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Sea lamprey (Petromyzon marinus)
Photo credit: Courtesy of Cory Genovese/PhotoYoop

Sea Lamprey

(Petromyzon marinus)

Report this Species!

If you believe you have found this species in its invasive range in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Sea lampreys are primitive, jawless fish that resemble eels. These aggressive parasites affect Great Lakes fish populations such as lake trout, rainbow trout, whitefish, and walleye because they can latch onto their victims for up to several weeks and feed on their blood and body fluids.


Sea lampreys have long, flexible, cylindrical, scaleless bodies with a deeply notched dorsal fin, separating it into two distinct parts. The body is 30-51 cm (12-20 in) long and weight can range from 227-369 g (8-13 oz). Adults have a disc-like mouth that contains circular rows of over 100 sharp, hooked teeth. Larval lampreys, called ammocoetes, have a very small, undeveloped mouth hidden between folds of skin. Juveniles have white undersides and uniformly colored blackish blue or silver backs. Adults can be olive-brown, yellow-brown, green, red, or blue mottled with a darker shade of the same color; or sometimes nearly black. The underside is typically white or gray.

Similar Species

Non-parasitic native lamprey species such as the Ohio lamprey (Ichthyomyzon bdellium), the American brook lamprey, (Lampetra appendix) and the northern brook lamprey (Ichthyomyzon fossor) are much smaller, lack the dark blotches on the body, and have a single, continuous dorsal fin.


Sea lampreys require three distinctly different habitats connected by free-flowing stretches of stream. Spawning adults are found in late May or early June in shallow pits near the upper end of gravel riffles. After hatching, the ammocoetes drift down to larger, slower moving streams and burrow into the sediment. After several years, they transform into parasitic adults in spring, and migrate into large bodies of water. They migrate back to tributary streams the following spring to spawn and then die shortly after.


In 1921, the sea lamprey appeared in Lake Erie, arriving via the Welland Canal. It took just 25 years for it to spread to the remaining Great Lakes.


Sea lampreys are native to the Atlantic Ocean, where natural populations moved into the freshwater areas of Lake Ontario and the St. Lawrence River to spawn. Now landlocked in the Great Lakes, the sea lamprey has distributed itself into the tributaries of those lakes. In Pennsylvania, sea lampreys are native to the Delaware and Susquehanna river basins in the eastern part of the state, and invasive in Lake Erie and Walnut Creek in Erie County.


Note: Distribution data for this species may have changed since the publication of Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015), the source of information for this description.

*To learn more about the native/invasive range of Sea lamprey in Pennsylvania, check out the information available for this species on our Invasive Here but Not There page.

Environmental Impacts

A single sea lamprey can destroy up to 40 pounds of fish during its adult lifetime. Under some conditions, only one out of seven fish attacked will survive. The sea lamprey population explosion in the 1940s and 1950s contributed significantly to the collapse of economically important Great Lakes fish species such as lake trout.

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