Blind Australian cave fish

Closest relatives in Madagascar

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Picture of blind cave fish.
Typhleotris pauliani (top), a Malagasy cave fish, and the newly discovered, pigmented relative (bottom). Image: AMNH/J. Sparks.

Sept. 4, 2012 — Through DNA analysis, a team of researchers from Louisiana State University and the American Museum of Natural History has discovered that two groups of blind cave fishes on opposite sides of the Indian Ocean are each other's closest relatives. The groups of fish occupy similar subterranean habitats in Madagascar and Australia, land masses separated by salt water for the last 100 million years. Their study, which appears in the journal PLOS ONE this week, adds to existing biological evidence for the existence of Gondwana, the prehistoric supercontinent that contained of all of today's southern continents, as well as Madagascar and India.

Picture of University of Louisiana professor Prosanta Chakrabarty
Prosanta Chakrabarty

"This is the first time that a taxonomically robust study has shown that blind cave vertebrates on either side of an ocean are each other's closest relatives," said Prosanta Chakrabarty, an assistant professor and curator of fishes at Louisiana State University's Museum of Natural Science. "This is a great example of biology informing geology. Often, that's how things work. These animals have no eyes and live in isolated freshwater caves, so it is highly unlikely they could have crossed oceans to inhabit new environments."

The cave fishes, of the genera Typhleotris in Madagascar, and Milyeringa in Australia, are small — less than 100 millimeters long — and usually lack pigment. These fish are really suited only for life in freshwater and complete darkness. For this reason, they have very restricted distributions within isolated limestone caves. It's also why the newfound genetic relationship between the trans-oceanic groups is an exciting geological find.

"The sister-group relationship between cavefishes from Madagascar and Australia is a remarkable example of Gondwanan vicariance—a geographical split dating back to the Late Cretaceous some 100 million years ago," said John Sparks, a curator in the Division of Vertebrate Zoology at the American Museum of Natural History. "The interesting thing about Madagascar's extant freshwater fish groups, with the exception of a single species, is that all exhibit relationship patterns that are in time with the Mesozoic breakup of Gondwana — some are related to groups in India/Sri Lanka, and others to groups in Australia. Only a single freshwater species has its closest relative in nearby Africa."

One of the new forms discovered by the researchers, which will be named in a future publication, is a novelty among cave fishes because it is fully and darkly pigmented. The authors of the study believe their analysis suggests it evolved from a pigment-free ancestor, which they feel indicates that some subterranean forms can "reverse" themselves for this character.

"It is generally thought that cave organisms are unable to evolve to live in other environments," Sparks said. "Our results, and the fact that we have recently discovered new cave fish species in both Madagascar and Australia belonging to these genera, are intriguing from another perspective: they show that caves are not so-called 'evolutionary dead ends.'"

"Only two specimens of the new pigmented form were recovered from the first cave we searched in Madagascar, despite the fact that we spent hours in this sinkhole," said Chakrabarty. "Even the locals hadn't been inside of it before."

Because remote locales with caving opportunities exist all over the world, the researchers are eager to pursue other opportunities for discovery.

"Conducting this research really developed my love for caving," said Chakrabarty. "You don't always find something exciting. But, when you consider how isolated many of these caves are, especially in places like Madagascar, and how unaffected they have been by the passage of time, you know that the fish in there are going to tell a really good story."

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Adapted from materials obtained from the AAAS