Sea lampreys' sexy secret? Bile salt, baby

The sea lamprey has rows of teeth similar to a shark.
A sea lamprey's sharp teeth — which grow in rows, similar to a shark — are used to rasp into the skin of fish to parasitize them.

Nothing says "I'm single and ready to mingle" like a dose of salty digestive fluid. For sea lampreys, anyway.

Male sea lampreys use a type of bile salt, which is more typically known for digesting fats, to advertise their genetic fitness to females and readiness to mate, new research shows. This stands in contrast to silver lampreys, which use the enzyme for digestion and other nonsexual functions, said Weiming Li, a researcher at Michigan State University.

The research could be used to help find a way to thwart the mating habits — and thus reduce the population — of sea lampreys, an invasive species that has taken a major toll on a variety of fish species in the Great Lakes, Li told LiveScience. [Photos: The Freakiest-Looking Fish]

Larvae of silver lampreys, which are native to the Great Lakes region and don't pose a major threat to local fish, excrete the bile salt in high enough concentrations for migratory females to sense it, Li said. This serves as a signal to females that an area is a good place to lay eggs, Li said. But the chemical signal remains nonsexual, he added.

A group of sea lamprey, an invasive species in the Great Lakes.
A group of sea lamprey, an invasive species in the Great Lakes.

Not so for sea lampreys, which evolved from silver lampreys. The research of Li and his colleagues, the latest chapter of which was published this month in the journal Proceedings of the Royal Society B, shows that what was originally a nonsexual signal (in silver lampreys, which are ancestral to their invasive cousins) became sexual.

A very rough parallel to this phenomenon can be seen with cologne and perfume use in humans. The scents were once used to cover up body odor, but once daily showers and the like became common after the advent of the industrial revolution, people began to use cologne to advertise their sexiness, Li said.

Toward the end of the sea lampreys' 1.5- to 2.5-year life, the animals migrate from lakes or the ocean to streams to mate. During this time they do not eat and become starved for energy. But male sea lampreys excrete large quantities of the bile salt, the chemical name of which is 3KPZS, to demonstrate how fit they are.

"It takes a lot of energy — and it's been starved for months and migrated for a long time over a long distance," Li said. "Every drop of energy is important to them. They go this far to advertise their sexuality, the whole animal turns into a machine."

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