A Chemical and Ecological Approach Sheds Light on the Urticating System of Marine Fireworms

Marine fireworms (Annelida, Amphinomidae) hold stinging dorsal bristles (chaetae) that cause injuries to divers and bathers. Hermodice carunculata is the most notorious species and it has recently attracted interest as a potentially invasive fireworm with few predators and uncharacterized defensive capacities. To date, the only acute inflammation inducer isolated from an amphinomid is “complanine”, a trimethylammonium compound. The main goal of this study was to promote an ecological understanding of H. carunculata defences through a multidisciplinary approach. The occurrence of complanine within tissues and its mode of delivery were assessed combining chromatographic steps and high resolution LC-MS/MS. The exact mass and retention time of complanine were detected in the extracts of the main ectodermal (dorsal body wall, gills, dorsal chaetae, ventral chaetae) and endodermal (gut, pharynx) tissues of H. carunculata. The role of complanine in trophic interactions was assessed offering the ectodermal tissues towards a predator (the fish Chromis viridis) and two relevant prey species (the anemones Anemonia viridis and Aiptasia diaphana). Only the dorsal chaetae were effective against predators and prey: they strongly deterred fishes and induced paralysis in the anemone tentacles. Dorsal chaetae treated with organic solvents lost their deterrence against fish predators and an inner hollow cavity suitable to vehicle toxins could be viewed by ESEM. These findings support a synergy between the mechanical injury of dorsal chaetae penetration and the release of complanine. This unique feature could support the success of fireworms in marine benthic environments and significantly improves knowledge on the chemical ecology of amphinomids.