Two golden silk orb weavers lived on the ISS for two months; most space-built webs became nearly symmetric, but under light the spiders used brightness as a new sense of up

Every astronaut aboard the International Space Station had to get used to microgravity. Two spiders had to solve the same task, and the way they did it astonished the scientists watching. A study titled "Spiders in space-orb-web-related behavior in zero gravity," published in Science and indexed on PubMed, describes an experiment with two young adult golden silk orb weavers. The two arachnids were sent to the International Space Station, each in its own separate habitat, for two months in 2011, while two more of the same age and species stayed behind on Earth in identical, separate habitats as a comparison group. Scientists wanted to see how the absence of gravity would affect spiders' ability to make their signature webs and whether they could notice any differences between the two groups.

The species built for asymmetry

On Earth, golden silk orb weavers build impressively irregular webs. Their centers, or hubs, tend to be more towards the top edge, rather than the middle, taking advantage of gravity to help them pounce on any unwary insect that dares venture into their midst. Researchers believed their webs would be even less symmetrical in space, since the astronauts could not easily observe the spiders' movements. And since the spiders are usually positioned at the top of their web facing downward, they had to be placed near the top of the box. Researchers chose this species because previous studies showed that its webs are consistently asymmetrical on Earth. The scientists may have been especially careful because an earlier mission had failed. The first space-station spider experiment in 2008 used two different spider species; it wasn't canceled but was ultimately compromised when one spider escaped its enclosure and got into the other's habitat, and its web-building interfered with and partially destroyed the first spider's web. On top of that, the flies intended as their food proliferated, producing two webs that were difficult to interpret. The two spiders on board in 2011 were kept in separate enclosures to avoid such a problem.

What actually happened when gravity disappeared

Scientists analyzed 100 webs built in zero gravity over two months, along with more than 14,000 images of spiders in their resting positions. They found that most webs built by spiders in zero gravity were more symmetric than typical webs built by the same species on Earth. The hub of the web was closer to the center, and the overall shape had fewer irregularities. Not all zero-gravity webs were symmetric. Some were still asymmetric, which led scientists to examine another factor affecting web building: the habitat lighting.

Light as the spiders' new sense of up

The habitat's lamps were at the top of the enclosure, not on the sides. When researchers divided the webs up by whether construction began while the lights were still on or after they'd been switched off, an uneven pattern emerged. Compared with webs built after the lights went off, webs built while the lights were on were more imbalanced, with the hub pulled toward the top of the enclosure, where the lamps were mounted. It was as if a magnet had replaced Earth's gravity. The spiders that spun their webs in darkness oriented themselves more or less randomly around the hub. The spiders appear to have used the overhead light as a stand-in for gravity. Samuel Zschokke, a biologist at the University of Basel, said he had not expected light to affect the webs because spiders had evolved under gravity. Because the lamps happened to be mounted at the top of the habitat rather than on the sides, the researchers were able to observe exactly how the lights affected the balance of web construction.

How we understand animal senses

The finding goes beyond showing a useful adaptation that helped spiders cope in an unfamiliar environment. Instead, it offers insight into what happens within a living system when one sensory cue is unavailable. Spiders' asymmetric web building and gravity-based orientation are similar to other sensory behaviors in which animals use one cue to detect a stimulus, such as vibrations from insects trapped in their silk. The spiders did not build webs at random when their primary orienting cue was removed; instead, they used another cue in their environment, a process scientists rarely observe directly. The study also shows the value of the International Space Station for investigating fundamental biological processes relevant to more than one species. The next time such an experiment is launched, researchers will be especially interested in signs that animals may use other sensory inputs when their preferred one is lost.