ to free-living fish in coral-rubble habitat at Dump Reef. In the lab, I offered captive wild-caught supermale bluehead wrasses, Thalassoma bifasciatum, and sergeant majors, Abudefduf saxatilis, artificial foods containing CaCOɜ as either food pellets or food strips. I found that CaCOɜ levels characteristic of those in 0. echinata do not significantly deter reef fish. In the tethering experiment there was almost complete predation on brittle stars during the daytime, which supports the results that CaCOɜ does not significantly deter predation. In field food strip palatability assays, reef fish showed little preference for foods with lower CaCOɜ levels. In laboratory food pellet assays, sergeant major damselfish did not prefer foods with lower CaCOɜ levels. Bluehead wrasses accepted all food pellets but those with 100% CaCOɜ. However, in lab food strip assays both species of fish preferred control foods and there was a decrease in consumption with an increase in CaCOɜ level. There was also no significant difference in predation on two forms of CaCOɜ in artificial foods, spicule or powdered.