Male Bahamas mosquitofish (Gambusia hubbsi) that live in predator-heavy waters have larger genitalia and more aggressive mating techniques than their peers that live in safer "neighborhoods."

The researchers found the fish that lived in regions where they faced a pressing predatory threat had longer gonopodium tips, which transfer sperm, than members of the same species living in areas with less of a threat, a North Carolina State University press release reported.

These gonopodium tips are usually only one millimeter long, so the difference is miniscule, but researchers were able to identify a pattern of size linked to where the fish lived.

The team believes the fish living in regions with more predators developed better ways to impregnate females under the dangerous conditions.

"When predators are around, G. hubbsi males spend a lot of time attempting to mate with females because of the high mortality rate," NC State Ph.D. student Justa Heinen-Kay, a leader of the study, said. "We hypothesize that G. hubbsi have evolved these bonier and more elongated gonopodium tips as a way to copulate even when females don't cooperate."

"Essentially, males need to transfer as much sperm as possible as quickly as possible, and this shape difference could help facilitate that," assistant professor of biological sciences R. Brian Langerhans, who also led the study, said.

The researchers observed fish in "blue holes" in the Bahamas. These watery "test tubes" are underground caves that have been filled with water for thousands of years. Langerhans called them "aquatic islands in a sea of land."

The "blue holes" were perfect for the study because some contain predators while others don't, so researchers were able to mosquitofish in a variety of environments.

"Comparing Gambusia across blue holes reveals that predation is associated with evolutionary changes in male genital shape," Langerhans said. "It's a beautiful and elegant system to study the causes and predictability of evolutionary changes."