How human behavior impacts these sex-changing fish
When a male pearly razorfish is snatched from the sea, a female is forced to transition their sex too early. Scientists say this may have a wide-ranging impact on the fish’s DNA.
Every pearly razorfish is born female and guards her own little territory on the sea floor of the Atlantic Ocean or the Mediterranean Sea, where she hunts for shrimp and hides in the sand at night to sleep. Yet if she survives long enough, she will eventually transition to become a male that defends the territory of the multiple females he now mates with.
This arrangement offers the females protection, structure, and stability, and reduces the amount of infighting in the group since the male regularly reminds everyone who’s in charge, preventing the females from transitioning sex too soon.
But pearly razorfish have a big problem: “When the open season starts, we have this mad rush,” says Margarida Barcelo-Serra of the Mediterranean Institute of Advanced Studies on the island of Mallorca, Spain. “Families are going out to fish for them, and everyone wants to eat them.”
Around the Balearic Islands, for example, these fish are a very popular target. Fishing for them during the summer mating season is no longer allowed there.
But from September 1 onward, they’re fair game, and prices can spike as high as lobster. In some areas, up to 60 percent of the pearly razorfish are caught in just a couple of days. These fish are especially likely to approach anything that moves, so as soon as a baited hook appears, the dominant male—the largest fish in the neighborhood—often immediately grabs it and ends up in a bucket on deck instead of defending his group of female fish. This means young females often transition early, when they are still relatively small.
Bad news for young fish
A study led by Barcelo-Serra published in the journal Philosophical Transactions of the Royal Society B this week found that in protected areas without any fishing at all, pearly razorfish are, on average, 6.85 inches long and about 4 years old. However, in areas with heavy fishing, the average length was about an inch less, and the average age only 2 years old.
This is bad news for individual fish, says Barcelo-Serra, since larger females produce many more eggs than smaller ones. “Every extra year as a larger female means many more offspring for an individual female, so transitioning early is not good for them.”
By looking at DNA collected from the fins of 120 individuals, the team found intense fishing may have other impacts too. Kees van Oers, of the Netherlands Institute of Ecology, and his team did a DNA analysis that compared fish from three marine reserves where no fishing is allowed with fish from three fished areas. “We found almost 300 differences in chemical marks on the DNA that are unlikely to be a consequence of the difference in average age,” he says. “And several of these may affect gene activity.”
Pearly razorfish in fished areas and reserves had different marks on genes known to influence things like energy use, the excitability of nerve cells, and the regulation of the immune system in other fish, and it’s conceivable these changes are caused by the stress that arises when males disappear. “But more work is needed to find out what the genes do in this species,” says van Oers.
The DNA differences between the populations are small and affect only a tiny fraction of the fish's DNA, says Soojin Yi of the University of California, Santa Barbara, who has studied similar phenomena in other species, but was not involved in this research. “I think the results are interesting, but the evidence is not yet strong enough to draw firm conclusions,” she says.
For now, the most important message may be that fishing can have a significant effect not only on population numbers, but also the lives and bodies of the survivors. “This is an exciting finding,” says Malin Pinsky of the University of California Santa Cruz, who studies genetic changes in fish population, but was not involved in this study. “I am not surprised that large differences in the environment, such as from fishing, would affect the DNA in this way.” But he adds that “the study opens important new areas of research to find out why fishing has this effect.”
Hope for the fish
The new study suggests that the model of the Balearic reserves, where no fishing is allowed year-round, is working. “The population in these reserves is more diverse, but genetically very similar to that in the fished areas,” says Barcelo-Serra. This is due to how razorfish reproduce: Males and females release sperm and eggs at the same time, eggs are fertilized in the water, and then they drift around for weeks before the tiny fish settle down somewhere. This means that a healthy population in a reserve can replenish other areas too.
The existence of these populations that aren’t fished at all—so that females can grow larger before transitioning and large males can compete for dominance—may solve another problem too. In fished areas, the genes that may be most important for fulfilling the crucial male role are no longer regularly “tested” by natural selection, because most males die young. This means they may become less useful over time, as the most vigorous males continue to be caught.
In the reserves, however, males are almost never caught and can express the full range of their natural behavior, allowing them to test each other’s strength, and the fittest ones produce the most offspring. When these males fertilize the eggs of the females they've carefully guarded, the eggs will drift on the sea currents for about a month, and many of the larvae that emerge will settle in fished areas too.
“I love these little fish,” says Enric Sala, the director of National Geographic's Pristine Seas project, who was born nearby in Girona on the Spanish coast. “This study reveals that marine protected areas do more than just allow the fish to grow larger and reproduce more. They are genetic reservoirs that protect the entire molecular toolkit that the fish need to adapt to survive.”
Still, recreational fishers in the Balearic Islands notice they aren’t catching quite as many razorfish as they used to, says Barcelo-Serra, and the fish they do catch are smaller than those in past decades. “People are very concerned about the conservation of this species,” she says. “They want to keep fishing.” For the pearly razorfish, that may be a mixed blessing.
