Tiny, eyeless spiders naturally sense light so they will stay in the dark, study says

(CN) — A family of eyeless spiders dwelling in dark caves can sense light.

Approximately one to three millimeters in size, Leptonetela genus spiders live in the karst landforms of southwest China and the Balkan Peninsula in Europe. In their study published Wednesday in Science Advances, researchers from Hubei University and the University of Iceland focused on 10 Leptonetela species from caves or cave entrances in the Yunnan-Guizhou Plateau, China.

Unlike most spiders that have eight eyes, Leptonetela species have only six, which the study says range in size from highly reduced to eyeless. The spiders lack anterior median eyes, which are generally used to identify and stalk prey. But to spiders that live mostly in the dark, those eyes may not be useful.

However, the researchers found through genetic analyses that regardless of whether they lived near the cave entrance or cave depths, all 10 of the studied species had nearly complete phototransduction pathway genes ("PPGs"). Phototransduction is when light converts into electrical signals in the retina and is understood by the rest of the nervous system.

To understand why the spiders had PPGs, the researchers collected five entrance-dwelling species and five cave-dwelling species before placing them in boxes throughout the Manwang Cave site.

According to the study, 85% to 100% of individuals from each of the 10 tested species chose to stay in the dark areas of the boxes, regardless of the degree of eye degradation. And while 35% to 60% of individual spiders did not show a preference for dark areas if the boxes were transferred into the deep cave, a significantly greater number of individuals for each Leptonetela species remained in dark areas when exposed to the cave entrance.

Initially, the researchers thought that Leptonetela spiders retained light perception, and their photophobic behavior, as a survival mechanism for their weak drought resistance. According to the study, the cave-dwelling spiders forced to live in the cave openings without additional water began to die off in a way that suggested a higher sensitivity to dry air. However, the need for water did not account for the cave-dwelling spiders' tiny size, because they risked death if so much as a single water drop from the deeper parts of the cave fell on them.

Thus, the researchers theorized that cave-dwelling spiders, and their entrance-dwelling counterparts living in high relative humidity environments, preferred relatively lower humidity environments as a balance to keep them safe from death by dry air or a drop of water.

Study coauthor Dr. Jie Liu of Hubei University said the team believes that natural selection allowed cave-dwelling spiders to detect light so they could avoid the dry habitats outside of the caves.

“Our results offer evidence that they probably retained the original ocular phototransduction system derived from eyed ancestors, and this allows them to sense light, even without a functional eye structure,” said Liu via email. “However, we were unable to determine whether extraocular photoreception existed in eyeless or eye-degraded cave-dwelling Leptonetela spiders, and thus further studies are required in the future. Since many cave-dwelling Leptonetela spiders are found in caves within primitive forests, the collection process can involve unpredictable dangers. We are well aware of the challenges of field research work. With this in mind, we would like to express our respect to all field research workers.”

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