Acanthobdella peledina

The taxonomy of Acanthobdellida has been contentious for many years,[3] but it is now recognized as the sister group to leeches.[4][5] Acanthobdella peledina was first described by the German zoologist Adolph Eduard Grube in 1851.[1] For a long time, it was believed to be the only species in the order, but in 1966, another species, Paracanthobdella livanowi, was described from the vicinity of the Kamchatka Peninsula in eastern Siberia.[6]

A. peledina differs from leeches in not having a well-developed sucker at its anterior end; instead, it attaches to its host with about forty hooked chaetae (chitinous bristles) borne on its first five segments.[7] It is in fact unique among Hirudinea in having chaetae at all, and also in having a coelom (body cavity) divided by septa into a discontinuous channel.[7] Other ways in which it differs from other leeches are that the body is divided into 29 segments, it lacks a prostomium and a peristomium, and the nephridia do not have funnels. It has a sucker at the posterior end, and moves in a leech-like fashion by alternately extending its anterior end forward, then clinging on with its hooked chaetae while it brings its posterior sucker close to its front end.[7] The leeches are cylindrical, olive green and about 23 mm (0.9 in) in length.[8]

The species has a boreal distribution and is known from freshwater locations in northern Europe and Asia, and from Alaska.[7]

One of the fish in Alaska parasitized by A. peledina is the sardine cisco (Coregonus sardinella). The worms were found just behind the pelvic fins with their anterior ends embedded in the skin and muscle tissue below.[8]