Many tourists travel to the Portuguese island of Terceira each year to marvel at Algar do Carvão, a 3,200-year-old volcanic vent that drops 90 metres into the earth. While admiring the geological spectacle, the very astute among them might spot a tiny, pale spider clinging to a web on the cave's wall. This is Turinyphia cavernicola, and Algar do Carvão is the arachnid's last hope for survival.

Turinyphia cavernicola is in serious decline. The subterranean spiders can be found in two other caves on Terceira, but their populations there are too small to be sustainable. That means the species now meets the IUCN's Red List criteria for a 'Critically Endangered' listing. 

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Male of Turinyphia cavernicola from Algar do Carvão (Terceira, Azores. Image: Paulo Henrique Silva

In an effort to fast-track an assessment from the IUCN, which could help mobilise species-saving conservation efforts, a team of researchers from the University of Azores teamed up with colleagues from the University of Barcelona and the Finnish Museum of Natural History to create a Species Conservation Profile for the spiders. Published in the Biodiversity Data Journal, the profile outlines data about the plummeting population trend of Turinyphia cavernicola and, if all goes to plan, should ultimately help ensure that the species gets added to the Red List.

It's the critters' troglodytic love for humid lava tubes that has put it in such a perilous situation. The arachnids can survive only in volcanic pits like Algar do Carvão, and their home is under severe threat as a result of increased farming activity, road construction and the throngs of tourists that visit the caves each year.

"These spiders never leave their underground habitats, which are strictly humid lava tubes and volcanic pits. There they build sheet webs in small holes and crevices on the walls of the caves," a press release from Pensoft explains.

Turinyphia cavernicola was discovered just eight years ago, and not much is known about the distribution of the species, but researchers believe that significantly larger populations existed in the past.

Fifteen other cave sites were thoroughly examined in the area, but no trace of the elusive spiders was found. In the past, many of the caves were covered by dense native forests, but as the natural vegetation has been systematically wiped out to make way for grazing pastures, cave environments have been altered.

For a species that thrives in muggy conditions, the destruction of native forests has had a big impact. Humidity levels and availability of resources in many of the island's caves have been reduced, pushing the spiders closer to extinction. For the spiders living in Algar do Carvão, however, tourism is the greatest threat. As more and more people flock to the popular geological attraction, the delicate balance of the cave system is distrupted and the spiders suffer as a result.

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The volcanic pit Algar do Carvão (Terceira, Azores), the main location of the species Turinyphia cavernicola. Image: Pensoft

So is there hope for the lava tube-loving arachnids of Terceira? Experts suggest that protecting the spider's habitat and carefully monitoring their population trends is vital to saving the species.

The area around Algar do Carvão was classified as a "Regional Natural Monument" in 2004, and since "pasture intensification is one main threat, this might be important to safeguard the species survival in the future and should be extended beyond the current area, possibly allowing the recovery of other caves to original conditions where the species might be reintroduced," the researchers explain.

For now, the future of the species remains uncertain, but researchers hope that a critically endangered status will help get it on the map and ensure its survival.

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Female of Turinyphia cavernicola from Algar do Carvão (Terceira, Azores). Image: Pedro Cardoso