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The fish tale is legendary. On December 23, 1938, South African museum curator Marjorie Courtenay-Latimer got a phone call from her friend Hendrik Goosen, a trawler captain who often let her look through the day’s catch for anything peculiar. Goosen had just returned with a new haul, including a very unusual large fish that he thought would pique Courtenay-Latimer’s interest. It certainly did. The fish—blue when caught, but dark gray by the time Courtenay-Latimer arrived—looked primordial and had odd fleshy fins. Biologist J. L. B. Smith later identified the fish as a coelacanth, a species thought to have been extinct for more than 66 million years. It was like dipping a net into the ocean and dredging up a plesiosaur.
In 1997, almost 60 years after the coelacanth’s revival, a second species of coelacanth was spotted by biologists at a Sulawesi fish market in Indonesia. This and other similar discoveries have cemented fishers and fish markets as important sources of information for marine biologists itching to unearth new species. The world’s oceans are so vast and so deep that keeping an eye on what’s being brought to market can tip biologists off to species they’d likely be unable to spot or distinguish in the animals’ natural habitats.
The task can be a little slippery, says Hawai‘i Institute of Marine Biology researcher Sarah Tucker. “Markets can be difficult because fish are often sourced from various places and exchange hands through many different fish collectors,” she says.
Obtaining the precise, critical information about where a fish was caught can be hard in such circumstances. Some researchers develop relationships with specific fishers, just as Courtenay-Latimer did, to help get essential information about where fish are being taken. More than that, Tucker adds, “talking to local fishermen and fish collectors can be super helpful to understand what they are observing and the local names of those species.”
Sometimes the presence of something new on a trawler or in a market is obvious. In 2018, marine biologists named a new species of deep-sea shark, Planonasus indicus, that had first caught researchers’ attention at a market near India’s southwest coast.
But many new fish species are cryptic—difficult to tell apart based on anatomy alone. To get around this, biologists often survey fish at the markets using DNA barcoding, an effort that sometimes turns up previously unknown species, like a new deep-water snapper named earlier this year. “It’s becoming more common in scientific publications that you need both genetic and morphological analyses,” Tucker says, with genetic clues sometimes being the first hint that there’s something new among the fish market stalls.
And it’s not just fish that biologists are interested in perusing.
“We prefer to buy fish species that have not yet been investigated intensively, or at all, if our focus is finding new parasite species,” says parasitologist Stefan Theisen at the University of Rostock in Germany. That fish tend to have parasites specific to their species means that new fish likely have new parasites, he adds, with specialists checking anatomical hotspots like the heart and esophagus for tiny hangers-on. Usually, Theisen adds, a sample of about 35 fish is needed to detect all the parasites that might affect that species, which means a lot of time fishing among the stalls for something remarkable.