A tiny fish is more genetically diverse than we thought. Here’s why that matters.
On land, rivers and mountain ranges can divide species into genetically distinct populations. In the vast expanse of the ocean, where there is seemingly little to stop fish and other sea creatures from going where they please, scientists have long expected marine species to find it easier to mix. But ongoing research shows there’s more than just geographic barriers keeping populations separate, and marine species often have a higher genetic diversity than anticipated.
Hannes Baumann, a marine scientist at the University of Connecticut, says that for years the prevailing notion was that species in the ocean didn’t form separate populations. “But the last 20 years has demolished that concept,” he says. “Now everywhere we look we see differentiation.”
Protecting that genetic diversity is a focus of conservationists. At a recent meeting of the United Nations Convention on Biological Diversity (CBD), the agency’s members adopted a new framework setting overarching goals for conservation efforts, including preserving genetic diversity within species to safeguard their ability to adapt to changing conditions.
“Genetic diversity is especially important for resilience,” says Sebastian Nicholls, from the Pew Charitable Trusts’ ocean conservation program, which works closely with CBD member states to help them meet their commitments on marine conservation issues. “If there is too little diversity, a species may be susceptible to a single pathogen or environmental stressor.”
A strong example of the value of that diversity comes from the recent discovery by Baumann and his colleagues that the northern sand lance, an important forage fish, is actually two populations.
By sequencing the genomes of hundreds of northern sand lance living from Greenland to New Jersey, the scientists found that the fish population is split in two—one group dwells north of the Scotian Shelf, off the east coast of Canada, and one lives farther south.
There is something curious about the Scotian Shelf, says Baumann. No obvious barrier prevents fish from crossing the divide and mixing with their neighbors, but it seems that their offspring do not survive when they do. Baumann suspects a change in water temperature centered around the shelf is to blame—the southern waters are too warm for the cold-adapted northern fish, and vice versa. The shelf also separates populations of other species, including lobsters, scallops, and cod. “This confirms with yet another species that the Scotian Shelf is almost a universal genetic barrier,” says Baumann.
More than a curiosity, the genetic minutiae of this little fish is surprisingly important. Sand lance are a cornerstone of ocean ecosystems. Just about everything eats the slender forage fish, including 72 species of fishes, birds, and mammals.
Theoretically, the existence of a population adapted to warmer water should help the species weather the stresses of climate change because it is more likely to thrive and spread northward as the ocean warms. But that doesn’t mean we should give up on their northern neighbors, since other unique adaptations could become important in the future, Baumann says. “Even if we don’t know which variant is the important one, we need to preserve all of them.”
The problem is, scientists know very little about the genetic diversity of most marine species, especially in the deep sea, says Nicholls. Many marine ecosystems are remote and difficult to get to, making it challenging to understand what diversity actually exists. “We don’t really know what’s out there; we’re discovering new species all the time,” he says, “so it’s even harder to get information about genetic diversity.”
Nicholls says the best tools to preserve both the genetic diversity we know about, and that which we don’t, are strong networks of marine protected areas. At the CBD meeting, members also agreed on a target of protecting 30 percent of coastal and marine areas by 2030. “If we protect enough of the ocean, populations can replenish themselves and spill over into adjacent areas, maintaining diversity both within and outside their boundaries,” Nicholls says.
Popular Science, 7 March 2023