Spiders in Dark Caves Are Evolving to Go Blind

Spiders are some of nature's creepiest and crawliest creatures, but some species might be getting a little bit less scary: they're going blind.

New research published in the journal Molecular Phylogenetics and Evolution has found seven new species of funnel web spider across several caves in Israel, all of which are at various stages of evolving to be blind.

"In this study, we sought to understand the evolutionary relationships between funnel web spiders with normal eyes that are found at the cave entrance, with those that are further in the cave and are pigmentless, eye-reduced, and even completely blind," said ecologist and co-author of the paper, Shlomi Aharon, from the The Hebrew University of Jerusalem (HUJI), in a statement.

The researchers collected the spiders by hand, examining their body plan, and extracted DNA samples to compare them to other species to study how they evolved.

Five of the seven new species were found to have reduced eyes, and two were completely blind, with their eyes having almost entirely disappeared.

Across all the species of spiders studied, both previously known and new to science, the researchers found several caves occupied by spiders that exclusively inhabit the twilight or dark zones deep inside the caves, and other caves containing spiders living at the entrances of the caves that still relied on the light to some extent. Some caves contained both types of spider, getting increasingly blind the further you went into the cave.

Other animals have lost the ability to see over time, notably the Mexican blind cavefish.

In the dark depths of a cave, an animal has no dependence on having eyes. Therefore, if the metabolic cost of eyes is very high, or if they randomly mutate worse and worse vision over time—or both— there is no strong evolutionary pressure for them to keep their eyes.

"Any animal that lives in permanent darkness and doesn't need vision to find food or avoid predators won't really need their eyes or visual centers in the brain," Damian Moran, a researcher at Seafood Technologies Group, Nelson, New Zealand, told National Geographic in 2015.

Eventually, the species may evolve to be blind. This does not always happen, however, and there are many species that live in near-total darkness but have retained their eyes over evolutionary time.

"Among the spiders we found, five were unique to different caves, and the two other species were found in several caves in the Galilee and in caves situated at the Ofra karst field, which is now under threat due to construction plans," HUJI ecologist and paper co-author Efrat Gavish-Regev said in the statement.

The researchers also found that the spiders that had adapted to the dark sections of the caves by losing their eyes could not survive outside of the darkness of the cave, implying that there was very little mating between deep cave and cave entrance populations, even if these populations were physically close. Additionally, they found that the deep-cave spiders were more closely related to deep-cave species in the Mediterranean than they were to their much closer cave-entrance neighbors in Israel.

"One of the surprising findings in the study show that the new species are evolutionarily closer to species from caves in Mediterranean areas in southern Europe, than to species living in close proximity to them at cave entrances in Israel," said Gavish-Regev.

This led the researchers to suggest that a single species is the common ancestor of all the blind and semi-blind cave spiders, with the varying degrees of sight loss having evolved independently in each branching population. They also suggested that the deep-cave and cave-entrance branches were from separate populations rather than a single branch going deeper and deep into a cave and gradually losing its sight.

This is known as convergent evolution: when similar evolutionary traits emerge due to similar environmental pressures rather than a common ancestor with that trait. Other examples of convergent evolution are wings in bats, birds and flying insects, or alternatively, the use of echolocation (sound waves reflected back from objects) to detect prey and the surrounding environment in bats and dolphins.

The researchers theorized that the original species went extinct locally outside of the caves, leaving only their cave-dwelling descendants. This extinction event is thought to have occurred around five million years ago and have been as a result of climate change during the beginning of the Pliocene era.

"We are currently witnessing the effects of climate change on many habitats, which obliges us to consider, maintain, and promote programs that include the preservation of underground habitats—many of which are at immediate risk," said Professor Dror Hawlena from the Department of Ecology, Evolution, and Behavior at The Hebrew University of Jerusalem.

"We must protect Israel's unique nature, preserve its underground systems for the future, and further explore the processes that created these systems in the country."

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