The detection of specific prey cues triggers distinct predatory behaviour in Aurelia coerulea polyps (Cnidaria: scyphozoa)

Predatory behaviour of cnidarian polyps is poorly described, although their tentacles are equipped with thousands of stinging cells to capture preys. The predatory action is elicited by the tactile stimulation of the tentacles due to the physical encounter between the prey and the polyp. Micro-predation involves low-energetic movements of tentacles and fast ingestion of more small prey simultaneously. In contrast, macro-predation includes expensive actions of the whole body, with a duration of up to 15 min to ingest a large prey. When polyps are spatially aggregated, they can engage in a collective predation in which they first cooperate and then compete for the division of the prey. We hypothesized that an additional stimulation represented by specific prey-produced cues could serve as an additional trigger for predation. We provided three diets (1 large prey, 2 small preys) to groups of five polyps and later stimulated them with prey homogenates as prey-produced cues. Behaviours were first observed and categorized and later used to evaluate feeding activity, together with tentacle contractions and rapid movements. Polyps fed with the large prey and later stimulated with its homogenate showed 2.2 to 6.5 times higher number of tentacle contractions and rapid movements and 55.7% higher occurrence of the category "active feeding" in comparison to all the other combinations of prey/prey cue, resembling the predatory action necessary for macro-predation. These outcomes suggest the ability of polyps to discern the prey cues in the water medium and to modify their behaviour accordingly. Cnidarians are metazoans lacking a central nervous system; however, they are able to detect the presence of prey in their surroundings, process this information and adjust their behaviour accordingly, maximizing the benefits of predation.