Waterborne conspecific cues enhance larval settlement in the invasive jellyfish Aurelia coerulea

Abstract Gregarious settlement contributes to the formation of jellyfish polyp aggregations, but the mechanism by which larvae detect habitats with conspecifics is poorly understood. Here, we tested the hypothesis that Aurelia coerulea larvae settle gregariously by detecting organic compounds released into the water by polyps. First, we conducted choice experiments to confirm the preference of larvae for substrate with conspecifics and assess whether larvae detect physical or chemical cues from polyps. Then, we conducted settlement induction experiments to explore the mechanism underlying the detection of conspecifics and the nature of the chemical cues involved. We exposed larvae to seawater that had been in contact with polyps, and to substrate from which polyps had been detached to assess whether the settlement cues were waterborne or surface-bound compounds. Finally, we measured whether settlement cues persisted after 18 h to determine their stability and evaluated their organic nature by testing whether larvae still settled when exposed to seawater from which dissolved organic compounds from polyps had been removed via filtration. Larvae actively selected substrate inhabited by polyps, and settlement was enhanced with seawater conditioned with a high-density of polyps. These settlement cues were organic and active for at least 18 h. Our results provide compelling evidence of a larva-polyp interaction in A. coerulea mediated by stable organic waterborne compounds that promote gregarious settlement. This chemical interaction may have unexplored ecological implications beyond habitat selection, potentially facilitating the invasive success of A. coerulea and the formation of jellyfish blooms.