Northern Brook Lamprey

The northern brook lamprey (Ichthyomyzon fossor) is a small, non-parasitic jawless fish native to freshwater streams of the Great Lakes, St. Lawrence River, and parts of the upper Mississippi and Hudson Bay basins in North America. It is one of several non-parasitic brook lamprey species believed to have evolved from parasitic ancestors within the genus Ichthyomyzon, retaining the general body plan but losing the feeding adult stage. The species completes its entire life cycle in fresh water, spending most of its several-year lifespan as a burrowing larva before a brief, non-feeding adult phase focused entirely on reproduction. Because it closely resembles the larvae and juveniles of related parasitic species, the northern brook lamprey is most reliably identified as a mature spawning adult.

Northern brook lampreys are small stream lampreys best identified by their weak dentition and small adult size.

Key field marks:

• Small, weakly developed teeth on the oral disc, reduced compared to related parasitic species in the same genus
• Dark grey to blackish-brown back, fading to a pale cream belly
• Dorsal fins closely connected, with only a shallow separation
• Seven round external gill openings on each side of the head
• Adults typically 10-17 cm, notably smaller than parasitic Ichthyomyzon species such as the silver or chestnut lamprey

Distinguishing the northern brook lamprey from the larvae of related parasitic species in the same streams can be difficult and often requires examining a mature, breeding adult for weak tooth development and a regressed digestive tract.

Northern brook lampreys inhabit small, clean, cool freshwater streams and tributary systems within the Great Lakes and St. Lawrence River basin, with additional populations in parts of the upper Mississippi and Hudson Bay drainages. They favor stream reaches with a mix of gravel riffles for spawning and soft sandy or silty margins for larval burrowing, and are sensitive to habitat degradation and pollution. Because they never leave fresh water, northern brook lampreys depend entirely on stable, well-connected local stream systems throughout their life cycle. Larvae spend the great majority of the species' lifespan buried in sediment, with adults emerging only briefly in a localized area to spawn.

Northern brook lamprey larvae, called ammocoetes, live burrowed in fine stream sediment for several years, filter-feeding on algae, detritus, and microorganisms carried in the current. After metamorphosis, adults enter a brief non-feeding stage in which the digestive tract regresses, relying entirely on stored energy reserves. Adults move only short distances to nearby gravel riffles, where they build simple nests by shifting small stones with their sucking mouths, then spawn communally in groups before dying within days to weeks. Because adults never feed, the species has no parasitic relationship with other fish. Its larvae play an important role in stream nutrient cycling and serve as prey for other aquatic species.