Whence this fish?

The Chiapas catfish's chocolate-brown color and white chin barbels are truly enigmatic. Credit: Courtesy of John Lundberg In February 2005, John Lundberg, an evolutionary biologist at the Academy of Natural Sciences in Philadelphia, obtained molecular data from a tissue sample that

By | September 1, 2008

<figcaption>The Chiapas catfish's chocolate-brown color and white chin barbels are
                    truly enigmatic. Credit: Courtesy of John Lundberg</figcaption>
The Chiapas catfish's chocolate-brown color and white chin barbels are truly enigmatic. Credit: Courtesy of John Lundberg

In February 2005, John Lundberg, an evolutionary biologist at the Academy of Natural Sciences in Philadelphia, obtained molecular data from a tissue sample that he just couldn't believe. The sample came from a bizarre species of Mexican catfish that no one could identify. But the data said the fish was from Africa. He thought, "Wait, maybe someone switched the jars." To be sure of the findings, he wanted a second tissue sample. So, in May 2005, Lundberg headed approximately 2000 km south to the Lacantún River along the Mexico-Guatemala border. There, he found another fish, which confirmed the original conclusion: This catfish was, indeed, of African descent. So how on earth did it end up in Mexico?

The story began nine years earlier, when Rocío Rodiles-Hernández, a Mexican ichthyologist, was surveying the Lacantún River's aquatic fauna. Her fishhook pulled up a large mysterious catfish that wasn't in any of her taxonomic guidebooks. To help identify the fish, Rodiles-Hernández, now at the College of the Southern Frontier in San Cristóbal de las Casas, Mexico, sent a preserved specimen to Lundberg and his colleague Dean Hendrickson at the University of Texas at Austin. Right away, the two catfish experts were taken aback. "Just looking at it from across the room I knew that we had something special," says Lundberg.

The fish's chocolate brown body and bright white chin barbells made it stand out to the naked eye, says Lundberg, but what really set it apart from all other catfish were its distinctive gill arches, unusually shaped swim bladder, and two unique bones in its head. "It was really an enigma." Perplexed, the researchers assigned the fish to a new family and gave it the name Lacantunia enigmatica, or simply "Chiapas catfish," after the southern Mexican state where the fish was found (Zootaxa, 1000:1-24, 2005).

How did this freshwater fish get from Africa to Mexico?

Lundberg then enlisted the help of his postdoc, John Sullivan, who was already carrying out a phylogenetic analysis of all the major catfish lineages based on a 3.6 kb stretch of nuclear DNA spanning the rag1 and rag2 genes, involved in immune cell maturation. Sullivan tossed the Chiapas catfish sequence into the mix, and ran the analysis again.

"I can still recall with that first sequence when I put it in and got the result after doing the Bayesian analysis," says Sullivan. "I was quite surprised." The analysis showed almost unequivocally that the Chiapas catfish was nested in among the African species, not the North or South American species as the researchers had anticipated. Through fossil-calibrated molecular clock dating, the researchers concluded that the Chiapas catfish branched off from its African relatives between 75 and 94 million years ago (Proc Acad Nat Sci Phila, 156:39-53, 2007).

After the findings were confirmed, the researchers set about determining how the fish had traveled from Africa to Mexico. Nearly all catfishes are freshwater species, so a marine route was highly improbable. The simplest solution would be for the Chiapas catfish to diverge from the African catfishes when South America and Africa separated. But the last time the two landmasses were combined was in the southern supercontinent of Gondwana, which broke apart an estimated 100 million years ago, before the split of the Chiapas catfish.

According to the researchers, that leaves only two scenarios where the geological record provides evidence for a plausible land-locked freshwater connection: Either the Chiapas catfish traveled through Asia, across the Bering Strait, down through North America, and into southern Mexico; or, it traversed Europe, journeyed along an ancient North Atlantic land bridge, through Eastern Canada, and further on to Mexico. "Any of these scenarios seem somewhat outlandish, and it's a matter of choosing between hypotheses," says Sullivan.

Joseph Nelson, a fish systematist at the University of Alberta in Edmonton, Canada, thinks the North Atlantic land bridge explanation is more likely. "To me, it's the simplest, the most direct, and the shortest, so I would tend to favor it - but you never know," he says. With no living relatives or fossil remains, there's no way to say one way or the other, he admits. "The most direct way of solving it will be to find fossils."

Sullivan optimistically thinks that's a real possibility. "The discovery of Lacantunia in the first place was a huge surprise, and there could be other surprises in the future."


Avatar of: Greg Wooster

Greg Wooster

Posts: 2

September 5, 2008

Dear editor,\n Any chance the "Chiapas catfish" was a relatively recent introduction of an exotic invasive species from the aquarium trade \n(someone dumped a fish that got too big for an aquarium), the aquaculture industry or perhaps ship ballast? (Ship ballast has been implicated for introducing the infamous Zebra mussel, round goby, and other organisms in the Great Lakes). Did the researchers look for it anywhere in Africa? Just curious that only exotic sounding \ngeological explanations were discussed. Sincerely, Greg Wooster
Avatar of: John Lundberg

John Lundberg

Posts: 1

September 8, 2008

Dr. Wooster asks an important question. We have indeed considered the possible human mediated transplant of an extant African species, especially in light of our global inventory of catfish species . No sign of Lacantunia in Africa where ichthyological exploration has had a long and productive history; if Lacantunia were a transplant one would expect it to be reasonably common and known in Africa - it is a big edible fish. Also, if it were such a transplant it would be an old one (decades or more) since the local Mayan native Americans in Chiapas have a their own name for it. "Madre de juil", i.e. "mother of" one of the other catfish in the Rio Usumacinta. Tantalizing too is a Mayan legendary catfish that originated from transformation of a man.
Avatar of: ALAn Drake

ALAn Drake

Posts: 4

September 19, 2008

The Atlantic Ocean was far smaller at the estimated date of divergence. A number of species have been confirmed to be transferred by birds (see island biology).\n\nMight a pelican or albatross like bird carry some durable eggs across a narrow ocean ? Such an introduction would be characterized by limited genetic variation within the population (natural mutations from a brother-sister start could be modeled).\n\nAlan Drake

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