Sea Lamprey

The sea lamprey (Petromyzon marinus) is a jawless fish belonging to the ancient lineage Petromyzontiformes, a group that diverged from other vertebrates over 350 million years ago. Native to the North Atlantic and adjoining coastal rivers of Europe and eastern North America, it is anadromous, spending its adult parasitic life at sea before migrating into freshwater rivers to spawn. A landlocked population became established in North America's Great Lakes in the early 20th century, where it is regarded as a highly destructive invasive species due to its impact on native fish populations. Sea lampreys lack jaws, paired fins, scales, and a bony skeleton, instead having a cartilaginous frame — traits that place them among the most evolutionarily distinct of all living vertebrates.

Sea lampreys are easily distinguished from true eels and other fish by their lack of jaws and paired fins.

Key identification features:

• Circular, disc-shaped sucking mouth ringed with concentric rows of sharp, horn-like teeth and a rasping tongue
• Seven small round gill openings in a row behind the eye on each side, instead of a single gill slit
• Single nostril centered on top of the head
• Long, scaleless, eel-like body, mottled olive-brown to grey with darker blotching
• No paired pectoral or pelvic fins; only a low dorsal fin and small caudal fin
• Adults typically 60-90 cm, up to about 120 cm

The sucker mouth and blotched pattern separate sea lampreys from the smaller, plain-colored, non-parasitic brook lampreys and from true eels, which have jaws and pectoral fins.

Sea lampreys are native to the coastal North Atlantic Ocean, ranging from the Gulf of St. Lawrence and eastern United States to the Mediterranean and northern European coasts. As adults, they live in open ocean and coastal marine waters, attaching to host fish as they roam. To reproduce, they migrate into freshwater rivers and streams with clean, gravel- or cobble-bottomed sections and moderate current, where they build nests and spawn. After hatching, larvae (ammocoetes) burrow into soft silty or sandy stream substrate for several years before metamorphosing and migrating back to salt water. A self-sustaining, non-native population also persists entirely within the freshwater Great Lakes of North America.

Sea lampreys are solitary, opportunistic parasites as adults, using their sucker mouth to attach to the sides of host fish such as salmon, trout, and other large fish, rasping through skin with their tongue to feed on blood and body fluids over an extended period. A single lamprey can attach to several hosts over months at sea before its spawning migration. When ready to breed, adults stop feeding entirely, migrate upstream, and use their sucking mouths to move small stones and build shallow nests, where several individuals may spawn communally over gravel beds. All adults die shortly after spawning. Larvae live buried in stream sediment as filter feeders for several years before undergoing metamorphosis into the parasitic adult form.