It's a spring day in Interior Alaska and the ice is melting off a stream. In the water, a pair of eel-like fish is spawning. They're arctic lampreys, and they've come more than 2,000 miles up the Yukon River to this tributary where they were born almost a decade earlier to spawn and die. Lampreys are anadromous fish, like salmon, but that's where the similarity ends. Arctic lampreys begin their lives as mud-dwelling filter feeders and end as parasites.
Adult lampreys lack jaws and instead have a disk-like mouth filled with small sharp teeth and a sharp rasping tongue. They attach to other fish and rasp through the scales and skin of the host and feed on its blood and fluids. Some fish die from lamprey attacks, but in many cases, fish survive the experience. The lamprey releases and drops off after the meal.
Juvenile lampreys hatch in freshwater and look very different than adults. The worm-like juveniles, called ammocoetes, are eyeless, blind filter feeders that burrow into the silt and mud of shallow pools and backwaters of rivers and sloughs and filter microorganisms and organic matter from the sediment. Ammocoetes live as filter feeders in fresh water for three to seven years before metamorphosing into adults. They migrate to the sea and spend one or two years in the ocean as parasites before returning to freshwater to spawn and die.