Herrings, sardines, mackerel and tuna are fish with silvery scales that reflect the light in shallow oceanic water. For many years, scientists have wondered how their predators, such as octopus and squid, could find them. It is known that octopi, squid and other sea creatures such as shrimp have polarized vision. The scientists guessed that the polarized vision would allow octopi and squid to easily spot and catch the silvery fish.
In a study published in the journal Current Biology, researchers went scuba diving around the Great Barrier Reef of Australia to find out whether or not fish with excellent sight could locate fish that have a silvery, mirror-like appearance that makes them difficult to see under the water. The researchers used customized cameras with small polarizing light filters built into the sensor. They photographed the silvery fish from about 6 to 10 feet away.
The findings of this study suggest that it is not the polarization of the octopi and squids' vision that allows them to spot the silvery fish under the water. In the lab, the researchers used a mathematical model of visual perception and a measure of the brightness and polarization of the water and the fish. Their results showed that polarized vision does not help octopi, squid or other creatures to spot silvery fish from a greater distance than they could without the special vision.
Squid and octopi also tend to live in deep parts of the ocean that visible light does not reach. This would make the polarized vision ineffective because there would be no light for their eyes to filter. More research is needed in order to determine just what octopi, squid and other sea creatures are doing with their polarized vision. One hypothesis is that the animals use the vision to find suitable mates. The polarized vision might also be more effective at close distances than what the researchers were able to measure with their equipment.