Endangered fish might face a new threat — sexual harassment.

Male Trinidadian guppies (Poecilia reticulata) are jeopardizing the survival of the critically endangered Mexican fish Skiffia bilineata by constantly sexually harassing the endangered females, research now reveals.

The guppies, which invaded Mexican rivers and lakes roughly 50 years ago, physically resemble S. bilineata, but are genetically distant from them. Scientists knew that male Trinidadian guppies sexually assault females of their own species, and were concerned over whether they harassed the endangered fish as well.

The researchers found that when put in aquaria together, male Trinidadian guppies persistently tried having sex with the female S. bilineata, even when seven of their own females were present. The male guppies are drawn to large females, and endangered females are larger than guppy females, the scientists noted.

Normally S. bilineata have sex by simply uniting their genital pores in a copulatory embrace. Male Trinidadian guppies, on the other hand, have sex using a hooked organ — their "gonopodium." As such, they can damage the endangered females, and female S. bilineata have to spend a lot of energy dodging the guppies.

"Sometimes we observed harassed females jerking away from guppy males when the males appeared to have inserted their gonopodium," said researcher Alejandra Valero, a behavioral ecologist at the National Autonomous University of Mexico in Mexico City. "We are assessing several possible ways in which female Skiffia can be damaged by guppy sexual activity."

The guppies could be contributing to the decline of the Mexican fish. "Half of the species in the Goodeinae tribe to which Skiffia bilineata belongs are already at risk of extinction," Valero said.

However, Valero stressed the guppies were not intentionally out to harm the endangered females or drive S. bilineata to extinction.

"It may seem like their sexual behavior is part of a strategy to displace native species in order to gain exclusive ownership of the habitat," she told LiveScience. However, the guppies "are doing only what they have been doing for the last million years, which is being highly efficient at passing their genes down to successive generations — only in this case, the genes end up in the wrong females!"

Valero and her colleagues Constantino Mac?as Garcia and Anne Magurran detailed their findings online Jan. 23 in the journal Biology Letters.