Chestnut Lamprey

The chestnut lamprey (Ichthyomyzon castaneus) is a jawless, parasitic fish native to freshwater rivers, streams, and lakes of central North America, ranging across the Mississippi River basin, Great Lakes drainage, and parts of Hudson Bay tributaries. It belongs to the genus Ichthyomyzon, a group restricted to North America that includes both parasitic and non-parasitic brook lamprey species. Like the silver lamprey, the chestnut lamprey completes its entire life cycle in fresh water without a marine phase. Its common name derives from its characteristic reddish-brown, chestnut-like coloration. As an adult, it attaches to and feeds on a range of host fish species, playing a natural ecological role within its native freshwater range.

Chestnut lampreys are identified through their distinctive coloration and dentition pattern.

Key field marks:

• Reddish-brown to chestnut coloration on the back and sides, giving the species its common name
• Oral disc teeth with multiple sharp cusps (multicuspid), differing from the single-cusped teeth of the related silver lamprey
• Pale cream to yellowish belly
• Two dorsal fins that are closely connected or only shallowly separated
• Seven round external gill openings on each side of the head
• Adults typically 25-35 cm long

The reddish-brown coloring and multi-cusped teeth distinguish the chestnut lamprey from the silvery, single-cusped silver lamprey and from smaller, weak-toothed non-parasitic brook lampreys found in the same region.

Chestnut lampreys inhabit freshwater rivers, streams, and lakes across central North America, with a range centered on the Mississippi River basin and extending into parts of the Great Lakes and Hudson Bay drainages. Adults are typically found in medium to large rivers and lake habitats where host fish are plentiful. For spawning, they move into smaller tributary streams with gravel substrate and moderate current. Larvae burrow into soft sand or silt along low-velocity stream margins, remaining there for an extended period before metamorphosing into free-swimming adults. As with other Ichthyomyzon species, the chestnut lamprey never leaves fresh water throughout its life cycle.

Chestnut lampreys are parasitic as adults, using their toothed sucking disc to attach to a variety of host fish species and feed on blood and body fluids, often over an extended period before releasing the host. They are typically found in open river and lake waters during their feeding phase. As spawning approaches in spring, adults migrate into smaller streams, stop feeding, and construct shallow nests in gravel substrate by clearing stones with their mouths. Spawning occurs communally among multiple individuals, after which all adults die. Larvae live buried in soft sediment for several years, filter-feeding on organic material and contributing to nutrient cycling within stream ecosystems before eventually transforming into the parasitic adult form.