A champion sex molecule has turned up in an analysis of sea lampreys, and it may inspire new ways to defend trout and other Great Lakes fish against the invading bloodsuckers.
Long and skinny, the adolescent sea lamprey goes through a parasitic phase in which it clamps its toothy mouth onto another fish and feeds on the host’s blood for weeks. The sea lamprey is native to the Atlantic coast but wriggled westward in the past 200 years as shipping channels opened. Sea lampreys “have been the most devastating invasive species ever to move into the Great Lakes,” says Weiming Li of Michigan State University in East Lansing.
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Male lampreys release a sex pheromone, which Li and his colleagues identify in the April 5 Science. They report that the compound can catch the attention of females at least 65 meters downstream. Insects often trail plumes of their sex lures, but this is the first evidence for such a far-reaching chemical come-on in vertebrates, Li says.
Fisheries managers might use such a powerful attractant to lure invasive lampreys into traps or develop ways to foil their courtship, Li speculates.
“It’s a very exciting possibility,” comments Marc Gaden of the Great Lakes Fisheries Commission in Ann Arbor, Mich.
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In the 1940s and 1950s, the invading lamprey population boomed in the Great Lakes and fisheries crashed. Now, lamprey-killing chemicals, barriers, and operations to sterilize males keep lampreys down to about 10 percent of their nightmare peak, says Gaden. Control costs the United States and Canada together some $15 million dollars a year.
To develop new lamprey controls, Li’s team targets spawning. Male lampreys swim up streams and scrape depressions in streambed gravel that serve as nests. Earlier research suggested that males advertise these boudoirs to females by emitting pheromones.
Li and his colleagues worked out a pheromone-identifying strategy that they say might apply to many creatures. The scientists put in a bucket of water males almost old enough to produce sperm and put fully mature males in another. Then, in a series of tests, the researchers treated one side of a chamber with water from either the mature or not-quite-mature males. Ovulating female lampreys preferred the side washed with water from mature males but not the one treated with water from younger males.
The researchers then compared the compounds in the water from the two groups of males. Using analytical chemistry techniques, the researchers noticed a compound that mature males produced in abundance but younger males didn’t produce. The researchers then worked out the chemical formula of this substance. Only one small group of atoms distinguishes it from a bile compound of larval lampreys.
The new pheromone is interesting in its own right, regardless of its potential for controlling sea lampreys, says Li.
The two other known classes of fish sex pheromones are steroids and prostaglandins, so the lamprey’s is the first that’s a bile acid. The compound seems to be made specifically as a pheromone because adult fish don’t eat, and so they don’t need a digestive bile acid.
“Finding a bile acid was a big surprise,” says Li.