November 8, 2013
Scientists Discover Carolina Hammerhead Shark Species By Accident
April Flowers for redOrbit.com - Your Universe Online
Among biologists, discovering a new species is akin to hitting a grand slam. Joe Quattro, a University of South Carolina ichthyologist, led a team that recently cleared the bases.
Quattro's team describes a rare shark, the Carolina hammerhead, in a paper published in the journal Zootaxa. The shark has long eluded discovery because it is outwardly indistinguishable from the common scalloped hammerhead. In the face of relentless human predation, this new species, Sphyrna gilberti, underscores the fragility of shark diversity through its rarity.
Quattro's initial goal was not to discover a cryptic new saltwater species. As a new professor at USC, his focus was mainly on fish in the freshwater rivers that flow through the state before emptying into the western Atlantic Ocean.
Currently, Quattro's interests include conservation, genetic diversity and taxonomy. A desire to better understand evolution has been a driving force for his scientific curiosity. For mining insight into evolutionary history, South Carolina's four major river basins -- the Pee Dee, the Santee, the Edisto and the Savannah -- are a source of particularly rich ore.
After growing up in Maryland, Quattro earned a doctorate at Rutgers University in New Jersey. He then 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 followed them upstream.”
Rivers south of Virginia, in contrast, 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?”
Quattro and his team began with the pygmy sunfish to examine the genetic makeup of fish species within the ancient freshwater drainage systems. The banded pygmy sunfish was found in all South Carolina rivers -- and in nearly all the river systems of the US southeastern and Gulf coasts, starting from the plains of North Carolina, around Florida, and all the way to and up the Mississippi River.
Two species of pygmy sunfish are much rarer, however. The bluebarred pygmy sunfish is found only in the Savannah and Edisto systems, while the Carolina pygmy sunfish is found only in the Santee and Pee Dee systems. They both coexist with the common banded pygmy sunfish in these river systems, but they do not exist anywhere else in the world.
This is a noteworthy find from an evolutionary standpoint because these rare species are related to the widespread species. However, the details of the inter-relationships, such as which pre-date the others and are thus an ancestral species, still defy ready description. Finding both the rare and common species together in an ancient river system is important information in the ongoing struggle to define evolutionary history. In previous research, taxonomic charts were drawn almost solely on the basis of physical structure (morphology) and available fossils. The process is still in its early phases, but the genetic data revolution of recent decades is helping redefine biology in a more precise manner.
Quattro has been participating in this redefinition by slowly moving down the river systems to the ocean, collecting genetic data the whole way down. He has examined pygmy sunfish, other sunfish and bass in freshwater rivers, while closer to the ocean, he has examined short-nosed sturgeon, which spend most of their time in the estuary - where the river meets the ocean. Further downriver, Quattro studied shark pups.
Several species of sharks, including the hammerhead, use South Carolina as pupping grounds. At the ocean-side fringes of the estuary, the female hammerhead will birth her young. These pups will remain in the area for a year or so before moving out to the ocean to complete their life cycle.
Quattro's team uncovered an anomaly as they were examining hammerhead sharks in the estuary. They found that the scalloped hammerheads (Sphyrna lewini) had two different genetic signatures in both the mitochondrial and nuclear genomes.
A search of relevant literature revealed that Carter Gilbert, the renowned curator of the Florida Museum of Natural History from 1961 to 1998, had previously described an anomalous scalloped hammerhead in 1967 that had 10 fewer vertebrae than S. lewini. This hammerhead had been caught near Charleston. The sample is housed in the National Museum of History, which allowed the team to examine it morphologically. This specimen suggested that it constituted a cryptic species -- that is, one that is physically nearly indistinguishable from the more common species.
Quattro and his colleagues first published their preliminary genetic evidence for the new, cryptic species in Marine Biology in 2006. They followed up by making measurements of 54 cryptic individuals and 24 S. lewini hammerheads to fully describe the new species in the their paper. They named the new species, S. gilberti, in Carter Gilbert's honor. the defining morphological difference between the two species is 10 fewer vertebrae.
The team has established locations and genetic signatures for a number of closely related, yet distinct, species in the rivers, estuaries and coastal waters of South Carolina. The findings of this study will go a long way in furthering the efforts to accurately define taxonomy and evolutionary history for aquatic life.
The findings also reveal the rarity of the species. “Outside of South Carolina, we’ve only seen five tissue samples of the cryptic species,” Quattro said. “And that’s out of three or four hundred specimens.”
Over the last few decades, shark populations have greatly decreased. “The biomass of scalloped hammerheads off the coast of the eastern U.S. is less than 10 percent of what it was historically,” Quattro said. “Here, we’re showing that the scalloped hammerheads are actually two things. Since the cryptic species is much rarer than the lewini, God only knows what its population levels have dropped to.”