To characterize the sensitivities of both novel color visual systems [the way in which the eye itself perceives color] Morehouse used:

• Microspectrophotometry –– measures directly how photoreceptor cells in the retina absorb light differently. 

• Visual system modeling –– uses mathematical models to estimate how the retina perceives color. 


Morehouse has identified two distinct solutions for how these animals see color and both solutions are found within the retina of the principal eyes.

• Habronattus employs a red filter to create a third type of photoreceptor cell predominantly sensitive to red light.

• Maratus uses no filters, but has two additional types of photoreceptors –– one is blue sensitive, one is 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,” explains 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.”

A close-up of the octovision of a Maratus "peacock" jumping spider. The retinas of the principal eyes, the largest ones on the front of the head, are positioned at the end of long tubes and have multiple layers of cells, each sensitive to different colors of light. By angling each of their two primary front eye tubes just so, the spiders have binocular vision with excellent acuity and full color perception. The six secondary eyes on the side of their heads give them more or less 360-degree vision. photo/Jurgen Otto

Visual diversity shares common goal

Gene duplication is increasingly credited as a mechanism for evolving functions, Morehouse says, including new visual functions. In the case of the jumping spiders, color vision provides a valuable new trait for not only luring their mates, but is especially critical for successful foraging.

He explains these fuzzy fur balls as voracious predators eating a wide variety of small insects and some of their prey are brightly colored, which can signal their toxicity to other predators like birds and dragonflies. So fortunate spiders that can see these warning colors are also at an advantage for discriminating between brightly colored toxic prey and non-toxic prey.

“We think that more effective foraging is the major reason for the evolution of color vision,” says Morehouse.

But what about the bright colors of males? Wouldn’t this lead them to be more obvious to spider predators? Morehouse points out that bright male body colors are usually only observable from the front when the spiders are face-to-face.

The males may wave their legs around in a kaleidoscope of colors but Morehouse says it typically only shows on the underside. When birds observe these rare, multi-hued spiders from above they only see their mottled drab colors of brown, black and tan patterns on the surfaces facing upward.

The result is a clever solution: camouflage on top for predators and bright color displays in front for members of their own species.

Researchers in the Morehouse Lab capture the colorful and rhythmic courtship dance of a pair of North American Habronattus pyrrithrix jumping spiders. video/Zurek & Echeverri, Morehouse Lab

Morehouse hopes to continue this research by looking at a group of spiders in India that are brightly colored and unrelated to either of these two groups. “If we increase our knowledge of additional groups that have transitioned to more sophisticated color vision, we’re in an exceptional position to understand why color vision evolves in the first place and what consequences it has for color signaling behavior and the ecology of these species,” says Morehouse.

“And who knows, perhaps we will find inspirations for novel color sensing technologies.”

Funding:

• This work is partly funded by a current, NSF grant (IOS-1557549).

A female Maratus caeruleus "peacock" jumping spider spins a protective web around her eggs. photo/Jurgen Otto

Mating progeny of a male Maratus avibus and a female Maratus caeruleus. Both are hybrids of two species. photo/Jurgen Otto  

For this research: Nathan Morehouse acknowledges his appreciation for the tremendous contributions by his colleague photographers Thomas Shahan and Daniel Zurek for the North American Habronattus spider images and Jurgen Otto and David Knowles for the Australian Maratus spider images and video.

For more information:

• MorehouseLab
• Spectral filtering enables trichromatic vision in colorful jumping spiders 
• Flexible color learning in an invertebrate predator: Habronattus jumping spiders can learn to prefer or avoid red during foraging  
• Colour use by tiny predators: jumping spiders show colour biases during foraging