Far Eastern Brook Lamprey

The Far Eastern Brook Lamprey is a small, jawless fish belonging to the ancient lamprey lineage (Petromyzontiformes), distinct from true bony fishes. It is classified in the genus Lethenteron and is widespread across freshwater streams of Japan, the Korean Peninsula, northeastern China, and the Russian Far East. Unlike its parasitic relatives, this species is non-parasitic: adults do not feed on other fish and instead rely entirely on energy reserves built up during a multi-year larval stage. Populations are generally considered stable across their range, though local stream degradation and damming can fragment habitat. The species is of interest to biologists as a living example of one of the most primitive vertebrate body plans still found in modern freshwater ecosystems.

Field marks:

• Elongated, eel-like body lacking paired (pectoral/pelvic) fins
• Jawless, round sucker-disc mouth used for attaching to substrate, not prey
• Two low dorsal fins that may appear continuous, and a small caudal fin
• Seven round gill openings visible in a row behind the head on each side
• Uniform brownish-olive to gray coloration, paler ventrally
• Adults measure only 10-16 cm, noticeably smaller than parasitic lamprey species

It is distinguished from parasitic lampreys by its smaller adult size, reduced or non-functional teeth, and short, non-feeding adult lifespan. Larvae (ammocoetes) are blind, toothless, and burrow in soft sediment, making direct identification difficult until metamorphosis.

Far Eastern Brook Lampreys inhabit small, cool, well-oxygenated freshwater streams and headwaters with sand or fine gravel substrate, where larvae burrow into soft sediment for several years. The species ranges across Japan, the Korean Peninsula, northeastern China, and parts of the Russian Far East, typically in temperate lowland to montane drainages. Adults remain in freshwater their entire life cycle, never entering brackish or marine water. Clean, silt-free stream beds with stable flow are essential for larval survival, and the species is sensitive to channelization, pollution, and barriers that disrupt stream connectivity.

This lamprey spends the majority of its multi-year life cycle as a burrowing, filter-feeding larva buried in soft stream sediment, feeding on detritus and microorganisms. After metamorphosis, adults emerge with functional eyes but do not feed, living only long enough to spawn. Spawning occurs in shallow gravel riffles, where males and females construct simple nest depressions and release eggs and sperm before dying shortly after, a semelparous reproductive strategy typical of lampreys. As larvae, they play a role in stream nutrient cycling by processing organic matter; as short-lived adults they contribute biomass to the ecosystem after death.