Brook Lamprey

The brook lamprey (Lampetra richardsoni), also called the western brook lamprey, is a small, non-parasitic jawless fish native to coastal streams of the Pacific Northwest of North America, from northern California to British Columbia. It belongs to the ancient lamprey lineage (Petromyzontiformes) and is one of many "brook lamprey" species worldwide characterized by a life history in which adults never feed. Unlike its larger, parasitic relatives such as the Pacific lamprey, the brook lamprey completes its entire life cycle in fresh water, spending several years as a burrowing, filter-feeding larva before a brief non-feeding adult stage devoted solely to reproduction. Because larvae strongly resemble those of parasitic lamprey species, brook lampreys are most reliably identified as spawning adults.

Brook lampreys are small, slender lampreys best distinguished from parasitic species by their reduced adult dentition and small size.

Key field marks:

• Small, weakly developed teeth on the oral disc, blunt compared to the sharp, well-developed teeth of parasitic lamprey species
• Circular jawless sucker mouth, smaller in diameter relative to body than in parasitic species
• Dark grey-brown to olive back, fading to a pale cream or yellowish belly
• Two closely spaced, low dorsal fins near the tail
• Seven round external gill openings on each side of the head
• Adults typically 10-16 cm, notably smaller than parasitic relatives such as the Pacific lamprey

Adult brook lampreys can be told apart from parasitic species mainly by their small size, weak dentition, and a digestive tract that degenerates after metamorphosis since they no longer feed.

Brook lampreys inhabit small, clear, cool coastal streams and tributaries throughout the Pacific Northwest, favoring reaches with a mix of gravel riffles for spawning and soft sandy or silty margins for larval burrowing. They generally avoid larger rivers and lakes, preferring headwater and low-order stream habitats with stable flow and good water quality. Larvae spend the majority of the lamprey's multi-year lifespan buried in fine sediment along stream margins, only emerging briefly after metamorphosis to spawn as adults. Because they never migrate to the ocean, brook lampreys are entirely dependent on the health of local freshwater stream systems throughout their life cycle.

Brook lamprey larvae, called ammocoetes, live burrowed in soft sediment for several years, filter-feeding on algae, detritus, and microorganisms drawn in with water currents. After a lengthy larval period, they undergo metamorphosis into a short-lived, non-feeding adult form whose digestive system regresses, meaning all of an adult's energy reserves must sustain it through spawning and death. Adults migrate short distances to gravel riffles, where males and females build simple nests by moving stones with their sucker mouths, then spawn communally, releasing and fertilizing eggs before dying shortly afterward. Because they do not feed as adults, brook lampreys have no parasitic interaction with other fish, and their primary ecological role comes from their larvae, which contribute to nutrient cycling and serve as a food source for other stream animals.