River Lamprey

The river lamprey (Lampetra fluviatilis), also known as the lampern, is a small anadromous jawless fish native to coastal waters and rivers of Europe, from the British Isles and Scandinavia south to the Mediterranean and Black Sea drainages. It is a parasitic species, though its feeding period at sea is shorter and its diet more restricted than that of the larger sea lamprey. River lampreys typically feed in coastal and estuarine waters rather than venturing far into the open ocean, attaching to smaller host fish such as herring and sprat. After a relatively brief marine feeding phase, adults migrate into rivers to spawn, after which they die. The species has declined across parts of its range due to river barriers and habitat degradation, though it remains more widespread than several related lamprey species.

River lampreys are mid-sized parasitic lampreys, larger than non-parasitic brook lampreys but smaller than the sea lamprey.

Key field marks:

• Circular sucker mouth with well-developed, sharp teeth adapted for parasitic feeding
• Bluish-grey to brownish back with a metallic or bronze sheen, transitioning to a silvery-white belly
• Two dorsal fins set close together near the tail, becoming more separated in breeding adults
• Seven round gill openings on each side of the head
• Adults typically 25-40 cm, distinctly smaller than the sea lamprey (60-90 cm) but larger than most brook lampreys

River lampreys are best separated from the sea lamprey by smaller size and more uniform coloration lacking heavy dark blotching, and from non-parasitic brook lampreys by their larger size and sharper, more developed dentition.

River lampreys are anadromous, spending their parasitic feeding phase in coastal seas and estuaries around the British Isles and continental Europe rather than the open ocean favored by sea lampreys. They inhabit shallow coastal and brackish waters where small schooling fish are abundant, before migrating into connected river systems, typically in autumn and winter, to reach spawning grounds. Spawning takes place over gravel substrate in flowing river and stream reaches with clean, well-oxygenated water. After hatching, larvae burrow into soft silty or sandy margins of the river for several years before metamorphosing and migrating back downstream toward coastal and brackish feeding grounds.

As parasitic feeders, river lampreys attach to smaller fish such as herring, sprat, and flounder using their sucker mouth, rasping the skin with a toothed tongue to feed on blood and body fluids over a period of weeks before releasing the host. Feeding activity is concentrated in coastal and estuarine waters over a relatively short marine phase compared to sea lampreys. As the spawning season approaches, adults cease feeding, undergo physical changes including fin separation, and migrate into freshwater rivers, often traveling at night. Spawning occurs communally over gravel beds, with adults using their sucking mouths to clear nest sites; all adults die shortly after spawning. Larvae remain buried in river sediment as filter feeders for several years, forming an important part of stream food webs before their own metamorphosis and seaward migration.