Spying on the sex lives of wild fish - 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 that it's not always a father-knows-best kind of world. There are other males on the prowl, and these males use deceptive tactics to ensure that they, too, have small fry. For instance, just 85% of sunfish studied were nest-builders, which scientists call bourgeois males. The rest used their unusual sexual development to trick the bourgeois males and reproduce.

There are two types of nonbourgeois males--sneakers and satellites. Sneakers are adult males disguised as immature males. Besides their smaller size and immature appearance, the sneaker males have another difference--dissections reveal that their testes are three times as large as those of a bourgeois male. "They're not much more than a swimming sack of sperm," DeWoody explains. As their name implies, sneakers approach a nest guarded by a bourgeois male and, when the moment is right, dart over the nest. "With no cooperation from the female, as the two fish are spawning, this other male zooms in and releases his gametes."

The second type of parasite male is even more deceptive. The satellite males lack the colorful pigmentation of the bourgeois male sunfish and are smaller than a typical male. They look almost exactly like a female sunfish. The satellite sunfish approaches the nest as if it, too, wants to spawn with the bourgeois male, but, instead of releasing any eggs, it releases sperm.

The question for fish ecologists has always been how successful these alternate fertilization strategies are. Not very, the researchers discovered. They examined the genetics of the nests and found that the parasitic male only sired 1.3% of the embryos. Moreover, the genetic data showed that each of the nests studied had been spawned by an average of more than seven females.

Not all freshwater fish are so promiscuous. DeWoody and his colleagues found that the most-popular sport fish in the U.S.--the largemouth bass--is unusual for two reasons. Bass are largely monogamous (at least for one season), and both parents are responsible for guarding the nest. By using the same gene marker techniques employed in the other studies, they determined that just 12% of the largemouth bass nests contained any eggs fertilized by another male. "Cuckoldry by males was rare. The genetic data also revealed that some nests contain juveniles that were not the progeny of the guardian females, a finding that can be thought of as low-level cuckoldry," DeWoody points out.

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