Echinometra sea urchins on Caribbean coral reefs: Diel and lunar cycles of movement and feeding, densities, and morphology

Abstract

Two species of Echinometra are found on tropical western Atlantic reefs. E. viridis and E. lucunter. Both are grazers and bioeroders, and on some disturbed reefs E. viridis has become the dominant herbivore, particularly since the disease-caused decline of the much larger echinoid Diadema antillarum. Thus, it is critical to understand factors affecting herbivory by Echinometra, including foraging behaviors and distances, diel and lunar cycles of foraging, and densities of Echinometra, particularly E. viridis, across habitats and locations.Twenty-four hour tracking studies undertaken in the San Blas Islands of Panama, conducted at different times in the lunar cycle, revealed that on new moons both species of Echinometra primarily moved and fed at night, but the pattern reversed on full moons. These data suggest that vulnerability to predators and/or aggression by damselfishes during the night on full moons may be a significant selective force affecting diel foraging behavior. Daily distances moved (minimum estimate) averaged 19 cm for E. viridis and 11 cm for E. lucunter. The average foraging range for E. viridis was estimated to be 0.3–5% the size of that of D. antillarum, while per capita daily algal consumption by E. viridis is 20% that of D. antillarum.These results suggest more intense, localized grazing by E. viridis compared to D. antillarum.A review of published densities of E. viridis show that 50% of studied sites (location/habitat combination) have densities less than 5 m−2, a level unlikely to produce a strong herbivory effect. Densities greater than 15 m−2 were found at 25% of sites, including the San Blas Islands, Panama patch reefs in this study, which averaged 26 m−2. Where high densities are combined with low predator numbers due to fishing, E. viridis can have strong direct effects on algal assemblages, with potential indirect effects on corals and other benthic invertebrates. However, available published data indicate E. viridis has strong regional variation in abundance, as yet unexplained, that limits its potential Caribbean-wide role as a reef herbivore.Morphological traits of E. viridis that are likely related to vulnerability to predation varied across habitats. A newly-described melanic color morph comprised 4–16% of the population, with significant variation in frequency across habitat types. Body sizes (test diameter) also varied among habitats, with larger individuals much more common in habitats with fewer shelter sites. Similarly, in habitats offering less protection from predators, spine lengths were longer relative to test diameter. Morphological traits that reduce predator detection or vulnerability to predation, as well as the refuges afforded by habitat, will ultimately affect both the number of E. viridis on a particular reef and their ability to forage in the presence of predators.