The Mussel Worm (Nereis vexillosa) is among the more familiar polychaete ("bristle") worms along the west coast of North America. Like other worms in the family Nereididae, it has two palps on the prostomium ("snout") and four pairs of tenticular cirri arising from the sides of the prostomium in front. Nereis vexillosa is mainly gray in color, but with iridescent green, blue, or even pink. It is found in the intertidal zone with mussels and barnacles, as well as on pilings and under rocks and pieces of wood in quiet bays. (Kozloff 1993; Sheldon 1999; Carlton 2007)
Johnson (1943) investigated the life history of Nereis vexillosa and his publication should be consulted for detailed information on the life history and development of this species. The tiny eggs produced by Nereis vexillosa are about 0.2 mm in diameter; they are spawned in firm, irregular gelatinoid masses, somewhat translucent, with blue-green, greenish, or brownish tints. These colors are most noticeable in the freshly laid eggs due to their greater compactness prior to absorption of water by the capsular material. The firmness of the masses enables them to withstand a good deal of handling or washing about on the beach by waves without disintegrating. As a result of their robustness, they are often found in good condition on tidal flats where bits of seaweed or other debris collects at the waters edge. However, they seem never to be found in more than moderate quantities despite the fact that N. vexillosa is often quite abundant. According to Johnson (1943), as far as is known the eggs of other Nereis species, and of closely related genera, are deposited separately in the water or are only lightly attached to each other.
Like other polychaete annelid worms, Nereis vexillosa has bristly appendages on each of its many segments that help it move about. Its mouthparts, a pair of blackish pincer-like jaws studded with numerous small teeth, can be everted out of its head for feeding (a curious observer may gently press behind the head with thumb and forefinger to encourage the worm to extrude its pharynx when it is not feeding). (Kozloff 1993; Sheldon 1999)
Nereis vexillosa reaches a length of around 15 cm (Kozloff 1993; Sheldon 1999).
According to Johnson (1943), this species can reach 30 cm.
Nereis vexillosa is found along the west coast of North America from Alaska to California (Sheldon 1999).
Nereis vexillosa uses its strong jaws to bite off chunks of seaweed or soft-bodied animals (Brusca and Brusca 1978; Sheldon 1999).
When Nereis vexillosa worms become sexually mature (apparently at 1 year of age), their fleshy parapodia (the lateral, bristle-bearing protrusions on each segment) expand into paddle-like structures for swimming, and periodically during the summer ripe males and females swarm near the surface at night (this behavior can be observed from floating docks if the area is illuminated). The posterior part of the body is typically redder in females than in males. The worms "mate" by spewing sperm and eggs out through openings that develop in the body wall. Observations suggest that the worms die shortly therafter, within a few days or less. (Johnson 1943; Kozloff 1993) In experiments, females ripe with eggs were induced to spawn almost instantly when a few drops of sperm-laden water were added to the water in which they were isolated (Johnson 1943). A spawning female comes to or near the surface and suddenly exudes a mass of eggs, which instantlly clump together. She then then passively sinks to the bottom together with the mass. A few moments later she frees herself from the mass, which remains at the bottom and within a few hours swells to about three or four times its original size through the absorption of water. (Johnson 1943)