Spiders have been around Antarctica for 9 million years
Take a look under the rocks of the sub-Antarctic islands and you’ll probably see a spider crawling out. A study shows that they were not introduced by explorers.
“I had long formed the project of discovering the southern lands, or of making a voyage in the southern part of the globe, to try to find some lands in the immense space of the seas which surround the South Pole between Cape Horn, New Holland [Australia, ed. note] and the Cape of Good Hope”, writes Yves-Joseph de Kerguelen de Trémarec, ten years after the discovery of the islands that bear his name, in his book Relation de deux voyages dans les mers australes et des Indes.
Land he indeed found in the name of the King of France in 1772, and returned to regain possession two years later, before James Cook’s passage. He was able to do this thanks to the commander of L’Oiseau, Monsieur de Rosnevet, ahead of Le Roland, on which Yves-Joseph de Kerguelen was detained due to damage and bad weather.
The inhospitable, predominantly rocky landscapes offered little rest for the discoverers, who returned to Mauritius hungry, thirsty and tired. However, this was not the case for the small spiders that took up residence here as soon as they arrived some 5 million years ago.
The colonization of Antarctica by these eight-legged critters is believed to have started 9 million years ago, according to this summer’s issue of The American Naturalist. The research team led by Jonas Wolff, from the Greifswald Zoological Institute in Germany, and David Renault, from the Laboratoire Écosystèmes, biodiversité, évolution in Rennes, France, report that they are believed to have originated in Tasmania and southern New Zealand. Today, nine species can be distinguished between the various sub-Antarctic islands.
“Let themselves be carried away by air vortices”
“To our surprise, we found clear genetic partitioning of the specimens of Myro kerguelensis from the islands of Crozet, the Kerguelen, and Macquarie Island, showing that, in contrast to our initial hypothesis, the extreme distribution range of that species of more than 9,000 km is not the result of human-mediated translocation but of repeated natural long-distance dispersal followed by long-term isolation”, explain the authors.
Crossing oceans? Although a Chilean beechwood has already been found at Heard and McDonald, and these 5 mm predators can cling to floating objects, sea salt would not allow them to withstand long crossings. By tracing the genetic history of these arachnids, the researchers discovered that they gradually colonized the sub-Antarctic islands by following circumpolar currents.
Although spiders have also been observed on bird feathers, for David Renault, an ecologist and entomologist specializing in southern terrestrial introductions, ballooning may have played a decisive role. “Spiders are notorious for taking up the right position to let themselves be carried away by air vortices, which works over very long distances,” he explains. “A bird would only carry a few specimens, whereas with ballooning, hundreds of individuals can use the same currents.”
“In a cocoon of woven silk”
The Crozet archipelago would have been a refuge during periods of glaciation, during which other populations have formed, in Marion to the west, or Kerguelen, Heard and Macquarie to the east. The genome of Kerguelen spiders bears traces of long isolation.
Since their arrival, they have lived in the scree slopes of the lunar landscapes of the Southern hemisphere, feeding on springtails and soil mites. On these isolated bits of earth, wingless flies walk slowly and their larvae crawl. “The bite on the back of the neck is as quickly lethal as a lightning”, wrote naturalist Jean-Henri Fabre in Souvenirs Entomologiques about a spider on the hunt.
“They can spare the weaving of webs for their foraging and allocate it to reproduction,” David Renault explains. “The silk threads protect the egg-layers in a highly compacted cocoon, tightly woven to maintain the moist environment around the larvae.” The agile arthropods leave these “germ boxes” – “satin capsules”, wrote Fabre – under the stones of the sub-Antarctic archipelagos, which “will serve as their roof”.
As early explorers crossed their paths, the spiders’ diet undoubtedly diversified. The comings and goings of whalers, scientists and visitors brought new classes of prey. Facilitated access to the Antarctic Peninsula was also made available to them. Ever since, management and researchers have been fighting against the accidental introduction of new species.
Camille Lin, Polar Journal AG
With thanks to the French Polar Institute and CNRS for their images.
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