A Cobalt Blue Tarantula (Hapolpelma lividum), with brilliant cobalt blue hair-like setae on its legs. Credit: Bastian Rast

For what reason are a few tarantulas so vividly colored? Researchers have considered why these enormous, hairy spiders, active basically during the night and at evening time, would game such vibrant green & blue coloration—particularly as they were for some time thought to be not able to differentiate between colors, let alone possess — true color vision.

In an ongoing study, Scientists from Carnegie Mellon University (CMU) and Yale-NUS College discover uphold for new theories: that these dynamic blue colors might be utilized to convey between expected mates, while green color gives the capacity to hide among the foliage. 

Their study additionally recommends that tarantulas are not as color-blind as previously accepted and that these arachnids may be able to see the splendid blue tones on their bodies.

The study was published in Proceedings of the Royal Society B on 23 September & is included on the front page of the current (30th September 2020) issue.

The research was jointly led by Dr. Saoirse Foley from CMU, and Dr. Vinod Kumar Saranathan, in collaboration with Dr. William Piel, both from the Division of Science at Yale-NUS College. To understand the evolutionary basis of tarantula coloration, they surveyed the bodily expression of various opsins (light-sensitive proteins usually found in animal eyes) in tarantulas. They found, contrary to current assumptions, that most tarantulas have nearly an entire complement of opsins that are normally expressed in day-active spiders with good color vision, such as the Peacock Spider.

These findings suggest that tarantulas, long thought to be color-blind, can perceive the bright blue colors of other tarantulas. Using comparative phylogenetic analyses, the team reconstructed the colors of 110 million-year-old  ancestors and found that they were most likely blue. They further found that blue coloration does not correlate with the ability to urticate or stridulate—both common defense mechanisms—suggesting that it did not evolve as a means of deterring predators, but might instead be a means of attracting potential mates.

The team also found that the evolution of green coloration appears to depend on whether the species in question are arboreal (tree-dwelling), suggesting that this color likely functions in camouflage.

“While the precise function of blueness remains unclear, our results suggest that tarantulas may be able to see these blue displays, so mate choice is a likely potential explanation. We have set an impetus for future projects to include a behavioral element to fully explore these hypotheses, and it is very exciting to consider how further studies will build upon our results,” said Dr. Foley.

The team’s survey of the presence of blue and green coloration across tarantulas turned up more interesting results. They found that the blue coloration has been lost more frequently than it is gained across tarantulas. The losses are mainly in species living in the Americas and Oceania, while many of the gains are in the Old World (European, Asian, and African) species. They also found that green coloration has evolved only a few times, but never lost.

“Our finding that blueness was lost multiple times in the New World, while regained in the Old, is very intriguing. This leaves several fascinating avenues for future research when considering how the ecological pressures in the New and the Old Worlds vary,” said Dr. Saranathan. “For instance, one hypothesis would be differences in the light environments of the habitats between the New and the Old World, which can affect how these colors might be perceived if indeed they can be, as our results suggest.”