Body elongate, tapering posteriorly, dorsal surface convex.
Prostomium with a pair of antennae without cirrophores, a pair of biarticulate palps, longer than the antennae and 2 pairs of eyes in a trapezoid arrangement.
Peristomium achaetous, with 4 pairs of tentacular cirri originating from short cirrophores; dorsal tentacular cirri longer than ventral ones, second dorsal tentacular cirrus reaching chaetiger 5 (N. pelagica-detail).
Pharynx with a pair of denticulate jaws; oral and maxillary rings with conical paragnaths, group I: 2 (1-2); group II: numerous (10-20); group III: numerous (11-19); group IV: numerous (20-33); group V: absent; group VI: 4 large cones (3-7); groups VII-VIII: numerous in irregular rows, those furthest from mouth tending to be larger (N. pelagica-proboscis).
Parapodia of the first two chaetigers uniramous with 3 lobes.
Remaining parapodia biramous with four lobes: a dorsal notopodial lobe, a notopodial acicular lobe, a neuropodial acicular lobe and a ventral neuropodial lobe. Both notopodial lobes of similar lengths, ventral neuropodial slightly shorter and neuropodial acicular lobe the shortest; dorsal notopodial, notopodial acicular and ventral neuropodial lobes all broadly rounded in anterior chaetigers, both notopodial lobes becoming somewhat more elongate posteriorly. Neuropodial acicular lobe with a poorly developed postchaetal lobe. Dorsal cirri longer than the parapodial lobes, ventral cirri shorter or of comparable length as the ventral neuropodial lobe (N. pelagica-parapodium).
Notopodial chaetae are exclusively homogomph spinigers anteriorly, with additional homogomph falcigers in posterior chaetigers.
Neuropodial chaetae homogomph spinigers and heterogomph falcigers above and heterogomph spinigers and falcigers below the acicula (N. pelagica-chaetae).
Up to 150 mm for 100 chaetigers.
Living animal yellowish, greenish, golden brown, reddish brown, olive green or violet.
Body divided into two regions, an anterior region of 16 chaetigers in males and 18-20 in females, followed by chaetigers with modified parapodia. The dorsal cirri of the first 7 chaetigers in males and the first 5 in females are modified, as are the ventral cirri of the first 4-6 chaetigers in males and 3-4 in females. Metamorphosed parapodia from the posterior part of the body develop accessory flaps at the base of the dorsal cirrus, below the notopodial acicular lobe, on the posterior surface of the neuropodial acicular lobe and at the base of the ventral cirrus. The accessory flaps are better developed in males than in females. In males, crenulations develop along the lower edge of the dorsal cirri and a pygidial rosette is found. In both sexes, the ordinary chaetae are replaced by paddle chaetae in the modified parapodia.
Predominantly intertidal, though occasionally found sublittorally where the morphologically similar Nereis zonata tends to replace it. It lives in a mucous tube on the underside of half embedded stones and boulders or in the holdfasts of large algae.
In both the Atlantic and Pacific in both hemispheres, as well as in the Mediterranean.