Australasian Science: Australia's authority on science since 1938

Along Came a Spider

Credit: Nick Birks, Wildflight Australia Photography

Credit: Nick Birks, Wildflight Australia Photography

By Sophie Harrison

Genetic studies reveal that trapdoor spiders colonised Kangaroo Island after surviving a remarkable rafting journey from South Africa.

Spiders are not normally associated with oceanic voyages. Indeed, for much of the Southern Hemisphere’s flora and fauna – including that of Australia – it seems more likely that the current distributions of many species are a result of their presence on the ancient supercontinent Gondwana rather than by any remarkable feats of dispersal. When Gondwana broke into the land masses that are now known as Antarctica, South America, Africa, Madagascar, India and Australia, species that were already living on the supercontinent were taken with it – physically “inherited” at the point of separation.

Therefore, when we see related groups of organisms with poor dispersal ability living on these land masses, we assume in the first instance that their distributions are related to shifting continents rather than shifting organisms. Such distribution patterns are called “vicariant”.

However, advances in genetic techniques mean we can now investigate the evolutionary relationship between species and the timing of their separation, and make more robust inferences about their evolutionary history. In essence, we can use genetic similarity to calculate how closely related species and populations are to each other. As DNA usually mutates at fairly constant rates, these rates can be used to calculate when two taxa may have diverged, and therefore determine if this timing matches with historical events, such as the breakup of a land mass, the creation of a mountain range, or the formation of a river.

The Migidae are a family of trapdoor spiders that are often called tree trapdoor spiders even though many species within this family build burrows on the ground. Trapdoor spiders belong to the infraorder Mygalomorphae, which differ from other spiders by having parallel fangs and four book lungs.

While most spiders usually have annual or biannual life cycles, trapdoor spiders are very long-lived, with life cycles in excess of 10 years having been recorded. Instead of using silk to create webs to catch prey, they use it to line burrows in the ground, which they use for shelter and as a base for prey capture.

As a rule, they also have very limited powers of dispersal; once they have hatched and emerged from the maternal burrow, they usually disperse only a few metres, dig a burrow of their own, or find a pre-existing niche to form the basis of their burrow. They will then stay in this burrow for the duration of their lifespan, widening and lengthening it as they grow.

This sedentary lifestyle has led to the logical assumption that the presence of Migidae in both Africa and Australia is due to vicariance; that is, that ancestral populations were widespread across Gondwana and travelled on the land masses that then formed Africa and Australia. But is the story more complex than this?

We closely examined the evolutionary relationships among migid trapdoor spiders, including those species found in south-western Western Australia, Kangaroo Island, South Australia, and those found in South Africa. Surprisingly, our multi-gene evolutionary tree showed that the species from Kangaroo Island (Moggridgea rainbowi) is much more closely related to those from South Africa than to its Western Australian counterparts.

To further examine this relationship, we compared individuals belonging to the genus Moggridgea from Kangaroo Island and South Africa. We conducted rigorous morphological and genetic analyses to examine how close this relationship was, and discovered that M. rainbowi was not only very closely related genetically to African species but also extremely similar in appearance. The degree of genetic and physical similarity was so great that we even wondered if it was possible that the Kangaroo Island species had been inadvertently introduced by sealers who first colonised Kangaroo Island in the 1800s!

However, when we employed molecular dating techniques to estimate how long ago the Kangaroo Island species and African species had split, we found that this hypothesis was not supported by our results. Astonishingly, the African and Kangaroo Island species diverged 2–16 million years ago, much (much) later than the separation of Africa from the rest of Gondwana (~95 mya), and obviously much earlier than human arrival on Kangaroo Island.

Even when error margins around the estimated divergence date were doubled or even tripled, the timing of the divergence still did not match up with the breakup of Gondwana. Furthermore, disparate populations of Moggridgea on Kangaroo Island itself showed genetic differentiation that was consistent with their presence on the island for several million years.

The results of our study left only one likely hypothesis, and it is perhaps the most astonishing of all: that Moggridgea survived a remarkable journey across more than 10,000 km of open ocean from South Africa to Kangaroo Island.

Given trapdoor spiders’ inherent lack of dispersal abilities, the most logical way that they could have survived such a long-distance journey is via oceanic rafting. This is the term given to a large mass of earth, vegetation and debris that is washed out to sea during major flood events. Rafting is already thought to have facilitated trans-oceanic voyages in many groups of organisms, including geckos and some primates. However, this is the first long distance dispersal event to be recorded in a mygalomorph spider.

While this was clearly an exceptional feat, several precedents by mygalomorph spiders at smaller scales already exist, supporting the idea that they are indeed capable of dispersal across bodies of water. Indeed, mygalomorph spiders occur on the Galapagos Islands, Hawaii and the Comoros. These islands are all volcanic in origin, which means they have never been connected to continental landmasses; in all such cases, vicariance can be ruled out.

It is also worth noting that trapdoor spiders may be better equipped for oceanic travel than they initially seem, particularly in terms of dispersal via rafting. Moggridgea live in secure, silk-lined underground burrows that would provide a water-proof shelter and climate control. They also construct tightly fitting trapdoors that can be sealed shut with silk, further protecting them from the elements. They have a low metabolic rate and the ability to go for months without food, and dispersal of gravid females would lead to the arrival of numerous spiderlings in any new environment. Finally, they are also somewhat resistant to drowning due to their ability to use stored oxygen, which is a critical survival tactic when their burrows are temporarily flooded during storm events.

With their minimal sustenance requirements, sealable burrows and ability to “hold their breath”, these spiders may actually be the ideal passengers for oceanic rafting.

Sophie Harrison completed this research as a PhD candidate at the Australian Centre for Evolutionary Biology and Biodiversity, The University of Adelaide. It has been published in PLoS ONE (