Mussel Worm

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)

Nereis vexillosa is found along the west coast of North America from Alaska to California (Sheldon 1999).

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)

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)

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 uses its strong jaws to bite off chunks of seaweed or soft-bodied animals (Brusca and Brusca 1978; Sheldon 1999).

How to Distinguish from Similar Species: The large, strap-shaped upper ligule in the posterior notopodia is very helpful in identifying this species. In Nereis grubei and N. neoneanthes the upper ligule of the posterior notopodia is not strap-shaped. Nereis wailesi has no paragnaths on the distal proboscis. Nereis brandti has no homogomphfalcigerous setae on the posterior notopodia and the dorsal ligule of the parapodia is large and leaflike.

Intertidal with mussels and barnacles, on pilings, in sandy mud and cobbles, and in algal holdfasts.

Geographical Range: Alaska to San Diego; Pacific coast of Russia

Depth Range: Mid to low intertidal

As a member of Family Nereididae, few if any segments are longer than wide, the notosetae from the two sides do not almost meet along the dorsal midline, the prostomium does not extend as a dorsal caruncle back over the segments posterior to it, there is no circle of tentacles around the mouth, the prostomium has two antennae, and a pair of palps which are differentiated into two units, the distal unit of which is substantial even though it is smaller than the proximal unit. Some of the setae are compound. The pharynx is delineated into two parts, with a pair of stout jaws on the distal part and conical teeth on both parts. The peristomium has four pairs of tentacular cirri which arise on the anterolateral corners. Usually two pairs of eyes are present. The parapodia on the first two setigers are uniramous, and biramous on all subsequent setigers. Both the notopodia and the neuropodia have acicular lobes which are often small, plus one to three conspicuous lobes above and below the acicular lobes called ligules. The shape and arrangement of the ligules varies among species. In Nereis vexillosa the ventral cirrus of all parapodia is simple, the tentacular cirri of the peristomium are not constricted into several units, the segment behind the peristomium is not expanded into a large collar around the peristomium, all of the paragnaths on the eversible proboscis are conical, some paragnaths are present on the distal part of the proboscis, the posterior notopodia have homogomph falcigerous setae, and the upper ligule of the notopodia in the posterior region is much larger than the lower ligule and is strap-shaped, with a terminal cirrus. Color is greenish or greenish brown, often with blue tones; may appear iridescent in direct sun. Has 4 black eyes. The tentacular cirri are short. The animal is often large--up to 30 cm long and 1.2 cm wide.

Biology/Natural History: The large jaws on the distal portion of the eversible pharynx are used for seizing prey or tearing algae. The smaller denticles on the proximal portion are used for burrowing. Family Nereididae are called sea nymphs and are common polychaete worms in this area. Intertidal species are sometimes called ragworms. Nereids reproduce by releasing parts of their body as epitokes, which swim to the surface in mating swarms. Nereid epitokes are swollen with eggs or sperm, large parapodia, paddle-like chaetae, and large eyes. Day length is important in swarming of epitokes, and near-shore lights can affect the timing of swarms. In our area, spawning may occur shortly before midnight. The mating swarms release pheromones into the water which induces mating activity. The male epitokes swarm first and the females will not release their eggs unless in the presence of the males. The eggs are released into the water in the swarms, through ruptures in the body walls. In N. vexillosa the female releases an agglutinating material along with her eggs. Both male and female epitokes die after spawning. The mass, with eggs inside, sinks to the bottom and grows to about the size of a bluish-green chicken egg. Larvae remain as plankton for hours to months. In the Pacific Northwest the mating swarms usually occur in late winter or spring. This worm is often used for fishing bait. It squirms violently and everts its proboscis and jaws when captured, and may bite.

Nereis vexillosa (common name: mussel worm) belongs to the phylum Annelida, a group known as the segmented worms. It is generally iridescent green and can reach 30 cm in length. It can be distinguished by the size of the upper ligules on the notopodia of the posterior region of the body. The upper ligules are much larger than the lower ligules. It is also without a collar-like structure around the peristomium.