How to beat the sea lamprey with its own pheromones, bile salts

Researchers are now closer to using the invasive species's predilection for bile salts against it.

What could be more seductive than bile salts? Well, for a sea lamprey, bile salts are about as good a come-on as come-ons come.

The sea lamprey, an invasive and destructive species in the Great Lakes, has evolved to use bile salts, acids brewed in their livers to assist in digestion, as pheromones. That's right: female sea lampreys are so enthusiastic about digestive aids that they'll squiggle upstream in pursuit of the enticing males producing them.

It’s an unusual preference — so unusual, new research shows, that not even the silver lamprey, a cousin to the sea lamprey but a native to the lakes, shares the sea lamprey's predilection. That’s a find that could help researchers develop traps that use the sea lamprey’s affinity for bile salts against it, while sparing the native lamprey species. 

Understanding how the two species respond to bile salts is “very interesting when developing a pheromone program for sea lamprey control,” said Tyler Buchinger, a graduate student at the Michigan State University and an author on the paper, in an email to the Monitor.

“This research provides necessary data on the effect of sea lamprey pheromones on closely related non-target species,” he says. The findings are published in Proceedings of the Royal Society B.

Sea lampreys belong to an ancient order of eel-like fish, most of which, including the sea lamprey, are parasites on other fish. Sea lampreys’ gory lives begin as larvae hatched from fertilized eggs. Three to six years later, these larvae swim downstream toward the Great Lakes, smooching their mouths on unfortunate fish and sinking their rows of teeth into their flesh (skip the $13 admission price to a horror movie and pull up a photo of a sea lamprey’s jawless, circular mouth instead – you won’t be disappointed). Once attached, the parasites will then suck out the unfortunate fish’s bodily fluids.

If the fish is large, it will survive the encounter, with a bullseye scar to boot. If it is small, it will die, and the sea lamprey will snuggle up to another victim with its toothy kiss. After about a year of feeding, sea lampreys detach themselves to swim upstream, breed, and die.

There are five native lamprey species in the Great Lakes, but sea lampreys are not among them. Instead, these interlopers are believed to have arrived from the Atlantic via the new canals connecting North America’s water bodies in the early 1900s. The first invading sea lamprey was fished from Lake Erie in 1921. Over the next decade, the invader was spotted in each of the other four Great Lakes, ending its conquest with Lake Superior in 1938.

The sea lamprey's arrival was not welcome. Profiting from the invader’s advantage in an ecosystem unprepared to deal with it, the sea lamprey fed with abandon, bleeding out rainbow trout, lake trout, salmon, whitefish, and catfish, among others. As it fed, local fisheries – an industry now valued at about $7 billion – began to buckle.

In 1955, the Great Lakes Fishery Commission was founded as the control center to wage war against the marauder. Since then, the sea lamprey population has been reduced some 90 percent, largely through the use of lampricides. Though effective, lampricides are thought to be harmful to some other Great Lakes populations, including mudpuppies (aquatic salamanders). But the less destructive alternative, barriers placed en route to the sea lamprey’s spawning sites, capture just 40 percent of the migrating sea lampreys. That puts an insignificant dent in the population, since each female fish that gets through can plunk down between 60,000 and 100,000 eggs.

So what to do instead? Well, it has been long suggested that the female sea lamprey’s weakness for bile salts could be manipulated. In 1995, Weiming Li and colleagues published a paper in the Journal of General Physiology that identified bile salts, or 3-Keto Petromyzonol Sulfate (3KPZS), as the sea lamprey’s migratory pheromone. Female sea lampreys, it unfolded in subsequent papers, rely largely on this pheromone produced in the male’s liver to lead them upstream, where they are then incited into breeding.

Since then, Dr. Li, who is a co-author on this latest paper, and a vast army of forces – including Michigan State University, the US Geological Survey, the US Fish and Wildlife Service, the Department of Fisheries and Oceans Canada, and the Great Lakes Fishery Commissions – have published dozens of papers on the subject. Each one draws closer to the ultimate goal of using synthetic pheromones to create attractive traps that female sea lampreys chugging toward the scent can’t wait to swim into.  

“Research on the use of pheromones in sea lamprey traps is ongoing, but recent large scale management tests suggest that pheromone will increase the efficiency of the trapping program,” says Mr. Buchinger.

But there has been an outstanding question: what about the native lamprey populations? How do they feel about bile salts? In the latest research, the team compared the sea lamprey’s enthusiasm for the pheromone to that of silver lampreys, a much more ancient species. Were these lampreys also attracted to bile salts? Or could the pheromone be deployed to uniquely target just one lamprey species?

The team found that both silver and sea lampreys swam toward the bile salts, but just the sea lampreys began breeding upon uniting. This is because male silver lampreys do not release the pheromones in quantities detectable to their female counterparts, Buchinger said. Instead, female silver lampreys swim upstream in response to secretions from larvae. For the native silver lampreys, bile salts are homing signals, not mating signals.

But in sea lampreys, the male population has evolved over time to mimic the secretions of larvae and deploy them a super-powered cologne.

“Spawning male sea lamprey effectively ‘trick’ females by releasing a smell they already like and luring them to the nest to spawn,” says Buchinger.

The researchers propose that the difference in the two species use of bile salts is likely related to the two species’ different mating behaviors, he said. Female silver lampreys spawn as a group with multiple males, and the whole process is fairly un-dramatic. But female sea lampreys spawn with just one male on a nest, provoking serious competition between males for a mate – hence the need for cologne.

“So while a single male sea lamprey needs to invest heavily in luring a female to the nest," says Buchinger, "the task of luring females to the nest for silver lamprey is upon the whole group of males."