The Bone Cave harvestman: sounds kind of like a nickname for the Grim Reaper, doesn't it?

The ghoulish-sounding moniker, though, actually refers to a stone-blind creature smaller than your fingernail that spends its entire life hidden away in shrouded caves in one small corner of Texas.

A member of the "daddy longlegs" tribe of spider-like arachnids, the Bone Cave harvestman (Texella reyesi) comes suited in handsome orange and hunts other tiny invertebrates in the limestone underground of Travis and Williamson counties, well out of sight of humankind.

A harvestman pair deep in a cave in Travis County, Texas. Image: Piers Hendrie/Wikimedia Commons

You'd be forgiven for assuming this low-profile troglobite (or cave specialist) is also firmly out of mind. But the Bone Cave harvestman has turned into an improbable tiny little flashpoint of controversy.

The eyeless arthropod is currently at the centre of a lawsuit raising fundamental questions about the reach of the US Endangered Species Act (ESA), the conflict between conservation and property rights and our valuation of organisms in general.

The US Fish and Wildlife Service (USFWS) added the Bone Cave harvestman to the Endangered Species List back in 1988, along with several other endemic troglobites from the same area with similarly superstar names, the Kretschmarr Cave mold beetle and the Tooth Cave pseudoscorpion among them.

The current lawsuit stems from a 2014 petition to take the harvestman off the list. The petitioners included several local landowners and developers, as well as the non-profit American Stewards of Liberty, described on its website as "dedicated to protecting private property rights, defending the use of our land, and restoring local control."

The petition claimed that the Bone Cave harvestman had proved more widespread than initially believed, and was adequately protected in many of its cave abodes. It also argued that the critter's endangered status put onerous limits on landowners' abilities to use or develop their properties.

"The Service originally listed the harvestman in 1988 based on extremely limited information," the American Stewards of Liberty argues. "In light of new data collected over the last 26 years, it is now obvious that the listing was a mistake. The harvestman's endangered status is insupportable, and the Service's erroneous listing has done significant economic harm to landowners and local governments."

The Devils Hole pupfish was one of the first species protected by the Endangered Species Act, and it's been the focus of much debate over its conservation since. Image: Olin Feuerbacher / USFWS

The USFWS has responded to this petition twice: initially in 2015 and, because of the current lawsuit, in a court-ordered finding this May. Both times the agency wasn't convinced, concluding that ESA protection for the harvestman is still warranted.

To some extent, the case of the Bone Cave harvestman resembles that of many other small, rare and unassuming creatures. It's not news that your average person is more captivated by a tiger or elephant than a minuscule subterranean invertebrate whose entire geography could be described as a smidgen. As in the plight of the similarly obscure Devils Hole pupfish (less than 200 of which inhabit a single groundwater pool in a Nevada cavern), the Bone Cave harvestman challenges us to reflect on the inherent or existential value of an organism.

And while a cave-dwelling spider lookalike might seem like a perfect example of a "useless creature", it's worth looking beneath the "surface" of things (if you will). As Travis Audubon representative Joan Marshall told Fox News in March, "The real importance of the Bone Cave harvestman is that it's an indicator species. It signifies the health and well-being of the environment it occupies."

That environment – those nook-and-cranny voids in the local limestone basement – might appear pretty well insulated from the human ruckus above. But the ecosystem of karst caves is actually tangled up with that of the surface, and the highly specialised Bone Cave harvestman represents the delicacy of that tangle. 

You'll find the Bone Cave harvestman only in Travis and Williamson counties in Texas – its entire geography can easily be described as a smidgen.

The realms above and below are linked by drainage from runoff and groundwater. And the food web of the light-starved caves depends mainly on nutrient deliveries from the surface: from plant matter and animal carcasses, to the creatures that commute between the sunlit and subterranean worlds. In the karst country of central Texas, cave crickets are particularly important commuters – in fact, USFWS guidelines suggest a 105-metre radius around target caves to provide cricket foraging space. 

Keeping vegetation near cave openings intact also helps to regulate drainage and buffers the underground from extreme weather. Because they've adapted to the stable world of dim, dank grottoes, troglobites have lost some of the thermoregulation skills of their aboveground relatives, making them dependent on cool conditions and high humidity. In torrid summer months, the Bone Cave harvestman is known to beeline for the coolest, dampest innards of its caves. 

It's a fragile sunken environment – one that's easily disturbed by human action, from groundwater and runoff pollution to direct cave destruction due to development. In its most recent five-year review of the species, the USFWS concluded that the Bone Cave harvestman faced the same suite of threats that led to its original listing, including the spread of exotic species such as the red imported fire ant, a potential predator of the harvestman (and of cave crickets).

And then there's climate change, which the review recognises as an emerging problem for the species. Should global warming intensify drought and heatwaves in south-central Texas, the harvestman might well serve as something of a canary in the karst cave.

A freshly moulted harvestman in a cave in Travis County, Texas. Image: Piers Hendrie/Wikimedia Commons

In the face of such challenges – and the fact that we don't know a ton about the hard-to-survey harvestman – the USFWS emphasises the value of protecting enough populations across its small range. This, the agency reasons, helps to preserve a diverse gene pool – and safeguards the species should a catastrophe (like flooding, disease or a leaking pipeline) hammer or eliminate harvestman numbers in one area.

This is the thrust of the agency's response to the claim that the harvestman no longer warrants endangered status.

In its original assessment of the 2014 petition, the USFWS acknowledged that numerous additional harvestman populations have been found since the species was declared endangered. But the agency also contends that "species are listed under the Act based on threats and not just the number of sites or size of the range." 

As for the claim that there are plenty of adequately protected harvestman habitats already, it argues that many of the existing preserves aren't big enough to satisfy protection guidelines for endemic karst invertebrates (such as allowing for that buffet buffer of crickets).

Meanwhile, the petition to delist the harvestman is one thing; the lawsuit that arose from it tackles much bigger territory. In it, the American Stewards of Liberty and its fellow plaintiffs argue that the USFWS is overstepping its bounds by invoking the interstate commerce clause in managing the Bone Cave harvestman. The clause helps establish some of the constitutional authority for Endangered Species Act regulations, but its validity for animals like the harvestman, which are restricted to a single state, has become a major source of controversy and legal wrangling. (For an overview of the whole commerce clause/ESA imbroglio as it pertains to the harvestman, check out this Austin American-Statesman article.)

Williamson County rancher John Yearwood, whose 865-acre property includes three caves known to harbour Bone Cave harvestmen, is among the original petitioners, and a plaintiff in the lawsuit against the USFWS. He argues the threat of steep fines or imprisonment for potentially impacting his resident arachnids impinges on his freedom to use the land, and negatively impacts his property's value. 

"This isn't just about a cave bug," he told the Austin American-Statesman. "It's about the private property rights, about overreach from the government."

Meanwhile, as developers and conservationists debate the Bone Cave harvestman in Texas, a similar battle is being waged over the smallest species of prairie dog, found only in the state of Utah.

Last year, the Center for Biological Diversity warned of the broader implications of such efforts to undermine environmental safeguards. "Should this radical argument prevail, it would undermine federal protections for many other imperilled species," it noted.

It's possible the current harvestman-centred lawsuit could go to the US Supreme Court. In the meantime, the USFWS is undertaking a new five-year status review of the species, expected to wrap up at the beginning of next year.

In other words, a surprising number of eyes remain laser-focused on the eyeless (and blissfully unaware) Bone Cave harvestman.