WALTERS, LJ; SACKS, PE; LISS, JL; ROBINSON, NM; GLARDON, CG; GREVERT, JA; STINER, JL; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; Univ. of Central Florida, Orlando; : Urchins on Coral Reefs: Enhancement of Macroalgal Cover?
The sea urchin Diadema antillarum was the most important factor mediating competition between algae and corals on reefs in the Caribbean until the 1980s. Huge numbers of this keystone herbivore kept the algae cropped and reefs were dominated by corals. In 1983-84, Diadema nearly went extinct. This resulted in a shift from coral to algal dominance, with the brown alga Dictyota covering many reefs. This shift prompted research to better understand the biology of Dictyota. One finding was that this genus has the ability to clonally propagate via vegetative fragmentation. Even the smallest fragments can produce microscopic rhizoids within hours that enable them to adhere to sand grains and continue growing. Caribbean Diadema numbers are now starting to increase. Thus, the hypothesis tested was that urchins foraging on Dictyota create fragments and these fragments survive. We ran feeding trials with 4 species of urchins (Diadema antillarum, Echinometra lucunter, Lytechinus variegatus, Eucidaris tribuloides) and Dictyota cervicornis. Dictyota and urchins were placed in modified minnow traps (field) or plastic tubs (lab). Controls included no urchins. At the end of 24 hr, all fragments of Dictyota were measured and placed in compartmentalized boxes with sand. Fragments were checked after another 24 hr to see if they had sand grains attached to them. All tested urchin species produced fragments (up to 5 per urchin) and all fragments, including those 4 mm2, survived and had attached sand. Hence, if urchins create fragments, then this may help populate reefs with algae. This scenario is predicted to be most important in areas with low to moderate urchin densities.
