Although the animals of this species are mostly less than 40-50 cm long, specimens of over 1 m length have been found. In larger individuals the body may be 5-6 mm wide, with an elongate, rather flattened and generally wrinkled appearance, but even in smaller examples the worms appear rather more bulky in build than Emplectonema gracile . The head is rounded and spatulate in shape, it may or may not be wider than the adjacent trunk region. The arrangement of the eyes is similar to that of Emplectonema gracile , although their larger number generally leads to them appearing more densely clustered (E. neesii-detail).
In colour Emplectonema neesii presents a speckled or longitudinally mottled light to dark brown appearance dorsally, with a tendency to be paler on the anterior and posterior extremities. Closer examination reveals that the background is a pale yellowish-brown, straw or flesh colour, marked with irregular dark brown pigment specks or streaks which, though loosely arranged in longitudinal rows, are frequently interrupted and do not form distinct stripes. In darker appearing individuals the streaks are often less numerous but more boldly marked than in paler examples. The ventral surface is a pale pinkish-white or flesh colour. Young worms, from deeper water, may on occasion appear almost white or pale skin coloured, and uniformly pigmented dull orange to pale brick-red specimens have also been found. Cerebral sensory organs are small, far anterior to the cerebral ganglia. Cephalic glands are well developed but barely or not posteriorly extending behind cerebral ganglia. The species is reproductively active for most of the year, having been reported breeding at any time between January and October.
Emplectonema neesii is found intertidally and sublittorally to 30 m or more depth. On the shore it typically occurs in the mid- to lower-tidal levels beneath stones and boulders, in rock crevices and fissures, in laminarian holdfasts, among the byssus threads of Mytilus colonies, or on a variety of substrata (sand, silty-sand, shelly-gravel, shingle). Infrequently it may occur in deep narrow rocky clefts at or just above the Pelvetia zone.
With a much more restricted zoogeographic range than Emplectonema gracile , Emplectonema neesii has been recorded only from the Atlantic, Irish Sea and North Sea coasts of Europe, from Iceland to the English Channel, and the Mediterranean.