Body narrow, integument smooth.
Prostomium with a pair of antennae without cirrophores, a pair of biarticulate palps slightly longer than the antennae and 2 pairs of eyes in a trapezoid arrangement.
Dorsal tentacular cirri approximately twice as long as the ventral ones, second dorsal tentacular cirrus reaching chaetiger 8 (N. virens-anterior end).
Pharynx with a pair of denticulate jaws; oral and maxillary rings with conical paragnaths, group I: 1 (0-2); group II: 5-7 (4-10); group III: 10-12 (7-14); group IV: 17-19 (11-23); group V: 0 (occasionally 1); group VI: 1-2 (1-3); groups VII-VIII: 2 irregular rows of 20-26 (17-36) (N. virens-proboscis).
Parapodia of the first 2 chaetigers uniramous with 3 lobes. Remaining parapodia biramous, with 4 lobes. Lobes varying little along the body. Dorsal notopodial lobe the longest, leaflike, especially in large animals, notopodial acicular lobe bilobed, the lobe above the acicula always shorter than that below it. Neuropodial acicular lobe bilobed, the lobe above the acicula always shorter than that below it and with a prominent postchaetal lobe above the acicula; the difference in length between the two lobes of the neuropodial acicular lobe is reduced in posterior chaetigers. Dorsal and ventral cirri much shorter than the parapodial lobes, the dorsal cirrus arising from the dorsal edge of the dorsal notopodial lobe (N. virens-parapodium).
Notopodial chaetae all homogomph spinigers. Neuropodial chaetae homogomph spinigers above and heterogomph spinigers below the acicula. In anterior chaetigers of small worms, heterogomph falcigers also occur below the acicula in the neuropodium, but are absent posteriorly, and absent from large animals (N. virens-chaetae).
Up to 900 mm for 200 chaetigers.
In males, epitokous modifications occur, but only to a limited degree, whilst in females, no external changes are apparent. Metamorphosed parapodia from the anterior region have elongated dorsal cirri and in the posterior region males develop small accessory flaps at the base of the dorsal cirrus and on the posterior surface of the neuropodial acicular lobe. In males a pygidial rosette is found, and in the modified parapodia paddle chaetae are present together with the ordinary chaetae. Because of the relatively slight modifications of the parapodia, the exact chaetiger at which such modifications first appear cannot be ascertained with any accuracy.
A variety of sediment types, from soft mud to muddy sand to sediments containing a large proportion of gravel or small stones. It does not extend as far up in estuaries as Nereis diversicolor , reflecting its inability to cope with very dilute sea water.
North Sea, Skagerrak, Kattegat, Arctic, North atlantic and North Pacific.