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Bloodsucking Fish Follows Chemical Trail to Breeding Grounds


With its jawless toothy mouth adapted to suck the blood of fish, the sea lamprey is a ferocious parasite.

Like many of its prey, lampreys spend their early lives in rivers, where they are more protected than in the sea.

Scientists assumed that lampreys behave like other fish that return for breeding to the river where they were spawned. But this is not the case, new genetic research suggests.

Salmon and other fish that swim into rivers from the sea for breeding follow a regular circuit. They migrate from their natal river to marine food resources, then back to their nursery river for breeding, which they identify from the unique chemical signature of the river imprinted on them. Returning to the natal river increases the likelihood of choosing a habitat of established spawning success.

Not all rivers offer the right conditions for lampreys. They are reared in sandy, murky backwaters, burrowing into silt for years before becoming adults and migrating to sea.

In the ocean for as long as four years, lampreys suck the blood and tissue of salmon, bluefish, mackerel, tuna, swordfish, haddock, and even whales and basking sharks. Using rows of horned teeth to bore through scales or skin, they fasten themselves to their prey for as much as two weeks at a time.


Sea lampreys cling to fish by suction. The mouth of the fish contains numerous teeth (top photo).

Photos on this entry courtesy U.S. Fish and Wildlife Service 

Lampreys are transported wherever their hosts take them. They are widely dispersed across the ocean and at depths of as much as 13,000 feet (4,000 meters). Finding the original nursery river when it is time for them to breed is a big problem.

Because they expend considerable energy reserves before and during spawning and then die shortly thereafter, it is critical that lampreys choose a river suitable for breeding. If they don’t return to where they were spawned how do they know how to pick a river that has the right conditions for rearing their offspring?

Genetic analysis results released today indicate that the lamprey’s breeding strategy is to find “any suitable river,” according to scientists who conducted the research.

Chemicals released by lamprey larvae already in the rivers are detectable by adults in the sea, the researchers suggest. “Adult males that ascend to the … spawning grounds then use powerful pheromones to communicate their presence to females downstream,” researchers John Waldman, Cheryl Grunwald, and Isaac Wirgin, wrote in the journal Biology Letters. John Waldman is a biology professor at City University of New York. Cheryl Grunwald and Isaac Wirgin are with the Department of Environmental Medicine, New York University School of Medicine.

The discovery could be a way to control the parasite, the researchers suggest.

Lamprey infestation in North America’s Great lakes, Lake Ontario, and Lake Champlain has caused substantial economic damage to fisheries. If lamprey pheromones could be laced in rivers unsuitable for spawning and nursery purposes then breeding populations might be lured to where they would have little prospect of establishing a new population, the scientists suggest.

The research was based on the lamprey species Petromyzon marinus collected from 11 North American east coast rivers to test for genetic evidence of homing. The scientists think that other species of sea lamprey in other parts of the world have the same relationship with rivers.

Related National Geographic News story:

Bloodsucking Lamprey Found to Be “Living Fossil”