Catonetria caeca is a spider species endemic to Ascension Island. The species has been assessed as Critically Endangered under criterion B (CR B1ab(iii,v)+2ab(iii,v)) as it is known only from a single, tiny cave locality and is therefore has an extremely limited extent of occurrence (EOO) and area of occupancy (AOO) and occurs in just one location. The cave appears to be shifting in microclimate and, despite extensive survey effort in 2022/2023, this species could not be detected. As such, it is likely that environmental change has reduced C. caeca populations and is threatening the species with imminent global extinction.

Catonetria caeca is a cave spider endemic to Ascension Island in the South Atlantic Ocean. The species has only ever been observed in the dark zone of Packer’s Hole Cave (locally known as “Bat Cave”), which is within one of the most recently-active volcanic areas on the island.

Catonetria caeca is known from only four collected specimens: one taken in 1990 and three more taken in 1995 by the same surveyors (Ashmole and Ashmole 1997). Extensive surveys were undertaken in 2022 and 2023 to try and relocate this spider, but failed to detect even one individual. The surveys were conducted by a different surveyor who followed the methods of Ashmole and Ashmole (1997), and spent a total of over ten hours divided across multiple days in this search. This observed decline in C. caeca abundance represents a decline in population across the entirety of its known extent of occurrence, however it is not possible to determine the time period over which this decline happened: it may have occurred before the 10 year/three generation time period required for criterion A.

With the species only having been observed on two occasions, little is known about Catonetria caeca. The species is a member of the family Linyphiidae or sheet weaver spiders. It is thought to have evolved from a yet-undiscovered sister species, which arrived on Ascension via “ballooning” (Millidge and Ashmole 1992, Ashmole and Ashmole 2000). This dispersal behaviour, common in Linyphiids, is responsible for the large global diversity of the family. C. caeca has become adapted to cave habitat: it has no eyes, and is therefore blind, and lacks pigmentation.

The sole cave where this species has been recorded from comprises a collapsed volcanic fumarole with an 18 m deep opening cleft. The cavity of the cave is 49 m long but the dark zone, where this species was found, is separated at one end by a narrow fissure and is itself around 8 m long. On the otherwise barren cave floor, (exo-)skeletal remains of land crabs (Johngarthia lagostoma) and black rats (Rattus rattus) are common. The cave is notable for being the most elevated (185 m) fully dark space on the island. It is unlikely that this small cave is the only suitable habitat for C. caeca, which may be able to utilize the tiny subterranean cracks running through the volcanic landscape. However, other caves may be experiencing changes similar to those recorded in Packer's Hole (see the Threats section).

This spider's webs have been described as “tiny” and “filmy” (Millidge and Ashmole 1992). It could be inferred that due to the spider’s size (1.25 mm) and the low diversity of invertebrates in its cave locality, the species predominantly feeds on endemic springtails (Pseudosinella lava). These springtails were by far the most abundant invertebrate in the dark zone of the cave where this spider was found.

All existing information on Catonetria caeca suggests that the spider is immediately threatened with global extinction. The most significant threat is a measured increase in air temperature in the sole cave that this species is known to inhabit, between the years of 1990 and 2023. In March 1990, at 10 am, a temperature of 24.4°C was recorded by Ashmole and Ashmole (1997). Multiple temperature measurements were taken at 10 am on multiple March days in 2023, at different places in the inhabited cave dark-zone, and all returned 25.0°C. A rise of just 0.6°C is likely to be highly significant to such a tiny species, which is well-adapted to the stable microclimate of a cave interior, and therefore represents an observed decline in habitat quality. Packer's Hole, where C. caeca has been recorded from, is the most elevated fully-dark cave known on Ascension. As such, there are no large caves spaces at higher elevation which might provide a naturally cooler cave microclimate to the species. It can therefore be concluded that habitat quality has reduced across the species' entire extent of occurrence.

Other emerging threats are posed by invasive species. Ascension has become overwhelmed by invasive plants and invertebrates after centuries of both intentional and unintentional species introductions. The habitat directly above the cave remains natural: barren with sparse grasses. Mexican thorn trees (Prosopis juliflora), however, are spreading quickly across those barren areas, and it is likely that they will reach the cave area if left unchecked. This has the potential to further alter microclimate inside the cave, and roots growing through the cave ceiling may introduce non-native invertebrates to the habitat.

Live rodents appeared to be absent from the cave, with recent jam baits remaining untouched (and removed). This is positive, as rodents have caused significant ecological damage elsewhere on the island. There is significant potential for rodents to access the cave in the future. Human visitors to the cave have introduced a moderate amount of rubbish, including broken glass, plastic bags and used cigarettes. These items do not seem likely to directly reduce habitat quality for C. caeca, but edible rubbish risks luring rodents into the cave space. Rodents pose the potential threat of modification to habitat structure, and small species may even predate C. caeca.

There is no trade or use of this species

In consideration of the specific cave locality from which this species has been detected, there are limited options in mitigating climate change impacts at such a small scale. We suggest that maintaining volcanic substrate structure across the island and particularly at higher elevations is the only feasible solution. This would potentially provide suitable microclimate space for Catonetria caeca to persist or move into under inevitable climate change.

Preservation of C. caeca will therefore rely on invasive species control on Ascension. Broadly, non-native vegetation must be prevented from spreading over the barren, volcanic areas of the island. This should maintain the presence of tiny crevices and other subterranean spaces which might accommodate C. caeca, and prevent the accumulation of organic matter and other changes in substrate structure. Invasive species management should also limit further potential threats posed by non-native species, including direct predation and resource competition.