The bearded fireworm (Hermodice carunculata) is a marine worm, although “worm” doesn’t mean much in biology, as worms embrace several distinct categories or phyla, to be more precise. Indeed, the fireworm belongs to the phylum Annelida and to the class Polychaeta. This means, in a nutshell, that its body is composed of a number of ring-like segments called metameres (annelids), all identical but the first and the last, each with a pair of appendices called parapods with a tuft of white bristles or chaetae (polychaetes). Moreover, small orange gills are found along the metameres, which allow it to breath – see picture.

The bristles are the problem. Beside serving for the locomotion, they are an extremely effective defensive weapon. Very narrow and fragile, yet hard because chitinous, they break like glass needles at the slightest contact with a potential predator, causing pain and inflammation too because of a neurotoxin that they release. This happens with the unlucky or dumbly curious man, yet with other marine species too, as the fireworm seems to have no actual predator.

Described first in the Caribbean and in the Red Sea, the earliest sightings in the Mediterranean go back to the beginning of the 1800’s (in Eastern Sicily and in the Aegean Sea). Here its distribution, initially confined to the southernmost areas, is lately moving north fast, up until the Adriatic Sea and the French coast. And even in the South, where we live, we do notice from year to year an increased quantity.

The point is that the fireworm fancies warm waters (well, who doesn’t?!?). Below 19 °C it seems that it becomes rather inactive, and its larvae need more than 22 °C to survive. This is why the reproduction takes place in the heart of the summer : males and females raise ¾^th of their body length like cobras and, slowly oscillating, release their gametes. These in turn get fertilized and start to roam pelagically for a long time, probably several weeks. This is indeed the secret of the fireworm’s wide spreading, the long pelagical larval duration that allows it to reach distant and scattered spots. Provided the water’s warm enough, which we all know is increasingly true…

Beside all this, the fireworm’s recent “fame” comes from the fact that it eats… basically everything! From dead fish to echinoderms (sea stars and sea urchins, among which the delicious Paracentrotus lividus); from cnidarians (sea anemones, jellyfish, and sea-whips, among which the emblematic Paramuricea clavata and Eunicella singularis e cavolinii – see picture) to molluscs (nudibranchs, chitons – see picture), and even tunicates (ascidians). I mean, it spreads destruction all over the place, and if you cut it in half it will even be able to regenerate its tail !

Speaking of sea-whips and anthozoans: it’s thought that, beside eating their polyps, the fireworm is also a vector of pathogen bacteria from the order Vibrio, which in turn infect the sea-whips. This has been documented, for instance, for Oculina patagonica, a colonial madrepore mostly known in the Eastern basin of the Med, which has suffered bleaching triggered by our beloved worm-friend.

Painful bristles, no predators, impressive spreading, extreme voracity – all this has recently put the fireworm under the spotlight. This is because it is the emblem of how dramatically can global warming be a lethal threaten to our sea.