Spying on the sex lives of wild fish.

• Position: Reproduction - Brief Article

New insights into the reproductive behaviors of freshwater fish have been discovered by scientists who utilized genetic tools first developed for use in humans. By employing genetic fingerprinting techniques such as those used to identify criminals, Andrew DeWoody, assistant professor of forestry and natural resources at Purdue University, West Lafayette, Ind., and his colleagues have been able to glean new knowledge about an underwater world of peculiar liaisons.

Most freshwater fish familiar to sport fishermen reproduce by building nests and laying their eggs in the spring. Unlike birds and reptiles, in many freshwater fish species it is the male that builds and tends the nest. "They try to build an attractive nest to entice females to come and spawn with them," DeWoody points out.

Nearly all freshwater fish are external fertilizers, which means that, while the female deposits some or all of her eggs in a nest, the male swims over the nest and fertilizes the eggs. The female leaves--often to find another male to spawn with--while the guardian male waits, fanning his tail to aerate the eggs, protecting the nest from predators, and hoping to entice another female to add more eggs to the nest. There is also a problem with other males. "Occasionally, a second male will join the spawning pair and attempt to `steal' fertilizations from the primary male," DeWoody says.

It's a long, lonely vigil hovering over the nest, and the males can't leave for any reason, even to forage for food. "As soon as they leave the nest, minnows or some other fish would come in and destroy the nest by eating the eggs," he indicates. Instead, the males survive by eating a few of the eggs themselves. DeWoody and his colleagues wondered if the males cannibalize just the eggs fertilized by other males, or if they are indiscriminate? So, the researchers captured male darters and sunfish to extract undigested eggs from their stomachs, then used DNA fingerprinting techniques to compare the DNA of the adults with that of the embryos.

They discovered that the males were unable to distinguish between the eggs that were their kin and those that weren't. "These findings have implications not only for fish ecologists, but also other areas of biology," DeWoody maintains. "The question of how well animals are able to recognize their kin is a question that many biologists are asking."

Cannibalism isn't the most-bizarre part of the spring spawning season. Even for fish, it turns out...