杏吧原创

An island spider decides which of its three kinds of webs to make depending on location and perhaps individual preferences.

Spiders usually make only one kind of web, but the Wendilgarda galapagensis spider 鈥� which lives exclusively on Cocos Island, about 550 kilometres off the western coast of Central America 鈥� can make three different webs.

High above ground it makes 鈥渁erial鈥� webs attached to nearby stems and leaves. Nearer to the ground it makes 鈥渓and鈥� webs with long horizontal strands secured between branches and with a series of vertical strands anchored to the ground. Finally, over pools it makes 鈥渨ater鈥� webs that are a bit like the land webs, but with the vertical strands attached to the water surface itself.

Darko Cotoras at the California Academy of Sciences in San Francisco wondered whether this flexibility in web construction indicates the spider is undergoing speciation, splitting into three distinct species, each with unique behaviours and exploiting a different food source. So with his colleagues he ran genomic analyses on 142 of the spiders.

To the team鈥檚 surprise, the results revealed that all of the spiders belonged to the same species, says Cotoras. This means they haven鈥檛 genetically diversified since arriving on the newly formed volcanic island perhaps as long as two million years ago, when their ancestors were probably carried in by air currents.

The researchers then marked the 2-millimetre-long spiders with drops of fingernail polish and moved them to different locations on the island to track their behaviour. For example, they took water-web-making spiders away from water sources and placed them in high bushes nearby. Again, the researchers were surprised to see that the spiders often built a new web with the architecture suited to the new location.

鈥淭his level of individual differentiation in terms of architecture, construction behaviour and microhabitat is unparalleled compared with other spiders,鈥� says Cotoras.

The species probably developed such flexibility because it helps the tiny spiders thrive on such a small, isolated island, he says.

鈥淚t seems they adapt their behaviour to perform multiple roles, sort of like people do when they live in a small town as opposed to a big city,鈥� says Cotoras.

That doesn鈥檛 mean the spiders necessarily like the idea of getting moved, however. In the experiment, Cotoras found that when some spiders were moved to a new microhabitat they promptly crawled back to the microhabitat they had been occupying before the experiment.

鈥淭hey might have intrinsic preferences, sort of like humans do, and maybe a preferred kind of web,鈥� he says.