This scale worm can be either free living or commensally with other invertebrates ( most commonly in the tubes of tubeworms). Halosydna brevisetosa has eighteen pairs of overlapping scales. It is the only species of scaleworm in the Pacific Northwest adorned with 18 pairs of scales.
The colour is variable with black and/or white spots on the scales. As free living animal, it attains a length of 8 cm, commensal it grows up to 11 cm. The eighteen scaled worm can be found on rocky beaches, among mussel clumps and among sea grasses.
Halosydna brevisetosa, commonly known as armored scaleworm, is a predator and feeds on other worms.