Spiders in the family Migidae don’t get out much. Known as “tree trapdoor spiders,” they are unapologetic homebodies, spending nearly their entire lives chilling in a single burrow. Unlike their close, but much more famous relatives the tarantulas, tree trapdoor spiders are teeny, with most species small enough to fit on a fingernail. Just a few meters away from where they originally hatched, they build silk-lined tubes within the bark of trees and hide inside, waiting for prey to come close enough for an ambush attack.
And yet, despite the simple, sedentary habits of the tree trapdoor spider, findings in a newly published paper in the journal PLOS ONE suggest that one variety of these humble hermits has accomplished a seemingly impossible feat—voyaging across an entire ocean.
The intrepid arachnid in question is Moggridgea rainbowi, a tree trapdoor spider found only in the woodlands of South Australia’s Kangaroo Island. Recent research has determined that this spider is somewhat of a black sheep among its closest family, being the only Moggridgea species found in Australia. Where are the rest of its brethren? Africa. Literally thousands of miles away across the Indian Ocean.
The explanation for dramatically split distributions like this has generally revolved around the fact that, up until about 150 million years ago, the southern continents of Australia, Africa, Antarctica, and South America were all part of the supercontinent “Gondwana.” The imprint of former Gondwana exists in the distribution of many groups of organisms even today, including Migidae. The assumption about the lone Australian Moggridgea spider had been that it was a relict species of a bygone era, separated from its African relatives by continental drift.
It’s a perfectly reasonable explanation, but not one that had been directly tested. To confirm that the continental split up was behind this familial fragmentation, the timing of the separation of the African and Australian spider populations would need to line up with the timing of the African and Australian landmasses. Luckily, modern genetic techniques can allow scientists to estimate just how long ago different species diverged from one another, and this is exactly what a team of Australian scientists did with these tree trapdoor spiders.
Trapdoor spider may have dispersed across the ocean from Africa to Australia. Image Credit: Nick Birks
The research team took DNA samples from several species of African Moggridgea spiders, the Australian oddball spider (Moggridgea rainbowi), and some other close, Australian trapdoor spider relatives (Bertmainius). In analyzing the DNA, the researchers found specific differences between several genes in the spiders, and compared them to ascertain the evolutionary relationships between species. This verified that the Australian Moggridgea species was indeed most closely related to the African spiders. Then, to figure out when all these species separated from each other, the scientists used a “molecular clock,” which relies on the fact that DNA mutations accrue at a predictable rate.
The results showed that the Australian Moggridgea species is fairly young, separating from its African relatives only about two million years ago. This is way too recently to support the idea that Moggridgea rainbowi is an Aussie because of the supercontinent shake up—Africa and Australia broke apart from each other 110 million years ago. At the same time, the divergence is also much older than any occupation of Kangaroo Island by 19th century explorers or European settlers, which theoretically could have introduced the spider from Africa via ships.
Two million years ago, the most logical way for a trapdoor spider to get from Africa to Australia was by going right across the Indian Ocean. Unlike some other spiders, which can use their silk to “balloon” and even direct their aerial path over huge ocean distances, trapdoor spiders are firmly grounded. This means that their most likely method of transport was passively floating there, sailing the high seas on a mat of plant debris. This phenomenon—called “rafting”—is a wild card way for living things to spread across the globe. Madagascar appears to have received many of its mammals this way, as does South America in regards to burrowing worm lizards. We’ve even seen it occur in real time, when hurricane debris allowed iguanas to colonize a Caribbean island.
Somewhat ironically, the trapdoor spider’s stationary lifestyle is probably what made its epic journey possible. Cozy in their burrows, they would be oblivious as a storm or landslide sent their wooden domiciles into the sea. Basically, these spiders are less “Bilbo fleeing Bag End for a grand adventure,” and more “guy accidentally becoming a stowaway by passing out in a ship’s cargo hold.”
While this rafting scenario is still a hypothesis, it is the only explanation so far that fits with the timeline of divergence of the African and Australian species. The proposal would be solidified if future genetics research on the other island spiders also shows an evolutionary timeline that doesn’t match up with geologic events, or if trapdoor spiders are discovered in a vegetation raft at sea.
This study’s revelations highlight the burgeoning role of modern genetics in uncovering the epic and unexpected origins of what we thought were familiar organisms. Just this year, genetics research also upended how we think of modern elephant relationships, and helped reveal a species of praying mantis with sexes that look nothing alike. As newer genetic methods develop, our understanding of the Tree of Life will continue to radically change.
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