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(O.F. Müller 1774)

Lineus ruber is mostly less than 7-8 cm in length, although larger individuals may occasionally be encountered. Specimens longer than 10-12 cm must be regarded as very uncommon. The head is bluntly rounded at the front, with 2-8 small brown to black eyes forming an irregular row on each dorsolateral margin (L. ruber-detail). In juveniles there is often only a single pair of eye-spots. Behind the lateral cephalic slits the body is of a more or less uniform width and is somewhat dorsoventrally flattened. In colour Lineus ruber is typically light to dark reddish-brown, with a paler ventral surface. The cephalic margins and sides of the body often appear as translucent white. Variations in the colouration include violet, greenish-red and yellowish-brown forms, and specimens in which the pigmentation is darkest in the anterior regions. The cerebral ganglia usually show pink or red through the dorsal surface. In sexually mature animals the gonads appear as whitish spots along the lateral intestinal margins. Dermis without distinct connective tissue layer. Cephalic glands are well developed. Proboscis with two muscle layers and two muscle crosses. Body wall musculature without a diagonal layer.
When mechanically irritated Lineus ruber contracts without coiling into a spiral; this behaviour enables it to be distinguished from Lineus sanguineus , which contracts in a tightly coiled fashion.
Reproduces from January to May, laying gelatinous eggstrings which may commonly be found adhering to the underside of rocks and boulders. It is an exceptionally hardy species in captivity and individuals have been kept without aeration or food for more than twelve months.

A very common British nemertean, typically found intertidally on muddy sand beneath stones and boulders from the Fucus spiralis level downwards. Often several individuals can be found under a single rock. It is also found in mussel beds, crawling between barnacles, on rock-pool algae (especially Cladophora and Ceramium ), among the holdfasts of the larger brown seaweeds, in estuarine muds and sublittorally on a variety of bottom types. Not uncommonly occurring in brackish waters, the lower limit of salinity tolerance of the species appears to be about 8 ä.

Lineus ruber is one of the most commonly recorded nemerteans and has a circumpolar distribution in the northern hemisphere, being reported from Atlantic, Mediterranean and North Sea coasts, both Pacific and Atlantic coasts of North America, Madeira, Greenland, Iceland, the Faeroes and the coast of Siberia. It is also recorded from South Africa.

Lineus ruber