Diversity of Fishes
Case Study: Dr. DeSalle Identifies Sturgeon Species
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Rob Gets a Visitor

In the early 1990s, Rob DeSalle, a curator at the American Museum of Natural History, received a visitor from Russia. "I'm sitting in my office one day, and this Russian guy comes in," Rob recounts. "He says, 'I work on these fish,' and hands me some pictures. They were these amazing things—bizarre—ancient looking."

His visitor was Vadim Birstein of the Sturgeon Society, and the fish he showed Rob were sturgeon. Sturgeon are known as "living fossils" because they have been around for an astounding 250 million years. They had dinosaurs for neighbors, and they look it. Sturgeon are covered with armor and grow to be 10 to 12 feet long. The largest fish can weigh one and a half tons and can live 150 years.

There are about 25 species of sturgeon (the exact number depends on whether you count certain subspecies as one or two species), and Birstein came to Rob and wanted help sorting them out.

Sturgeon have been around for 250-million years.
Sturgeon have been around for 250-million years. Note the distinctive bony scales on the back and sides of the fish.

The American Museum of Natural History's research focuses on systematics—determining relationships between and among species, illustrated in evolutionary charts called phylogenetic trees. Forked lines show which species are most closely related, and which groups branched off of which. Related species are arranged by shared, derived traits, or characters, because species that share a given mutation likely descended from a common ancestor.

Sturgeon are bony fishes, Class Osteichthyes.
Phylogenetic tree of the evolutionary relatedness of organisms. Sturgeon are bony fishes, Class Osteichthyes.

Rob's research on the DNA of organisms is exceedingly useful in systematics. Biologists traditionally sorted species by physical traits. For example, whales were sorted by the shape of their fin. But with modern DNA-sequencing technology, scientists can spot genetic mutations that are invisible to the naked eye. These mutations may involve as little as a single base pair in the organism's DNA. Since DNA sequences contain billions of base pairs, researchers use computer programs to sort through the data.

Although Rob had never studied sturgeon before, he was intrigued by the opportunity to research a fish for which so little was know about its genetic diversity and so he agreed to study them in the Museum's molecular lab.
 

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