It’s easy to think that the globe’s vast oceans would be effective barriers to the movement of land animals. An elephant can’t swim across the Pacific, after all. But 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. That voyage took them from Africa all the way across the Indian Ocean to Australia.
Moggridgea rainbowi spiders can be found on Kangaroo Island. It sits off the south coast of Australia. These trapdoor spiders build a silk-lined burrow in the ground with a secure-fitting lid, notes Sophie Harrison. She is a biologist in Australia at the University of Adelaide. The burrow and trapdoor provide these spiders with shelter and protection. It also offers them an out-of-sight spot from which to await approaching prey. And it means that the spiders don’t really need to travel more than a few meters (yards) over the course of a lifetime.
There is evidence, though, that the ancestors of this species might have traveled millions of meters to get to Australia — from Africa. That isn’t as unlikely as it might at first seem. Australia used to be connected to other continents, long ago, as part of a supercontinent called Gondwana. And humans have been known to transport species all over the planet (think bed bugs). But there’s a third option. The spiders might have rafted long distances across the sea.
A spider family tree
To figure out which story was most likely true, Harrison and her colleagues looked at the spider’s genes. They turned to six genes that have been well-studied by biologists seeking to understand links between spiders. As species evolve, their genes change. And scientists can look at the differences between genes to determine how closely related certain species are. They even can figure out when the most recent common ancestor of two species lived.
Harrison’s group looked at those genes in seven M. rainbowi specimens from Kangaroo Island. They also looked at five species of Moggridgea spiders from South Africa. Finally, they analyzed the genes of seven species of southwestern Australia spiders. They belonged to the closely related genus Bertmainius.
From these data, the researchers built a spider family tree. It showed which species were most closely related and how long ago their most recent common ancestor had lived.
M. rainbowi was most closely related to the African spiders, the genes showed. The Australian and African spiders split off from a common ancestor some 2 million to 16 million years ago, the data indicate. Australia’s Bertmainius spiders are more distant relatives.
Harrison and her colleagues detailed their analysis August 2 in PLOS ONE.
The Australian spiders diverged from their African cousins long after Gondwana split up. This also was long before either the ancestors of Australia’s aboriginal people or later Europeans showed up on the Australian continent. It seems impossible that a colony of spiders survived a journey of 10,000 kilometers (6,200 miles) across the Indian Ocean. However, that is the most likely explanation of how trapdoor spiders got to Kangaroo Island, the researchers now say.
Such an ocean voyage would not be unprecedented for spiders in this genus, Harrison’s team notes. Three species of Moggridgea spiders live on islands off the coast of Africa. Two of the islands were once part of the mainland. Their Moggridgea spiders might have diverged at the same time that their islands separated from Africa. But a third species, M. nesiota, lives on a volcanic island nation in the Indian Ocean known as the Comoros. Its spiders must have traveled across 340 kilometers (200 miles) of seas to get there.
Trapdoor spiders may be well-suited to ocean travel, Harrison’s team contends. If a large swatch of land washes into the sea, laden with arachnids, the spiders may be able to hide out in their nests throughout the journey. Plus, they don’t need a lot of food. And they can resist drowning. They can even “hold their breath” and survive on stored oxygen during periods of temporary flooding, the researchers note.
arachnid A group of invertebrate animals that includes spiders, scorpions, mites and ticks. Many have silk or poison glands.
biology The study of living things. The scientists who study them are known as biologists.
colleague Someone who works with another; a co-worker or team member.
continent (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
evolve (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genus (plural: genera) A group of closely related species. For example, the genus Canis — which is Latin for “dog” — includes all domestic breeds of dog and their closest wild relatives, including wolves, coyotes, jackals and dingoes.
Gondwana One of Earth’s two supercontinents that existed during the Devonian Period, which ran from roughly 416 million years ago to 360 million years ago. It’s when some of the earliest plants emerged, initially with no leaves or roots. By the end of this period, ferns and seed plants had evolved. Some shellfish and trilobites shared the oceans with various fishes. And many wingless arthropods (ancestors to spiders and insects) colonized the land.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sedentary Not physically active; an adjective for activities done largely while sitting.
silk A fine, strong, soft fiber spun by a range of animals, such as silkworms and many other caterpillars, weaver ants, caddis flies and spiders.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
spider A type of arthropod with four pairs of legs that usually spin threads of silk that they can use to create webs or other structures.
Journal: S.E. Harrison et al. Across the Indian Ocean: A remarkable example of trans-oceanic dispersal in an austral mygalomorph spider. PLOS ONE. Published online August 2, 2017. doi: 101371/journal.pone.0180139.