Avoiding predation: alarm responses of Caribbean sea urchins to simulated predation on conspecific and heterospecific sea urchins

Alarm responses to the extracts of conspecifics and hetero-specifics were measured for the Caribbean sea urchins Echinometra viridis, E. lucunter, Lytechinus variegatus, L. williamsi, Tripneustes ventricosus, Diadema antillarum, and Eucidaris tribuloides collected along the Caribbean coast (9°3314N; 78°5523W) during October 1984 and July–December 1985. Responses to seawater and extracts of the gnathostomate echinoid Clypeaster sybdepressus were used as controls. The intensity of the response resulting from exposure to sea-urchin extracts was measured by: (1) the percentage of individuals that responded by moving away from the extract and/or towards shelter, and (2) the mean distance moved. Echinometra viridis, E. lucunter, and L. williamsi responded to sea-urchin extracts by moving towards nearby shelter sites. The distance that individuals of each species moved in the first minute following exposure to conspecific extracts was correlated with the distance that species moved from shelter while foraging. L. variegatus and D. antillarum, living in microhabitats not providing protection from predators, responded to extracts of conspecifics and heterospecifics by moving away from the direction of the extract. Eucidaris tribuloides did not exhibit alarm responses to the extracts of con- or hetero-specifics. E. tribuloides secures itself with its stout spines into protected sites within corals. Similarly, L. variegatus living in long, dense seagrass that provided protection from detection by predators, and D. antillarum occupying crevices, showed no alarm responses to extracts of conspecifics. Presumably, in these situations, sea urchins cannot increase their defenses against predation by moving away from an injured neighbor. T. ventricosus showed a weak response to extracts of L. variegatus, but no response to extracts of other species including conspecifics. The reasons for this lack of an alarm response are unclear. For the five species that demonstrated an alarm response to sea-urchin extracts, the intensity of the response varied depending on the type of extract used. L. variegatus, L. williamsi, and D. antillarum responded most strongly to extracts from conspecifics, while Echinometra viridis and E. lucunter responded strongly to extracts from both conspecifics and congeners. The weakest responses were shown to the extracts of T. ventricosus and Eucidaris tribuloides. Habitat overlap, overlap in predators, and phylogenetic relationships did not consistently explain patterns of alarm responses to the extracts of heterospecific sea urchins.