The toothsome animal pictured below is a sea lamprey, a jawless, limbless fish native to the Atlantic Ocean that invaded the Great Lakes and caused the collapse of its fishery in the 20^th century. Scientists have been studying invasive sea lampreys out of USGS’s Hammond Bay Biological Station for decades. There, perched on the shores of Lake Huron in northern Michigan, they have developed the science behind one of the most successful invasive species control programs in the world, with sea lamprey populations today holding steady at only 10% of what they were at their peak. But that program is labor- and resource-intensive, so managers are eager for new approaches to controlling sea lamprey in the future.

That’s why USGS scientists and partners are studying the sex lives of sea lampreys. Understanding sea lamprey life cycles and reproduction can help scientists identify innovative strategies to disrupt that cycle and control their populations, protecting native ecosystems in the Great Lakes.
 

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The Facts of [Sea Lamprey] Life

Like salmon, sea lampreys start their lives as tiny larvae in freshwater streams. At this stage, they don’t have those iconic teeth. They are filter feeders, trapping tiny particles that drift by as they burrow in the mud. They stay in the streams where they were born for about 4-5 years.

Then they undergo a dramatic change, transforming into parasitic juveniles with suction-cup like mouths filled with tiny, sharp teeth (strictly speaking, they are more like tiny horns than teeth, since they are made of keratin, like our hair and fingernails). Scientists aren’t sure what triggers the transformation, but once it is complete the now-parasitic sea lampreys head off downstream to open water, where they attach themselves to fish and feed on their blood. In their native range, that’s the vast Atlantic, full of big populations of big fish that can survive or evade sea lamprey attacks. In the Great Lakes, most fish attacked by sea lampreys die. The juveniles stay in their parasitic phase for about 18 months, consuming up to 40 pounds of fish apiece.

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Once they are fully grown, they detach from their hosts and migrate upstream to spawn. They can travel hundreds of miles, swimming mostly at night. As they migrate, their intestine and liver shut down their digestive functions—the sea lampreys will never eat again.

How do they find their way? The migrating sea lampreys follow pheromones: chemical cues produced by larval lampreys. Since there are already young lampreys there, it’s a signal that there must be good spawning habitat if you follow the pheromones.

Good spawning areas have gravel bottoms and swift currents. The males usually arrive first and begin building the nest, using their suction-cup-like mouths, previously used to parasitize fish, to move rocks, sometimes ones as big or bigger than their heads. They also use their tails to move finer sediments, shaping the gravel into a U-shaped nest.

Once the male has begun to build a nest, he emits a sex pheromone of his own. Female lampreys can smell it for miles, and they follow it upstream to find a mate.

When the females arrive, things start to heat up. Males develop a ridge of tissue that is filled with energy producing cells that heat up when females are around. Both sexes have heat-sensitive cells in their mouths, which they use to sense their prospective mate in the nest.

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Sea lampreys are mostly nocturnal, and most spawning activity happens at night. In other words, they do it in the dark.

Females then use their mouths to attach to a rock or log or other object, and the male attaches to the back of the female’s head. They coil tails. Together their movements create a cloud of eggs, sperm, and sediment. The eggs develop sticky bottoms so they can attach to rocks in a position that helps sperm fertilize them.

After multiple bouts of spawning in pairs or groups, the adult sea lampreys die. But they make it worthwhile: each female produces 60,000-100,000 eggs, leading to an estimated 6,000 or more offspring. Remember that during its parasitic juvenile phase, each of these offspring can consume up to 40 pounds of fish—it’s easy to see how quickly they became invasive and drove the collapse of native fish populations.

Sex Signals: the Future of Sea Lamprey Control?

The details of the sea lamprey life cycle may sound dramatic and goofy, but understanding sea lamprey sex lives has led researchers to develop and test new strategies for sea lamprey control. Take those pheromones we discussed above--the chemical cues that attract adults to spawning streams, and the ones that attract females to males. Pheromones are a great target for intervention: they are typically species-specific, so you are unlikely to affect native species, and if you understand how they work, you can potentially stop or slow sea lamprey reproduction, like a form of sea lamprey birth control.

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USGS scientists and partners first tested whether synthesized versions of these pheromones could be used to attract sea lampreys into traps, acting like a bait. One of these pheromones was registered as the first ever vertebrate pheromone biopesticide in December 2015. More recently, they’ve explored whether sea lamprey pheromone biology could be used to directly disrupt the spawning process.

In a study published in September 2023, scientists from USGS, Michigan State University, and the Great Lakes Fishery Commission discovered that the sex pheromone released by males is made up of multiple chemical components that must be present in the correct combinations for maximal attraction. In a series of experiments, they found that if you change the ratios of these components or introduce a similar (but not identical) chemical, the whole process of attraction is disrupted. Females are no longer attracted to the signals the males give off. These altered pheromone cocktails are called pheromone “antagonists.”

In one part of the study, scientists introduced a pheromone antagonist to spawning grounds. Since most spawning happens at night, so the researchers conducted their trials of the pheromone antagonists after nightfall. They attached LED lights to female sea lampreys so they could follow their movements in the dark and see whether they swam upstream, found nests, and spawned. They found that pheromone antagonist and reduced spawning by 97 percent!

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If an intervention like this were used, that means the females may not find their way to males and nests, and no tail-hooking sea lamprey frenzy will occur. That’s bad news if you’re an invasive sea lamprey, but good news if you’re a native whitefish that will face a lower risk of having its blood sucked by a juvenile parasitic sea lamprey, and good news for Great Lakes fisheries.

Pheromone antagonists are just one tool that USGS scientists are exploring with the Great Lakes Fishery Commission, Michigan State University, and other partners. Other current research projects are looking at how migrating sea lampreys respond to light, or whether a “fish sorter” that distinguishes sea lampreys from native fish using artificial intelligence can allow native fish to pass over a barrier while sea lampreys are blocked and trapped. Recently, scientists began a project to track the entire life cycle of sea lampreys in the lab, beginning to end. Sea lampreys live for several years, so this could take a while—but who knows what secrets of sea lamprey life it will reveal? If there’s one lesson to take away from the sex lives of sea lampreys, it’s that studying an animal’s basic biology can lead to practical tools that you might never have considered.