Aritra Biswas is a PhD student at the Indian Institute of Science. He is a evolutionary biologist with special focus on the processes that generate and maintain biological diversity. Here, Aritra shares his recent work on the biogeographical history of tarantulas.
Aritra Biswas in the field looking for tarantulas in the forests of the Western Ghats.
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Institute. Indian Institute of Science
Academic life stage. PhD student
Major research themes. I am broadly interested in studying biogeography and macroevolution. Specifically, I am interested in the processes that generate and maintain biological diversity and distribution over large timescales at both global and local levels.
Current study system. My PhD focuses on understanding the historical biogeography and macroevolutionary dynamics of tarantulas, which are spiders belonging to the family Theraphosidae. They represent one of the most diverse groups (~1060 species) of spiders, distributed worldwide. Tarantulas are portrayed as scary and harmful to humans through literature, movies, and ancient folklore. The most unique characteristic of tarantulas is that they have urticating hairs that can shoot towards the predator as a defence mechanism. They are the only arachnid group with this amazing ability. They come in all sorts of colours, and all tarantulas are venomous. The presence of all these traits make them an exciting model system for macroevolutionary studies. Apart from all these, there is a tarantula named after one of my favourite singers, Johnny Cash (Aphonopelma johnnycashi), making them a little bit more interesting (at least for me)!
Recent JBI paper. Biswas, A., Chaitanya, R., & Karanth, K. P. (2023). The tangled biogeographic history of tarantulas: An African centre of origin rules out the centrifugal model of speciation. Journal of Biogeography, 50(8), 1241–1351. https://doi.org/10.1111/jbi.14678
Motivation behind this paper. My initial plan was to focus on the origin and biogeography of Indian tarantulas. However, while surveying the literature, I found that the group’s overall historical biogeography is unclear. I found two studies suggesting two different centres of origin for these spiders. The first study by Opatova et al. (2020) proposed an African origin. In contrast, Foley et al. (2021) suggested a South American origin. These two scenarios are mutually exclusive and cannot be true simultaneously. Also, we identified problems with the fossil calibrations employed in the previous studies. Despite being an ancient group, tarantulas represent a fossil-poor lineage, with just two fossils available to date. None of the earlier studies have employed any of these fossils. That was the motivation behind performing an improvised molecular dating with reliable fossil calibrations and an elaborate historical biogeographic analysis which resulted in our recent paper.
Key methodologies. To infer historical biogeographic processes like dispersal and extinction, different programs or packages are used (like BioGeoBEARS or RASP). A dispersal multiplier matrix tells the program the probabilities of dispersal between different biogeographic units. These probability values are often subjective. We used plate tectonic reconstruction data integrated with the dispersal ability of the model organism to come up with an informed dispersal multiplier matrix which reduces the subjectivity significantly. Moreover, we have manipulated the probabilities of dispersals in the matrix to develop different matrix variants to explicitly test different geographic origins and dispersal scenarios not done in conventional historical biogeographic studies. Our work proposes an improvised pipeline for reconstructing biogeographic histories when plate tectonics are involved. Playing around with the dispersal multiplier matrix and testing different values of probabilities is essential and might produce a better solution than the program’s default settings.
Sampling tarantulas along roadside mud bunds
Unexpected challenges. We expected a South American origin since most of the extant tarantula diversity is found on this landmass. We were thus quite thrilled when African origin emerged as the most likely scenario!
Major results. The results show that South America, despite harbouring around 70% of the diversity of extant tarantulas, is not the centre of origin for this group. Africa is the most likely centre of origin (like many other groups). The movement of continental plates played a significant role in shaping the modern-day distribution of tarantulas as they reached Southeast Asia via the drifting Indian plate. Basically, tarantulas have boarded a ship called India from Africa to reach SE Asia and Australia. So, a combination of transoceanic dispersals and plate tectonics has generated the extant distribution patterns in this old group of spiders.
A Thrigmopoeus tarantula doing threat display and flashing its fangs.
Next steps for this research. The next step in this research is to investigate why tarantulas are so diverse in South America. This paper shows that the tremendous diversity of tarantulas in South America has resulted from a single dispersal followed by diversification. Now I am investigating what has caused such disproportionate diversity of tarantulas over the South American landmass. Stay tuned for the result!
If you could study any organism on Earth, what would it be? I became interested in evolutionary biology mainly because of the “Jurassic Park” franchise. However, I have never wanted to be a palaeontologist. As a result, I have always been fascinated by extant giant carnivorous reptiles. If I get an opportunity, I would love to work on large constrictor snakes (pythons or anacondas) or crocodiles (the closest relative of dinosaurs). Especially, I am very much attracted to saltwater crocodiles and would love to work on them for my postdoctoral research.
Anything else to add? I had Arachnophobia during my childhood. I always used to run away whenever I came across any spider. It has been an exciting journey from being arachnophobic to choosing tarantulas as the model system for my PhD. Also, I saw a live tarantula for the first time only after a few months of submitting the first draft of this paper to the Journal of Biogeography. That was almost after spending one year into the PhD. So, yeah, I wrote a manuscript on the historical biogeography of my model organism without even seeing the animal ever!
Habitat of Western Ghats large burrowing tarantula