Trapdoor spiders are supposed to be creatures of forest floors and grassy banks. They dig burrows into soft earth, cap them with a hinged lid and wait in the dark for something edible to walk past. So when entomologist Max Tercel swept his headtorch across a bare stretch of wave-battered coastal rock on Ilot Vacoas, a one-hectare islet off the south-east coast of Mauritius, he wasn’t expecting to see a familiar shape: a spider’s legs reaching out of a burrow.

“It was dark by then and I was carefully searching through crevices with my headtorch when, to my astonishment, I noticed a familiar pattern: a spider’s legs extending from its burrow – something I’d only ever seen before in lush forests and grasslands,” Tercel told Discover Wildlife. “I was completely stunned.”

That sighting in May 2023 has now become a formal species description. Published in the journal Arachnology and announced by the Durrell Wildlife Conservation Trust in late October 2026, the animal is named Idioctis marevallum, the Mauritian tidal trapdoor spider. Its species name comes from the Latin for “sea wall”.

A home the sea floods every day

What makes I. marevallum remarkable is where it lives. Every nest the team found sat in coralline rock in the intertidal zone: rock that is either fully underwater at high tide or hit by splashing waves every day. Most were on the southern faces of the islets, where wave action is strongest. The researchers searched every other accessible part of both islands at night and found no further nests.

The spider doesn’t dig its own burrow, either. It moves into tubes in the coralline bedrock that are thought to have been bored by worms and molluscs thousands of years ago. The researchers describe nests up to about 12 centimetres long, lined with webbing and sandy material. Each one is capped with a trapdoor made from whatever is on hand (rock, sand and algae) that covers the entrance completely and blends almost perfectly into the surrounding stone. A Wildlife Preservation Canada volunteer who helped with the surveys described the doors as “only about the size of a quarter” and “nearly invisible”.

The team ran a simple field test to see whether those doors really keep the sea out. They poured seawater directly over a nest for about 20 seconds, then quickly opened the trapdoor with forceps to look for water inside the tube. They found none. They also noticed that the spider seemed to pull down on the door while the water was poured, which may tighten the seal. The nests themselves faced in all directions: some pointed straight down, some ran sideways and some pointed up.

Idioctis is a genus of spiders in the family Barychelidae that is known for living by the sea, with relatives on Pacific and Indian Ocean coasts. The new species is distinguished from its ten described relatives by details of the female reproductive anatomy: it lacks the secondary lobes on the spermathecae that the others have. The authors suggest it is most closely related to I. eniwetok.

“Someone screaming at me to come over”

Barychelids are mygalomorphs, the same broad group as tarantulas, and the people at the scene understood straight away how unlikely the find was. “The thing I remember most is Max screaming at me to come over because he’d just seen a mygalomorph – a trapdoor spider,” said Nik Cole, Durrell’s Mauritius country manager. “I couldn’t believe something from the same group as tarantulas could be living in a place like this.”

Cole says he was up all night, holding his phone in the air to get a signal and searching for any record of such a spider in Mauritius. At first the team wondered whether they had rediscovered Tigidia mauriciana, a species described in 1892 and detected only once since. It turned out to be something new, despite what Cole called “decades of nighttime reptile surveys on these islands”.

Soon after the first find, the team found the spider on a second islet, Ile aux Fouquets, also off the south-east coast. The holotype, the specimen that formally defines the species, is a female dug out of a horizontal burrow about 10 to 12 centimetres deep on Ile aux Fouquets on the night of 31 May 2023. It is now held in Durrell’s collection in Jersey.

Found because someone was restoring the islands

The discovery came out of conservation work rather than a dedicated spider hunt. The surveys were part of the Mauritius Island Restoration Programme, a partnership between Durrell, the Mauritian Wildlife Foundation and the National Parks and Conservation Service, with support from the Forestry Service. The programme works to restore the small offshore islets that serve as refuges for native Mauritian wildlife.

The same paper shows how much was waiting to be catalogued. Alongside the new trapdoor spider, the authors record 14 spider species new to the Mauritian archipelago, which they say represents a 27.45 per cent increase in the country’s known spider diversity. They call I. marevallum a “flagship endemic”: a species found nowhere else that can draw attention to the islets it lives on.

That raises an obvious concern. The spider has so far been found on only two very small islets, and only on their most exposed rock. Its whole known world is a narrow band of tidal stone between land and sea. That is just the kind of habitat that rising sea levels, coastal disturbance or a single bad storm season could change.

For now, the Mauritian tidal trapdoor spider is a reminder that new discoveries don’t always come from remote rainforests. Sometimes they turn up on the bare rock of a one-hectare islet, behind a door no bigger than a coin, waiting for the tide to go out.

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