Student Life Archives (2001-2008)

For fish: Is bigger really better?

Courtesy of Brian Langerhans

Is being big really worth it?

If you were a male mosquito fish, you might not be so sure, according to the new research of a Washington University PhD candidate. Males with a larger gonopodium, the technical name for the reproductive organ of that species of fish, tend to attract more attention from females. However, these well-endowed fish are also easy prey. The drag from an extra-long gonopodium decreases a fish’s burst speed when escaping from predators.

“The larger the gonopodium the lower the burst speed,” said Brian Langerhans, a PhD candidate in evolutionary ecology at the University. “If you compare a cross population, it can be as much as a 20 percent difference in speed. It’s actually quite an impact, but so far it’s hard to say exactly because there are a lot of other traits acting at the same time,” Langerhans said.

Male mosquito fish are one of the few fish belonging to the family poeciliid, including guppies and other fish that bear live young. Unlike most fish, which spray semen on already laid eggs, poeciliid males physically inseminate the female. “The gonopodium is a modified anal fin that’s basically a sperm transfer organ,” Langerhans said. “After copulation occurs, the females bear live young [that] develop from egg to larva to actual juvenile fish inside her body.”

Langerhans raised and mated fish in captivity first to prove that gonopodium size is indeed a heritable trait. “Basically, I showed that the pattern in the wild is also maintained when they were raised in the lab,” Langerhans said.

Focusing on the evolution of fish body shape, Langerhans compared mosquito fish in predator-free zones in captivity to the survival-of-the-fittest conditions of the wild. “Fish at a site without any type of predatory fish at all- they have a pretty big gonopodium. And the gonopodium is, on average, 15 percent larger than at a site you find with predators,” he explained.

Langerhans developed a mate-choice experiment for the female fish in captivity. Just as in the wild, the females exhibited preference for the male with the larger gonopodium, spending 81 percent more time directly interacting with him than his smaller-sized counterpart. Females also approached the large-gonopodium male 28 percent more times than the small-gonopodium male while flitting around the tank.

Yet predators like the sun fish and large mouth bass in Texas and the barracuda in the Bahamas constantly threaten mosquito fish with excessive baggage. Thus the mosquito fish dilemma; pre-mating sexual selection drives favors larger gonopodia, whereas predation, another aspect of natural selection, favors some kind of reduction.

“Everything I study revolves around similar questions; trying to understand the ecological mechanisms that drive evolutionary changes in body shape. We have to figure out why organisms have the traits they have,” Langerhans said.

Langerhans plans to continue this research by investigating evolutionary trends in related species. He hypothesizes that the female partiality for larger gonopodia might help explain the evolution of the swords-like caudal tail fins in male swordtail fish, a theory he plans to test in future research.

Print This Post Print This Post

No Comments Yet

You can be the first to comment!

Student Life is the independent student newspaper of Washington University in St. Louis. Keep in touch with Washington University by subscribing to an RSS feed of our stories or an RSS feed of our comments. Privacy Policy | Comments Policy | Web Policy