Mating progeny of a male Maratus avibus and female Maratus caeruleus. Both are hybrids of two species. [Image 6 of 15 related images. See Image 7.]
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Nate Morehouse, a biologist at the University of Cincinnati (UC), is studying tiny spiders no larger than a ladybug that have the rare ability to see color, including the bright and bold color patterns on the males' bodies. Morehouse is looking inside the many eyes of two groups of vividly colored spiders in the family Salticidae--better known as jumping spiders. These two groups--the tiny arachnids classified as the Habronattus jumping spiders of North America and the Maratus "peacock" jumping spiders of Australia--are no larger than a ladybug.
Males in both groups are talented dancers that display fancy footwork and have bright physical displays of orange, pink, red and yellow. The peacock spiders are additionally equipped with an elaborately decorated abdomen flap that they raise up and down like a flag.
"Itís rare to see bright colors on most spiders, as they donít usually have the visual sensitivity to perceive color beyond drab blues, greens and browns," says Morehouse. "But certain groups of jumping spiders deviate from this pattern. They not only possess a unique ability to see reds, yellows and oranges, but the males display those same bright colors on the exterior of their faces and other body parts [that] they use in their elaborate courtship dances." Morehouse discovered that these two groups see color using completely different mechanisms.
The Habronattus spiders possess a red filter on their retina that combines with their green sensitive retinal cells to be able to see reds, yellows and oranges. The Maratus spiders, on the other hand, have evolved a completely new type of retinal cell that is sensitive to red and does not require a filter.
Morehouse says this is a remarkable discovery, since the two groups of jumping spiders evolved on opposite ends of the Earth. "Both have the rare ability to see long wavelength colors like red, orange and yellow, but each group has arrived at independent solutions for seeing the color," says Morehouse. The peacock spiders see color through ultraviolet, blue, green and red sensitive cells within their eyes, which is most similar to birds.
To characterize the sensitivities of both novel color visual systems Morehouse used microspectrophotometry (measuring directly how photoreceptor cells in the retina absorb light differently) and visual system modeling (using mathematical models to estimate how the retina perceives color). He found that Habronattus employs a red filter to create a third type of photoreceptor cell predominantly sensitive to red light and Maratus uses no filters, but has two additional types of photoreceptors, one blue sensitive and one red sensitive.
"These additional photoreceptor cells are likely a product of a gene duplication that has subsequently evolved to be sensitive to a different range of colors, similar to the way humans and other higher primates evolved to see color," says Morehouse. "Somewhere early in primate evolution the gene responsible for the protein that gives us our green sensitivity got duplicated into two copies. One of these genes called an opsin gene mutated without affecting the other and those mutations eventually led to one of the copies becoming red sensitive. This may be what happened in the Maratus."
This research was supported in part by the National Science Foundation (grant IOS 15-57549).
To learn more about this research, see the UC Magazine story Tiny dancers--When jumping spiders show their true colors, UC biologist looks through the lens for the reasons. (Date image taken: Unknown; date originally posted to NSF Multimedia Gallery: May 1, 2017)