Whence this fish?

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?