Jawless Lamprey Fish Attract Females by Releasing Smelly Aphrodisiac Pheromone in Semen

The jawless sea lamprey fish attracts mates by releasing a smelly aphrodisiac chemical in its semen, according to scientists. The semen, or milt, of the sea lamprey fish is a rich source of pheromones, or chemical cues, which are thought to help it engage in special "lek" mating, study co-author Weiming Li, Professor in Environmental Physiology at Michigan State University, told Newsweek.

Lek mating involves two or more males performing courtship displays, and is seen in animals including birds, insects, and mammals like fallow deers. Females watch and pick the male they find most attractive for a short-term hook-up.

To mate, male lampreys gather on gravel patches in rivers and build nests which they defend as they release sex pheromones from their gills. Ovulating females meanwhile move between nests to mate.

The authors of the study published in the journal PLOS Biology used special equipment to study sea lamprey milt and fluid and eggs from ovulatory females. They also tested water containing females and males.

The team identified spermine—a compound first discovered in human semen—as a pheremone in the milt. At relatively low concentrations, the smell of the chemical can attract ready-to-mate females, but not males or non-ovulating females, according to the research.

Lampreys appear to have a receptor activated by spermine in the tissue they use to smell, the team said.

"Where examined, spermine is found in semen of all animals," explained Li. "Several species including humans are thought to detect spermine, although the potential effects of olfactory detection of spermine in other animals have not been examined." He said he hopes the study will spur further research on the pheromones present in the semen of different animals.

Lamprey fish,
A stock image of a lamprey fish, which use external fertilization. Getty

Li told Newsweek he was surprised to find a compound in human semen to be a pheromone released through lamprey semen.

"We were also surprised by the olfactory potency of spermine," he told Newsweek. Referring to the molar unit system used to describe concentrations in chemistry, Li explained: "At 10-14 Molar, it evokes olfactory as well as behavioral responses! At this concentration, it is equivalent of diluting 1 gram of spermine into 200,000 Olympic sized swimming pools (average volume 2,500,000 liter).

"To maintain spermine at 10-14 Molar, we would only need to meter 1 gram of spermine into the river over roughly 88 days to induce behavior responses from the female sea lamprey."

Next, research should investigate the hypothesis that spermine is a pheromone in areas where lampreys spawn, said Li. The work could help with efforts to protect the fish whose populations have declined steeply, as well as population control where needed, said Li.

In 2016, the U.S. Environmental Protection Agency announced plans to release a different pheromone in the Great Lakes region to attract and kill unwanted lampreys.

"We hope our discovery may help with conservation of sea lamprey resources along the Atlantic coast of Europe," he told Newsweek.

"Sea lamprey is an invasive species in the Great Lakes that severely impacts fisheries industry and ecosystem health. We believe spermine and its receptor antagonist may be useful for sea lamprey control."

sea lamprey, fish,
A stock image showing a close up of the mouth of a sea lamprey. Scientists have studied the species' semen.