All lampreys have snakelike bodies and smooth, slippery skin; a cartilaginous, boneless skeleton; lack an articulating lower jaw; and have a mouth that consists of a rounded sucking disk. There are no paired fins. There are 7 pore-like gill openings along each side of the head.
Unlike parasitic lampreys, brook lampreys (the group of nonparasitic lampreys) don’t have many teeth in their sucking disk, and the teeth they do have are poorly developed, especially near the outer edge of the disk. When expanded, the sucking disk is narrower than the head.
The northern brook lamprey has an undivided though shallowly notched dorsal fin, and all the disk teeth are poorly developed. In the innermost circle, all teeth are 1-pointed. Adults ready to spawn are darkish brown, becoming nearly black by the time spawning is completed.
The larvae (ammocoetes) of all lampreys resemble the adults but lack eyes, and the mouth is a horseshoe-shaped hood instead of a sucking disk. Larvae and new adults are grayish brown above, yellow on the belly and fins.
Similar species: Missouri has six species of lampreys; of these, four are nonparasitic (these are all called brook lampreys; our four species are the southern brook lamprey, northern brook lamprey, least brook lamprey, and American brook lamprey); our other two species of lampreys are parasitic (the chestnut lamprey and silver lamprey).
- Our four species of nonparasitic brook lampreys are never more than 8 inches long, they have narrow heads, and they have poorly developed disk teeth (the brook lampreys are bottom feeders and do not cling to fish). Missouri's other three brook lampreys are also found in Ozark streams, with the least brook lamprey being much more common than the other two.
- Our two parasitic lampreys are the chestnut lamprey and the silver lamprey. Both have well-developed, rasplike oral discs with plenty of teeth. The chestnut lamprey occurs in large streams and small rivers of the Mississippi River system, including the lower Missouri River (below St. Joseph) and the larger streams and reservoirs of the Ozarks. The silver lamprey is known only from the Mississippi River, where it is much less common than the chestnut.