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, where it can be reef forming. 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, possibly favored by limited dispersion ability of both sexual and asexual propagules. The venomous mauve stinger Pelagia noctiluca is a holoplanktonic jellyfish with a phosphorescent bell 3-12 cmwide. 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. 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 Mediterra nean Sea. Capture and ingestion of 20 observed jellyfish involved several polyps belonging to different colonies. 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. calycularispolyps first adhere to the bell after which sever al 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. The coral colonies display synchronized capture of the jellyfish. Collective synchronization of foraging provides immediate benefits to the fitness of engaged individuals with relatively little cost. These attributes allow for the selection of the observed behavior over evolutionary time through, e.g., the tendency of colonies to form aggregations. Thus, we consider the observed facultative mutualistic behavior a case of "protocooperation".