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(Müller, 1774)

Description:
In shape, size and eye arrangement Lineus viridis is generally very similar to Lineus ruber . It differs, however, being a dark olive-green to greenish-black in colour, although pale green individuals are sometimes found and in these the reddish cerebral ganglia are particularly evident through the dorsal body wall. The ventral surface may be marginally lighter in hue than the dorsal one. Mature adults are sometimes marked with pale-coloured slender annulations arranged at more or less regular intervals. In sexually ripe specimens the gonads and gonopores appear as obvious whitish spots arranged in a dorsolateral row on either side of the body. Eggs are laid during April.
Lineus viridis , most commonly under the name Lineus gesserensis , was for long regarded merely as a colour variety of Lineus ruber and not as a separate species. However, the two forms differ strikingly in their larval development and are regarded as distinct species. In Lineus ruber few (10-15) larvae of very variable size emerge from the egg-string after hatching and commence feeding immediately. They also exhibit a weakly positive, or, more usually, indifferent response to light which rapidly develops into a strong negative phototaxis. In contrast 400-500 small larvae emerge from the egg-strings of Lineus viridis and possess an initial period of strongly positive phototropism, lasting two to three weeks or more, during which they do not require food. The juveniles of both species are transparent and eyeless; in Lineus ruber the body pigmentation begins to appear after 2-3 months, but in the slower-growing Lineus viridis the colour takes longer to develop. Dermis without a 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.

Habitat:
Most often found on muddy sediments beneath boulders or stones from the mid-shore (Fucus vesiculosus , Ascophyllum nodosum ) level downwards, but may occur higher and on somewhat coarser substrata. Lineus viridis is also reported from estuaries.

Distribution:
The geographic distribution of Lineus viridis closely parallels that of Lineus ruber except that it is not known outside the northern hemisphere. The species does not appear to be as common as Lineus ruber , although it may be locally abundant.

Lineus viridis