Fish’s ‘polarized ornaments’ are a real turn-on

"We saw the female fish checking out the large males and looking at their polarization ornaments," says Viktor Gruev. (Credit: Jill R/Flickr)

Just like peahens are drawn to a peacock’s colorful feathers, female northern swordtail fish choose their mates based on patterns visible in polarized light.

Marine biologists at University of Texas at Austin used a bioinspired polarization camera developed by Viktor Gruev, associate professor of computer science and engineering at Washington University in St. Louis to make the discovery.

polarization on a swordtail fish
A video polarimetry image of a large male northern swordtail in false color shows the differences in polarization on its surface. (Credit: Washington U.-St. Louis)

The camera has been used in other applications in marine biology and is also now being used to help physicians and researchers see cancer cells very early in development.

Published online in the Proceedings of the National Academy of Sciences Early Edition, the new study shows female swordtail fish are attracted to certain patterns, called polarization ornaments, visible in polarized light in large male swordtail fish.

“There is a lot of social interaction among the fish,” Gruev says. “We saw the female fish checking out the large males and looking at their polarization ornaments. During the whole courting behavior, the more polarization a male has, the higher the chance for mating.”

Polarized sunglasses

Various animals can detect polarization, or the alignment of light waves in a plane. Previous research found polarization sensitivity in invertebrate animals, including octopus, but this is the first research to find polarization behavior in vertebrate animals.


The camera is similar to polarized sunglasses, which reduce glare by blocking polarized light. The camera is built with nanomaterials inside the camera, allowing it to capture the polarization properties of light in real time.

“We changed the polarization so that the large males with high contrast showed good contrast in their polarization ornaments,” Gruev says. “When we suppressed the polarization ornaments externally with light, the females didn’t pay attention to the males.

“When we changed the light sources to change the polarization signals on the fish body, the social interactions between female and male swordfish significantly increase.”

The camera is a replication of the visual system of the mantis shrimp, which is among the most sophisticated vision of all animals.

Gruev and several other researchers from Washington University also recently published a review paper in the Journal of the Proceedings of the IEEE on bioinspired polarization imaging sensors and their applications in biomedicine.

The Office of Naval Research, Air Force Office of Scientific Research, and the National Science Foundation funded the study.

Source: Washington University in St. Louis