Some baby spiders can float for tens or even hundreds of miles, buoyed by strips of silk and carried aloft by the wind. But even for these resourceful youngsters, making a journey of more than 6,000 miles across choppy ocean sounds fairly improbable. That’s why researchers have long assumed that one genus of spider found in both Africa and Australia must have ended up on both continents tens of millions of years ago, as supercontinent Gondwana slowly cleaved apart.
A new genetic analysis challenges that scenario, however. Remarkably, scientists now argue that these arachnids actually made the pilgrimmage via makeshift rafts, long after the modern continents had been established.
"While their survival of such a journey may be difficult to picture, these spiders may be actually better suited to dispersal via rafting than we’d initially think," says Sophie Harrison, a biologist at Australia’s University of Adelaide, of the Australian species Moggridgea rainbowi. The trait could make these inch-long arachnids such effective ocean voyagers is described in the name of their taxonomic family: trapdoor spiders.
Like many arachnids, trapdoor spiders make burrows to hide and rest in. But they also augment their burrows with a hatch that can be pulled tightly shut (hence the name). Within that sealed burrow, which is usually lined with silk, the spider enjoys a comfortable, relatively climate-controlled environment, Harrison said. Furthermore, trapdoor spiders have relatively slow metabolisms, meaning it's feasible that they could survive an extended ocean journey.
"In some ways, they are better suited to this type of dispersal than other species which have undergone transoceanic dispersal via rafting," says Harrison, whose findings were published today in the journal PLOS ONE. Indeed, scientists have found evidence of other spiders taking intercontinental cruises, such as the small arachnids from the genus Amaurobioides, which likely floated on bits of wood or plant from Chile to Africa to Australia.
Yet Harrison's proposal goes against what most scientists thought about how trapdoor spiders ended up in Australia, not to mention common sense.
Along with many other creatures, it had been thought that these spiders were separated from their companions by the slow breakup of the supercontinent Gondwana as the Earth's tectonics plates shifted. After all, Moggridgea rainbowi hardly shows any wanderlust, often making its own burrow just a few feet away from where it was born. And anyways, how could even the most adventurous spider traverse an ocean?
In 2013, Harrison was researching how armored trapdoor spiders have diversified into different species over time for her PhD. When studying some specimens she had acquired from a wildlife photographer and "trapdoor spider enthusiast" Nick Birks, she and her colleagues noticed that these trapdoor spiders appeared to be very closely related to their African ancestors—more so than one would expect for two species that had split so long ago.
Using genetic analysis of the two species, Harrison found that the two species appeared to have split off from each other in separate evolutionary paths somewhere in the range of 2 to 16 million years ago, long after Gondwana broke up around 100 million years ago.
Clearly, the slow tectonic shift scenario didn’t fit. But that timeline also had the spiders making the voyage long before the first humans arrived in Australia about 65,000 years ago, meaning that they didn’t hitch a ride like so many other species, either. "This left long-distance dispersal as the most plausible option for their method of arrival into Australia," Harrison says.
How this process started is a mystery, but it could have started with burrows of trapdoor spiders being knocked into the ocean by a landslide or uprooting tree, and then being carried by the ocean currents in their snug confines.
Miquel Arnedo, an evolutionary biologist at the University of Barcelona who has extensively studied the dispersal of species across long distances, questions the precision of some of the calculations used by Harrison's team, noting that he would have liked to see alternate methods of estimating the rate of genetic mutation . However, even accounting for the differences that other methods might produce, Arnedo says, the calculations still make it likely that the Gondwana breakup wasn't involved in the journey of these spiders.
Rafting spiders is not unprecedented; in 2014, Arnedo published a genetic study finding that a related species of trapdoor spider, Titanidiops canariensis, rafted its way onto the Canary Islands from mainland Africa. Still, he says that the distances trapdoor spiders would need to cover to make the leap Harrison describes would be astonishing.
"The evidence presented in this new [study] increased the traveled distance by 100-fold," says Arnedo, who was not involved in Harrison's research, via email. "All in all, I think the evidence presented is compelling."