Tetrastemma candidum lacks distinct external characters and, indeed, is an inadequately described species. Many of the records of the species must thus be of uncertain validity. It is possible that several species have been listed under the same name. Among the British tetrastemmids it may in particular be confused with Tetrastemma flavidum , although unlike this worm Tetrastemma candidum is a very restless and actively moving species.
Mostly reported as about 8-10 mm long and 1 mm or less wide, individuals with lengths in the 20-35 mm range have been ascribed to this species. The head is rather flattened, bluntly rounded and may be wider or narrower than the succeeding body regions (T. candidum). It bears four distinct reddish-brown, dark brown or black eyes (T. candidum-head). Posteriorly the body gradually narrows and ends in a blunt point.
The colour is extremely variable. Much of this variation appears to be attributable to the gut contents, as pale flesh-coloured or yellowish individuals have been described in which only the intestine is distinctly pink, brick red or bright green. Colour variations may also be due to different species being recorded as Tetrastemma candidum . Immature specimens, without obvious gut contents, possess a characteristic greyish-white translucent appearance with a faint greenish tinge. Ripe gonads may appear as a grey or yellow tinge. On the head an opaque whitish patch is sometimes present between the two pairs of eyes, and from this a slender median white longitudinal streak occasionally extends down the back. The cephalic furrows may be colourless or possess a brown pigmentation. The gonads are ripe in late spring and summer (April to August).
Occurring from about the mid-shore level down to depths of 55 m or more,Tetrastemma candidum is commonly obtained from rock pool algae (especially Ulva, Corallina and Cladophora ), from within the bladders of the wrack Ascophyllum nodosum , with Zostera , among colonies of hydroids, in old tubes of sabellid polychaetes, on submerged surfaces of hulks and boats, under rocks or in sediments such as shelly gravel and sand.
Tetrastemma candidum has a circumpolar distribution in the northern hemisphere. Apart from the British Isles, it is recorded from the coasts of Scandinavia, the North Sea, the Mediterranean, Madeira, the Faroes, Iceland, Greenland, the Atlantic and Pacific coasts of North America, the Caribbean and Japan. In the southern hemisphere examples identified as members of this species are reported from South Africa and Brazil.