Responses to predation cues and food in two species of sympatric, tropical sea urchins

We examined the responses of two tropical sea urchins, Lytechinus variegatus and Tripneustes ventricosus , to cues from predators, simulated predation events and food. Cues released from damaged conspecifics, heterospecifics and heteroclassics (holothurian) were used to simulate predation events in field experiments. Responses to the presence of seagrass, spiny lobster or both were tested in tanks. Findings were supplemented by natural history observations of dispersion patterns off Bermuda. In field experiments, neither species formed groupings in the presence of conspecific, heterospecific or heteroclassic cues. Flight responses were greatest in conspecific treatments; responses to heterospecific cues were intermediate to control and conspecific cues. Urchins in pre-assembled associations remained in groups in control trials but dispersed when exposed to predation cues. Lytechinus exhibited greater sensitivity to predation cues than Tripneustes . Cues from a damaged sea cucumber invoked a response from Lytechinus but not Tripneustes . Both species employed a two phased response to cues from damaged conspecifics: initially a rapid, but ephemeral (2 min), alarm response followed by a slower (approximate to 35 % lower) sustained flight phase for 6+ min, which in nature would disperse urchins downstream and away from a predator. In tank experiments, Lytechinus formed groupings only around food or food + predators. The presence of a predator reduced the aggregation response to food, suggesting that Lytechinus employed a risk aversion strategy. Tripneustes exhibited escape or refractory behavior in both control and experimental treatments in laboratory tanks.