Protocooperation among small polyps allows the coral Astroides calycularis to prey on large jellyfish

The coral Astroides calycularis (Scleractinia: Dendrophylliidae) is endemic to the Mediterranean Sea (Terr on-Sigler et al. 2016), where it can be reef forming (Musco et al. 2017: Fig. 1). In shallow water, it may cover up to 90% of rocky substrate from the surface down to 15 m depth where it is found in caves and on vertical walls. Astroides calycularis colonies frequently occur in dense aggregations (Goffredo et al. 2011), possibly favored by limited dispersion ability of both sexual (Goffredo et al. 2010) and asexual propagules (Serrano et al. 2017). High water movement promotes massive colony shapes with closely connected polyps usually forming continuous orange surfaces, leaving little space for the settlement of other benthic organisms (Casado-Amezua et al. 2013). Astroides calycularis is “polystomatous,” forming small colonies composed of polyps connected to each other as a single organism bearing several mouths. Colony morphology is also strongly controlled by food availability (Goffredo et al. 2011). Polyps are typically 4–5 mm in length but larger polyps can grow up to 8 mm long. Like other azooxanthellate corals, A. calycularis is an obligate suspension feeder (Cebrian and Ballesteros 2004) and it is assumed to feed primarily on zooplankton transported by water movement, although the identity of its most preferred prey is unknown. The venomous mauve stinger Pelagia noctiluca is a holoplanktonic jellyfish with a phosphorescent bell 3–12 cm wide. It is typical of warm water but currents may transport smacks of jellyfish into temperate and cold seas. Thus, it can be found from the tropics to the north Pacific and Atlantic, including the Mediterranean Sea. Pelagia noctiluca is usually pelagic and performs diel vertical migrations, albeit occasionally reaching the coast in large quantities. In the western Mediterranean, it forms persistent populations where it can be a nuisance to bathing people during periodic blooms in the warm season, raising increasing concerns as these blooms become more frequent (Canepa et al. 2014). Pelagia noctiluca is known to be preyed upon by fishes and turtles and parasitized by crustaceans. So far only one published picture of P. noctiluca seized by a polyp of a polystomatous coral is known (Aguilar 2007:30). Observations of predation of the mauve stinger, P. noctiluca, by the orange coral, A. calycularis, were made in 2010, 2014, and 2017 during three field survey campaigns carried out in different localities of the Mediterranean Sea (Appendix S2). Capture and ingestion of 20 observed jellyfish involved several polyps belonging to different colonies (Fig. 1a, Appendix S1: Fig. S1A–D, Video S1). The feeding behavior begins when a P. noctiluca jellyfish becomes trapped under an overhang with abundant A. calycularis. The pulsating swimming of the jellyfish moves the bell repeatedly against the overhang ceiling. Here A. calycularis polyps first adhere to the bell after which several polyps rapidly engulf the oral arms of the jellyfish, a process lasting between 1 and 5 minutes. Single polyps are able to ingest the oral arm tips preventing the jellyfish from escaping, while other polyps collaborate in ingesting pieces of jellyfish arms and umbrella (Fig. 1b, Appendix S1: Fig. S1D). Some jellyfish escape these attacks or become released as many were observed lying dead on the seafloor, with a hole on the top of the bell possibly caused by the predators. A similar behavior is shown when jellies are pushed by waves or currents onto colonies living on vertical walls along the shore (Appendix S1: Fig. S1A–D). Present data and available information do not allow us to quantify the predation pressure exerted by A. calycularis on the mauve stinger. It seems unlikely that predation by the