Caught in a trap? The surprising journey of the New Zealand trapdoor spider

Cantuaria burrow, complete with trapdoor. Image from Vikki Smith.

Back in 2015, my supervisor (Adrian Paterson)wrote about how I used tethered beetles to collect trapdoor spiders (genus Cantuaria) without harming the beetles, spiders, or their habitat. I needed to collect those spiders for my PhD research into their biogeography (where they live), ecology (how they live), and conservation (if they will continue to live) issues. Cantuaria are large (5-30 mm) trapdoor spiders (family Idiopidae) that live in burrows in the soil, often with a trapdoor from which they spring out of to capture unsuspecting prey. They were previously thought to live only in New Zealand, but recently some have been found on Tasmania by Dr. Mike Rix.

My PhD is now complete, and it’s been a blast. I had adventures  finding spiders all over New Zealand (detailed here), from wet, wild Deep Cove to semi-arid Central Otago and suburban Wellington. I was always amazed at the diverse places in which the spiders would turn up. At the beginning of my research, I thought of them as fragile relics, ancient reminders of a geological past; as my project progressed, I realised that couldn’t be further from the truth. These things have adapted and diversified!

While Cantuaria are mostly found in clay banks, I also found some populations in dry, rock-hard loess, mossy humus, salty beach sand, and rotting piles of bird guano. I was expecting to find them inhabiting areas with a narrow range of soil parameters, but soil moisture, pH, and soil type had little to no effect on whether or not Cantuaria were present.

Cantuaria specimens. Image from Vikki Smith.

Thankfully, human activity also had no detectable direct effect on Cantuaria presence or absence. The indirect effect, however, may be quite large. Presence or absence was linked to rainfall: Cantuaria were more likely to be present where the rainfall was below 1000 mm/year, and no populations were found in areas with more than 3000 mm/year. If high rainfall makes an area less suitable for Cantuaria, then the current models of future climate change in New Zealand do not bode well for populations on the West Coast, Tasman, Central Otago, and Southland, all of which are predicted to see increased rainfall.

The adaptability of Cantuaria was not the only surprising outcome of my research. Most of the researchers with whom I discussed my project suspected that Cantuaria’s presence in New Zealand dated back more than 80 million years to Gondwanan times. A Gondwanan history was a fair assumption due to Cantuaria’s lack of ability to move long (or even short) distances (dispersal ability), but recent research found evidence for a different scenario.

A team of Australian researchers created DNA phylogenies  (family trees) for Idiopidae (Cantuaria and its other trapdoor spider relatives). While most idiopid lineages separated from each other at roughly the same time as when their respective landmasses split apart, the Cantuaria lineage was younger than its landmass, meaning that they had come to New Zealand after it separated from Gondwana.

Cantuaria group of burrows. Image from Vikki Smith.

My DNA analyses found a Cantuaria phylogeny which also supported a long-distance dispersal scenario – at least until they got to New Zealand (about 16-18 million years ago). Cantuaria dispersal was probably aided by an unusual event, such as a large amount of debris, with Cantuaria ancestor burrows, falling into the sea and floating to New Zealand.

From their first landing point in Southland, they have steadily spread northwards, little by little. In general, the more deeply divergent (older) lineages are found further south. Lineages on the east of the South Island are also separated from their fellows in the west by 6-8 million years, which coincides with the rise of the Southern Alps mountain range, which likely separated and isolated the populations from each other. My tree supports the idea that Cantuaria are slow dispersers, but somehow undertook the long journey from Australia to New Zealand.

Trapdoor spiders may be dispersal-limited,in general they spread very slowly, but they are still capable of occasional long-distance dispersal. Throughout the course of my PhD, my perception of Cantuaria has changed: I no longer view them as fragile relics of a Gondwanan past. Instead, Cantuaria is as dynamic as New Zealand itself, adapting to different conditions as its distribution spreads over millions of years.

  Vikki Smith has just completed her PhD in zoology at the Department of Ecology, Lincoln University. Here she talks about her main results from her study. Vikki’s research was funded by the Brian Mason Trust, the Miss E L Hellaby Indigenous Grasslands Research Trust, the Mohamed Bin Zayed Species Conservation Fund, and Lincoln University.

 

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