Researchers from the United States and Slovenia have discovered a new, giant Nephila species (golden orb weaver spider) from Africa and Madagascar.
They have published their findings in the Oct. 21 issue of the journal PLoS ONE. Matjaž Kuntner, chair of the Institute of Biology of the Slovenian Academy of Sciences and Arts and a Smithsonian research associate, along with Jonathan Coddington, senior scientist and curator of arachnids and myriapods in the Department of Entomology at the Smithsonian's National Museum of Natural History, also reconstructed size evolution in the family Nephilidae to show that this new species, on average, is the largest orb weaver known. Only the females are giants with a body length of 1.5 inches (3.8 centimeters) and a leg span of 4 - 5 inches (10 - 12 centimeters); the males are tiny by comparison. More than 41,000 spider species are known to science with about 400 - 500 new species added each year. But for some well-known groups, such as the giant golden orb weavers, the last valid described species dates back to the 19th century.
Nephila spiders are renowned for being the largest web-spinning spiders. They make the largest orb webs, which often exceed 3 feet (1 meter) in diameter. They are also model organisms for the study of extreme sexual size dimorphism and sexual biology.
Giant golden orb weavers are common throughout the tropics and subtropics. Thousands of Nephila specimens that have been collected are in natural history museums. Past taxonomists collectively recognized 150 distinct Nephila species, but in his doctoral thesis, Kuntner recognized only 15 species as valid. Linnaeus described the first Nephila species in 1767, and Karsch described the last genuinely new Nephila in 1879. All more recent descriptions turned out to be synonyms.
"It was surprising to find a giant female Nephila from South Africa in the collection of the Plant Protection Research Institute in Pretoria, South Africa, that did not match any described species," said Kuntner, who first examined the specimen in 2000.
Kuntner, Coddington and colleagues launched several expeditions to South Africa specifically to find this species, but all were unsuccessful, suggesting that perhaps the Nephila specimen, first collected in 1978, was a hybrid or perhaps an extinct species. In 2003 a second specimen from Madagascar (in the Naturhistorisches Museum Wien in Vienna, Austria) suggested it was not a hybrid. No additional specimens turned up among more than 2,500 samples from 37 museums. The species seemed extinct. Then a few years ago a South African colleague found a male and two females in Tembe Elephant Park, and it became clear that the specimens were indeed a valid new species.
In the PLoS ONE paper, Kuntner and Coddington described N. komaci as a new species, now the largest web-spinning species known, and placed it on the evolutionary tree of Nephila. They then modeled evolution to test if natural selection had affected body size. They found strong evidence that it had, but only in females. Nephila females consistently through time increased in size and, mainly in Africa, a group of giant spiders evolved. Nephila males, in contrast, did not grow larger, but instead remained about five times smaller than their mates. Although males look like "miniatures" next to their mates, the males are actually normal-sized; the females are giants.
The new species was named after Kuntner's best friend Andrej Komac, who died in an accident at the time of these discoveries. "My friend, himself a scientist, encouraged me to tackle this PhD, but did not live to see the discoveries made," said Kuntner. "He was a big inspiration, and a great friend, thus it was logical to name this new species to his memory."
Kuntner and Coddington urge the public to find new populations of N. komaci in Africa or Madagascar, both to facilitate more research on the group, but also because the species seems to be extremely rare. "We fear the species might be endangered, as its only definite habitat is a sand forest in Tembe Elephant Park in KwaZulu-Natal," said Coddington. "Our data suggest that the species is not abundant, its range is restricted and all known localities lie within two endangered biodiversity hotspots: Maputaland and Madagascar."
Largest Web-Spinning Spider Discovered
Jeanna Bryner, livescience.com Yahoo News 21 Oct 09;
About the size of a standard CD, a newly described spider is now considered the largest in a class of web-spinners.
Scientists discovered remains of the species of golden orb-weaver, now called Nephila komaci, among museum collections in South Africa and recently more dead specimens from Tembe Elephant Park in South Africa. However, none of the discoverers knew at the time that the specimens belonged to the not-yet-described spider.
The oversized dimensions - a body length of 1.5 inches (3.8 cm) and a leg span of 4 to 5 inches (10 to 12 cm) - describe the females of N. komaci only. The males, which are considered normal-size spiders, are on average five times smaller.
Jonathan Coddington of the Smithsonian's National Museum of Natural History says the new species is "just a whisker" bigger - a few millimeters to be specific - than other Nephila species, which are known for their enormous body and web sizes.
(The largest spider in the world may be the Goliath bird-eater, or Theraphosa blondi, which has a leg span of up to about 10 inches, 25 cm, according to the Denver Museum of Nature and Science. This tarantula does not spin webs.)
Coddington and Matjaž Kuntner of the Institute of Biology of the Slovenian Academy of Sciences and Arts and the Smithsonian Institution describe N. komaci in the Oct. 21 issue of the journal PLoS ONE. They found that natural selection has nudged some of the females to larger and larger sizes, which allows them to lay more eggs and increase chances of passing on their genes.
Since nobody has knowingly seen a live spider of this species, even after several African expeditions, the researchers speculate the species could be endangered, or at least hiding out high in trees where grounded scientists have yet to look.
Evolving body size
The researchers placed the spider onto an evolutionary tree with other Nephila species and ran a computer simulation to figure out how size changed over evolutionary time and whether natural selection played a role.
While males showed no real jumps or dips in their size, hovering at about one-fifth the size of females, the female spiders consistently increased in size over time. The result is a group of giant spiders that evolved mainly in Africa.
As for why the females ballooned over time, Coddington says it makes sense from a reproductive perspective, because the larger the body size the more eggs she can lay.
"The problem with becoming bigger is it takes you longer to do it. You have to eat and eat. The longer you put off sexual maturity the more likely it is you will die," Coddington told LiveScience. "But if you're willing to take risks, you can become enormous and have thousands of eggs."
In addition, the female giants likely have few enemies, he added. In fact, reports suggest some Nephila spiders occasionally snag birds, bats and lizards.
Males, on the other hand, have the sole job of inseminating a female of its species, whose entire life is spent on a web. "Males are staying small because they can get away with it. If all you have to do is inseminate somebody and it doesn't matter how big you are, your best strategy is to become an adult as fast as you can," Coddington said.
Spider sex
One puzzler the team turned up: While the females were gaining in fecundity, males seemed to be losing their sexual prowess.
Sex between male and female Nephila spiders typically leaves females unable to re-mate and males castrated. "Males stick their business end up the females and then they break it off, and in doing so they plug the female," Coddington said.
The team compared the shape of the male pedipalps, or a pair of appendages used to transfer sperm, with that of the female genitals in Nephila species.
"These guys have long, whippy pedipalps, and over evolutionary time they seem to be gradually losing the ability to plug up females," Coddington said.