These spiders crossed an ocean to get to Australia

If you look at a map of the world, it’s easy to think that the vast oceans would be effective barriers to the movement of land animals. And while an elephant can’t swim across the Pacific, it turns out that plenty of plants and animals — and even people — have unintentionally floated across oceans from one continent to another. Now comes evidence that tiny, sedentary trapdoor spiders made such a journey millions of years ago, taking them from Africa all the way across the Indian Ocean to Australia.

Moggridgea rainbowi spiders from Kangaroo Island, off the south coast of Australia, are known as trapdoor spiders because they build a silk-lined burrow in the ground with a secure-fitting lid, notes Sophie Harrison of the University of Adelaide in Australia. The burrow and trapdoor provides the spiders with shelter and protection as well as a means for capturing prey. And it means that the spiders don’t really need to travel farther than a few meters over the course of a lifetime.

There was evidence, though, that the ancestors of these Australian spiders might have traveled millions of meters to get to Australia — from Africa. That isn’t as odd as it might seem, since Australia used to be connected to other continents long ago in the supercontinent Gondwana. And humans have been known to transport species all over the planet. But there’s a third option, too: The spiders might have floated their way across an ocean.

To figure out which story is most likely true, Harrison and her colleagues looked at the spider’s genes. They turned to six genes that have been well-studied by spider biologists seeking to understand relationships between species. The researchers looked at those genes in seven M. rainbowi specimens from Kangaroo Island, five species of Moggridgea spiders from South Africa and seven species of southwestern Australia spiders from the closely related genus Bertmainius.

Using that data, the researchers built a spider family tree that showed which species were most closely related and how long ago their most recent common ancestor lived. M. rainbowi was most closely related to the African Moggridgea spiders, the analysis revealed. And the species split off some 2 million to 16 million years ago, Harrison and her colleagues report August 2 in PLOS ONE.

The timing of the divergence was long after Gondwana split up. And it was long before either the ancestors of Australia’s aboriginal people or later Europeans showed up on the Australian continent. While it may be improbable that a colony of spiders survived a journey of 10,000 kilometers across the Indian Ocean, that is the most likely explanation for how the trapdoor spiders got to Kangaroo Island, the researchers conclude.

Such an ocean journey would not be unprecedented for spiders in this genus, Harrison and her colleagues note. There are three species of Moggridgea spiders that are known to live on islands off the shore of the African continent. Two live on islands that were once part of the mainland, and they may have diverged at the same time that their islands separated from Africa. But the third, M. nesiota, lives on the Comoros, which are volcanic islands. The spiders must have traveled across 340 kilometers of ocean to get there.
These types of spiders may be well-suited to ocean travel. If a large swatch of land washes into the sea, laden with arachnids, the spiders may be able to hide out in their nests for the journey. Plus, they don’t need a lot of food, can resist drowning and even “hold their breath” and survive on stored oxygen during periods of temporary flooding, the researchers note.