Bearded fireworms are usually between 5–10 centimetres (1.9–3.9 in) in length, but can reach up to 35 centimetres (13.8 in). They are endowed with a group of poisonous white bristles on each side, which are flared out when the worm is disturbed.
The bearded fireworm is usually found on reefs, under stones in rocky areas of the sea, and on some mud bottoms. Sometimes, they can be found to be hidden on moss-covered rocks.
It is encountered throughout the tropical western atlantic and at Ascension Island in mid-Atlantic. It can be found near ocean reefs and at least 150m. These worms have also been encountered in the mediterranean sea, in the coastal waters surrounding the Maltese archipelago.
The bearded fireworm is a slow creature, and is not considered a threat to humans unless touched by a careless swimmer. The bristles, when flared, can penetrate human skin with little difficulty, injecting a powerful neurotoxin producing intense irritation and an incredibly painful burning sensation, which gives the worm its name, around the area of contact, which can lead to nausea and dizziness. This sensation lasts only up to a few hours, but a painful tingling can continue to be felt around the area of contact. In a case of accidental contact, application and removal of adhesive tape will help remove the spines; applying alcohol to the area will also help alleviate the pain.