Red in a spider’s eyes

You probably never imagined the world through a spider’s eyes, but if you did, chances are you’d wonder how they consolidate the signals from eight different eyes. In that, however, they may not be as different from you as you might expect, because after all, we humans have to blend signals from parts of our eye that see differently: the peripheral vision that is better at low resolution motion detection, and our central vision that is better at high resolution colour images. Different animals have visual systems that differ in details, and yet perhaps their evolution responds to consistent selective pressures, or is constrained to find consistent solutions. And so, comparative biologists study many species to see if we can find the rules of visual evolution.

To better understand how vision evolves, a group of biologists led by Nate Morehouse of the University of Cincinnati have turned to jumping spiders (“salticids”), and in particular how they see colour. Jumping spiders have exquisite eyesight — as you can tell by watching a jumping spider watch the world — and yet it was an open question as to how well they could see colour. Studies suggested that some (most?) jumping spiders are as colour-blind as a dog, unable to distinguish red from green.

However, some species of jumping spider gave us a strong hint that they can see and distinguish red: their males have fancy plumes and scales and spurs full of colour, including red. These include the now-famous peacock spiders of Australia (Maratus) and the paradise spiders of North America (Habronattus). Here’s the wonderful Habronattus americanus.

Male Habronattus americanus

That the males display these ornaments to the females during courtship suggests that these females can likely distinguish red.

After Daniel Zurek, Morehouse and colleagues discovered a ruby-red filter in the eyes of Habronattus that gave them the ability to distinguish red, Nate pulled together an international team to see whether and how other jumping spiders could see red, including Megan Porter (University of Hawaii), Lisa Taylor (University of Florida), and myself.

What motivates us is this simple observation: There are only a few lineages of salticids with red male courtship ornaments, and those are scattered in isolated pockets on the evolutionary tree. Most groups of jumping spiders do not obviously (to our eyes!) use red in their male courtship ornaments, and so we might surmise they lack the ability to distinguish red. The scattered distribution of red ornaments hints that the ability to see red has evolved several times independently.

Independent evolution of a similar trait is magic to an evolutionary biologist, because it offers the possibility of discovering the rules of evolution: Under what circumstances does red-distinguishing vision evolve, typically? What are the consequences of evolving this ability?

Salticids give us a special opportunity to answer these questions. Our preliminary assessment is that red-distinguishing vision has evolved multiple times independently in jumping spiders, perhaps more than a half dozen times. That’s a remarkably large number for a group of terrestrial animals that has diversified in only the last 60 million years. And so, our team will survey across the evolutionary tree of jumping spiders to find out what colours their retinae are sensitive to, what light environments they live in, and how it affects their lives.

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