Tarantulas are among the most infamous of spiders, largely for their size and appearance – with hairy legs, large jaws and vibrant colors, tarantulas tend to make a splash every time they appear. But one thing puzzled researchers about tarantulas: why are they so widespread around the world?
Tarantulas can be found on all continents (outside of Antarctica), and they are spread over many parts of the world. This is especially surprising because individual tarantulas don’t really like to move.
“They are quite widespread and occur in the subtropical regions of every continent,” explains a research team led by Carnegie Mellon University bioinformatician Saoirse Foley in a new study.
“[Their] behavior does not predict that tarantulas would be successful propagators, but they have spread around the world and colonized strikingly different ecological niches. “
Females and juveniles rarely, if ever, leave their burrows, and males usually just venture out to find mates. So how are they so widespread?
To answer this question, Foley and colleagues followed two different paths: the biogeographic distribution patterns of tarantulas throughout history, as well as their genetic history (from transcriptome databases). They then modeled how tarantulas could have spread throughout their evolutionary history, building an evolutionary tree of tarantulas and calibrating it with known fossil evidence.
They found that tarantulas likely emerged some 120 million years ago during the Cretaceous Period, when tyrannosaurs, velociraptors and other dinosaurs roamed the planet. This led researchers to an interesting conclusion: Geology did a lot of dissemination work for the tarantulas.
“Previous studies estimated that tarantulas emerged between 150 Ma-71 Ma or ~ 107 Ma, which is compatible with Gondwana origin,” the researchers explain. “Indeed, some tarantulas (Selenocosmiinae) are presented as taxa from Northern Gondwana. “
At the time, many of the world’s current continents were huddled into a supercontinent called Gondwana, which encompassed Antarctica, South America, Africa, Madagascar, and Australasia, as well as the Arabian Peninsula and the Indian Subcontinent. However, some tarantulas also seem to be surprisingly skilled spreaders.
Some tarantulas from India remained local, but others migrated and spread across Asia and diverged while the Indian tectonic plate was still isolated and drifting towards Asia. These two genera colonized Asia 20 million years apart.
“Our results indicate that both Asian lineages diverged while the Indian Plate was still rafting to Asia … Interestingly, the two lineages also appear to be ecologically divergent,” note the researchers.
There are currently about 1,000 bird spider species, ranging in size from the size of a coin to the size of a plate. They exhibit remarkable variety, and since their fossil record is so sparse, there is still much we don’t know about their evolution and diversification – which is why studies like these are so important.
Ultimately, the study suggests that geology has given tarantulas a helping hand in spreading around the world, but it may also prompt researchers to rethink tarantulas’ distribution and adaptability.
“In the past, we didn’t 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.
Journal Reference: Saoirse Foley, Henrik Krehenwinkel, Dong-Qiang Cheng, William H. Piel. Phylogenomic analyzes reveal Gondwan origin and repeated colonizations from India to Asia by tarantulas (Araneae: Theraphosidae). PeerJ, 2021; 9: e11162 DOI: 10.7717 / peerj.11162