Sex-Shifting Parasitic Fish Chooses Gender Depending On Growth Rate, Study Says

Sea lampreys (Petromyzon marinus) have gained a notorious reputation in the North American Great Lakes. Although considered an endangered species in their native territories – Europe and the Pacific Northwest – their presence inspires terror in U.S. waters, threatening local fish populations and the fishing industry.

These parasitic eel-like fish thrive at the expense of other aquatic creatures, which have come to fear their gaping jawless mouths. Equipped with razor-like teeth, they attack and kill other water inhabitants, latching onto their prey and sucking the blood out.

But that’s not all. Lampreys hide even more secrets in their deadly arsenal of evolutionary traits. According to a new study published on March 29 in the journal Proceedings of the Royal Society B, the parasitic fish can change their gender depending on what their environment has to offer.

Peculiar Sex Determination Tactic

Researchers from the U.S. Geological Survey and Michigan State University set out to investigate how environmental conditions affect growth rates in lampreys. What they discovered instead was utterly unexpected: These “vampire fish” (as they are sometimes called) become male or female as dictated by how much they grow in their larval stage.

This sex determination method is unprecedented in nature, making lampreys the first creatures to undergo sex change based on the resources in their environment. In mammals, sex is established at a chromosomal level, while in reptiles gender assignment is governed by the temperature of the eggs.

The international science journal Nature reports lamprey larvae start out with undifferentiated sexual organs, developing gonads within a year and metamorphosing into adult fish after several more years.

Scientists led by Nick Johnson, a USGS biologist, surveyed the creature’s habitats in the Great Lakes area, including their tributaries and nearby situated reservoirs, and – after making sure there were no wild lampreys in the study sites – released between 1,500 and 3,000 larvae to monitor their developmental process.

More Food Equals More Females

All larvae had been tagged beforehand and so it was easy for the team to recapture them after metamorphosis and observe their sex. The experiment showed the environments with poorer food resources led to slower growth rates in lamprey larvae, which increased their likelihood of becoming male, rather than female.

Such unproductive sites, where prey wasn’t abundant, resulted in a male lamprey population of 78 percent, whereas more plentiful environments only produced 56 percent males.

“We were startled when we discovered that these data may also reveal how sex is determined because mechanisms of sex determination in lamprey are considered a holy grail for researchers,” said Johnson.

He believes the explanation for this species’ gender assignment system lie within larval density, as well as food availability, since a nutrient-rich environment offers better conditions for the fish to produce eggs.

Pest Control Alternative

The newfound data could help establish new ways to keep the lamprey population in check. These invasive and destructive parasitic fish have taken a great toll on the Great Lakes’ ecosystem, decimating native aquatic fauna and disrupting the multibillion-dollar fisheries based in this area.

Previous efforts to curtail the species’ prevalence involved using pheromone bio pesticides to lure females into baiting sites and collect them before they get the chance to breed.

Even though sea lamprey numbers have been reduced by 90 percent, specialists call for even more innovative pest control strategies to protect American lakes from their predatory attacks.

“The results of this study could open paths forward to novel technologies that can disrupt or modify gender in sea lampreys,” said David Ullrich, chairman of the Great Lakes Fishery Commission.

Photo: U.S. Fish and Wildlife Service Midwest Region | Flickr

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