鱼类学家发现一种新鲨鱼

Discovering a new species is, among biologists, akin to（近似） hitting a grand slam, and University of South Carolina ichthyologist（鱼类学者） Joe Quattro led a team that recently cleared the bases. In the journal Zootaxa, they describe a rare shark, the Carolina hammerhead, that had long eluded discovery because it is outwardly（表面上） indistinguishable from the common scalloped（扇形的） hammerhead. Through its rarity, the new species, Sphyrna gilberti, underscores the fragility of shark diversity in the face of relentless human predation. Quattro, a biology professor in USC's College of Arts and Sciences, didn't set out to discover a new cryptic species, let alone one found exclusively in saltwater. When he started as an assistant professor at USC in 1995, he was largely focused on fish in the freshwater rivers that flow through the state before emptying into the western Atlantic Ocean.

He has wide interests that include conservation, genetic diversity and taxonomy（分类法）. A driving force in his scientific curiosity is a desire to better understand evolution. As it turns out, South Carolina's four major river basins -- the Pee Dee, the Santee, the Edisto and the Savannah -- are a source of particularly rich ore for mining insight into evolutionary history.

Glacial influence had limits

Quattro grew up in Maryland, earned a doctorate at Rutgers University in New Jersey, and completed a post-doc at Stanford University. "New Jersey and Maryland, in particular, had huge glacial influences," said Quattro. "The areas where rivers now flow were covered with glaciers until 10,000 to 15,000 years ago, and as the glaciers receded the taxa（分类群） followed them upstream."

In contrast, rivers south of Virginia were not covered with glaciers. "In other words, these rivers have been around for quite some time," Quattro said. "The Pee Dee and the Santee are two of the largest river systems on the East coast. And we just got curious -- how distinct are these rivers from one another?"

Beginning with the pygmy sunfish, Quattro and colleagues examined the genetic makeup of fish species within the ancient freshwater drainage systems. They found the banded pygmy（矮小的） sunfish in all the South Carolina rivers -- in fact, this widespread species is found in nearly all the river systems of the U.S. southeastern and Gulf coasts, starting from the plains of North Carolina, around Florida, and all the way to and up the Mississippi River.

But two species are much rarer. The bluebarred pygmy sunfish is found only in the Savannah and Edisto systems. The Carolina pygmy sunfish is found only in the Santee and Pee Dee systems. Both species coexist with the common banded pygmy sunfish in these river systems, but are found nowhere else in the world.

From an evolutionary standpoint, it's a noteworthy finding. These rare species are related to the widespread species, yet the details of the inter-relationships -- such as which predates the others and is thus an ancestral species -- still defy ready description. The fact that a rare and a common species are located together in an ancient river system is important information in the ongoing struggle to clearly define evolutionary history. In the past, scientists drew taxonomic charts almost solely on the basis of physical structure (morphology) and available fossils. The genetic data revolution of recent decades is helping redefine biology in a much more precise manner, but the process is still in the early going.