(Forbes, 1838)
Description:
This is a variable species, which may reach 34 mm in length. There are two principal varieties, associated with their diet. One variety feeds upon the hydroids Nemertesia antennina and N. ramosa ; it has a dark brown body with paler, more crowded cerata. The other feeds on the hydroid Halecium muricatum ; in this variety the body is transparent white with brown pigment only on the head, while the cerata are pale cream or white. The cerata (up to 10 pairs) bear up to 12 concentric circlets of pale tubercles. An important diagnostic feature is the presence along each flank of an interrupted white line running from the rhinophore sheath to the tail-tip. The frontal margin of the head is somewhat dilated laterally. Each rhinophore sheath is widely flared and has white pigment around the lip.
Distribution:
It has been recorded from Iceland and Norway to the Mediterranean Sea (Distr. D. fragilis). There are unconfirmed reports from Greenland and from Japan.