Tarantulas are among the most infamous of spiders, in part because of their size, vibrant colors, and prevalence around the world. But one thing most people don’t know is that tarantulas are homebodies. Females and their young rarely leave their burrows and only adult males will wander looking for a mate. How, then, did such a sitting spider come to inhabit six of the seven continents?
An international team of researchers, including Saoirse Foley of Carnegie Mellon University, embarked on an ancestry.com-style study to find the answer to this question. They looked at the transcriptomes, the sum of all transcripts of the mRNA, of many tarantulas and other spiders from different time periods. Their findings were published online by PeerJ on April 6, 2021.
They used the transcriptomes to build a genetic tree of spiders and then calibrated their tree over time with fossil data. Tarantula fossils are extremely rare, but the software used in the study managed to estimate the ages of older tarantulas versus the ages of fossils of other spiders.
Two independent tarantula lines originated from India. Credit: https://dinosaurpictures.org/, 2021
They found that tarantulas are ancient and first emerged in the patch of land now considered America about 120 million years ago during the Chalk period of time. At that point, South America would be linked with Africa, India, and Australia as part of the Gondwana supercontinent. The spiders eventually reached their current destinations due to continental drift, with a few interesting departures.
For example, the nature of their entry into Asia suggests that tarantulas can also be surprisingly skilled spreaders. The researchers were able to identify two separate lines of tarantulas that diverged on the Indian subcontinent before it crashed in Asia, with one living mainly on the ground and the other mainly in the trees. They found that these genera colonized Asia about 20 million years apart. Surprisingly, the first group to reach Asia also managed to cross the Wallace Line, a border between Australia and the Asian islands where many species are abundant on the one hand and rarely, if at all, on the other.
Ancestral ranges of tarantulas estimated by researchers. Credit: https://mapchart.net/, 2021 (CC BY SA 4.0)
“We used to not see tarantulas as good propagators. While continental drift certainly played a role in their history, the two Asian colonization events encourage us to rethink this story. The microhabitat differences between those two sexes also suggest that tarantulas are experts in exploiting ecological niches, while at the same time showing signs of niche conservation, ”said Foley.
Reference: “Phylogenomic Analyzes Reveal Gondwan Origin and Repeated Colonizations from India to Asia by Tarantulas (Araneae: Theraphosidae)” by Saoirse Foley, Henrik Krehenwinkel, Dong-Qiang Cheng and William H. Piel, April 6, 2021, PeerJ
DOI: 10.7717 / peerj.11162
Other authors of the study include Willam H. Piel and Dong-Qiang Cheng from Yale-NUS College in Singapore and Henrik Krehenwinkel from University of Trier in Germany.