It is jawless with a round sucker-like mouth and has an outer circle of small teeth and an inner circle of large teeth (Ref. 88171). It has a typical eel-like shape with 2 dorsal fins and 7 gill openings behind the eye. It lacks paired fins. Young adults are uniformly greyish in colour. As it ages, the river lamprey becomes greenish-brown dorsally, golden yellow along the sides and white ventrally (Ref. 58137). In coastal waters of Germany, it can be confused with the sea lamprey (Petromyzon marinus), which is distinguished by having its teeth arranged in many consecutive circular rows (Ref. 88171). Other diagnostic features: Adults 8.6-49.2 cm TL. Body wet weight in individuals 18.0-49.2 cm TL, 30-150 g. Body proportions, as percentage of TL (based on 48 specimens measuring 10.8-38.6 cm TL): prebranchial length, 10.0-12.9; branchial length, 7.9-11.3; trunk length, 46.2-54.3; tail length, 24.1-30.3; eye length, 1.4-3.1; disc length, 4.6-7.0. Urogenital papilla length, as a percentage of branchial length, in 19 spawning males measuring 19.7-28.3 cm TL, 15.9-37.5. Trunk myomeres, 58-66. Dentition: marginals, 70-95; supraoral lamina, 2 unicuspid teeth; infraoral lamina, 5-9 either all unicuspid teeth or, more frequently, the lateralmost are bicuspid and the internal ones unicuspid; 3 endolaterals on each side; endolateral formula, typically 2-3-2, rarely 1-3-2 or 2-3-1; 1-2 rows of anterials; first row of anterials, 4-7 unicuspid teeth; exolaterals absent; posterials absent; transverse lingual lamina, 8-18, usually 12-14, unicuspid laminae straight or parentheses-shaped and each with 9-13 unicuspid teeth. Marginal membrane present. Velar tentacles, 4-10, with tubercles; no velar wings. Body coloration in recently metamorphosed individuals silvery; in preserved upstream migrants, bluish brown or lead gray on the dorsal aspect tending towards silvery on the lateral aspects and whitish or yellowish on ventral aspect. Early upstream spawning migrants returning from the sea have a bronze sheen. Dorsal fins of maturing individuals may have a purplish tint. Iris is golden yellow. Body coloration in the landlocked population in Lake Ladoga is completely black. Lateral line neuromasts unpigmented or darkly pigmented. Extent of caudal fin pigmentation, absent or trace in young adults and 75% or more in spawning individuals. Caudal fin shape, spade-like. Oral fimbriae, 84-112. Oral papillae, 11-20 (Ref. 89241).