Western Brook Lamprey

The western brook lamprey (Lampetra richardsoni) is a small, non-parasitic jawless fish native to coastal streams and rivers of the Pacific Northwest, from British Columbia to northern California. It belongs to the ancient lamprey order Petromyzontiformes and, unlike its parasitic relatives, never feeds as an adult. Instead, the species spends the vast majority of its multi-year life cycle as a soft-bodied, burrowing larva, called an ammocoete, living in silty streambed sediment. After metamorphosing into an adult, it swims upstream to spawn and dies within weeks without ever feeding again. Because it remains entirely within freshwater and requires clean, silt-bottomed stream habitat, the species is considered a useful indicator of stream health.

• Small, slender, scaleless body, olive to brownish-grey above, pale cream below
• Circular sucking-disc mouth with weak, blunt teeth, unlike the sharp teeth of parasitic lampreys
• Seven round gill openings behind the head
• Low, often continuous dorsal fin situated toward the rear of the body
• Adult body length rarely exceeds about 18 cm, smaller than most parasitic lamprey species

The western brook lamprey is best told apart from parasitic Pacific lamprey (Entosphenus tridentatus) by its much smaller adult size, weaker dentition, and lack of large piercing teeth, since it never needs to attach to prey.

Western brook lampreys live entirely in freshwater, occupying small, clear, cool coastal streams and tributaries from southern British Columbia through Washington, Oregon, and into northern California. Larvae burrow into soft sand, silt, or fine organic sediment in low-gradient sections of streams with slow to moderate flow, where they filter-feed for three to six years. Adults briefly move into gravelly riffle habitat with clean flowing water to spawn. The species is sensitive to stream degradation, siltation from erosion, and barriers to habitat connectivity, and its presence is often used by biologists as a sign of good water quality in small forested watersheds.

For most of their lives, western brook lamprey larvae remain buried in soft stream sediment, filtering algae, diatoms, and organic detritus from the water and surrounding substrate, rarely if ever emerging into open water. After several years, they undergo metamorphosis into a short-lived, non-feeding adult form with functional eyes and a sucking mouth disc used only for gripping stones and mates, not for feeding on prey. Adults migrate a short distance upstream to gravel riffles, where males and females together construct a shallow nest depression, spawn, and die shortly afterward. As larvae, the species plays a role in processing organic material within stream ecosystems and serves as a food source for other stream predators.