Spiders might have rafted across the Indian Ocean

Moggridgea rainbowi trapdoor spiders are homebodies. Found only on Australia’s Kangaroo Island, they quietly live out their lives underground, waiting patiently for lunch or dinner to happen across their doorsteps. Rarely do they travel more than a few metres from their birthplace.

How, then, could they have found their way across the Indian Ocean from Africa about 2 million years ago?

The question arises from a genetic analysis that points not only to the spider having African roots but having immigrated much more recently than previously thought, long after potential land bridges disappeared.

Moggridgea rainbowi shares a genus with 33 other species, almost all of which are native to Africa. This in itself is explicable. Both Australia and Africa were parts of the supercontinent known as Gondwana until about 110 million years ago, when Gondwana split and its component parts drifted apart. Moggridgea rainbowi, ran the theory, was simply on one side of the new continental divide, separated from its sister species.

To test this idea, a research team led by Sophie Harrison of the University of Adelaide, in South Australia, analysed six genes (five nuclear and one mitochondrial) from Moggridgea rainbowi and five African members of the Moggridgea family. The scientists ran the numbers on the genes to discover when the two groups had last shared a common ancestor.

The answer, that the lineage diverged 2 million years ago, came as a surprise – it was more than 100 million years too recent for the Gondwana-drift theory of trapdoor spider distribution to be true.

Human assistance was also ruled out, because the data indicates the spider arrived on Kangaroo Island well before people did.

Harrison and her colleagues suggest the spiders may well have made the journey by sailing – by inadvertently hitching a lift on drifting logs or vegetation.

“While it may be difficult to picture, these spiders may be actually better suited to rafting than we’d initially think,” Harrison says. “They construct burrows with secure, well-fitting lids, which creates a secure microclimate and offers them protection during the journey. They have a very low metabolic rate, which means they have low food and resource requirements during the trip. In some ways, they are better suited to dispersal via rafting on land/debris than other that have undergone transoceanic dispersal via rafting.”

There is circumstantial evidence to back up the idea. While the overwhelming majority of Moggridgea species live in Africa (where many are tree-dwelling, by the way), they are also found in the Comoros – volcanic islands 340 km off the African coast – and Socotra, an island in the Arabian Sea.

Both locations are plausible destinations for African driftwood.

The research is published in the journal PLOS ONE.

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