Mantis shrimp is a marine crustacean that is known to have a vision system that is much more complex than the eyes of many animals living on the ground. As explained here, it has 12 types of photoreceptor cells, can detect a wider frequency range than humans (into UV and infrared regions), and even polarization.
These animals live in dark underwater environments where sunlight cannot reach, so that a vision system would not seem to be useful. Why is this present?
My own thoughts are that even though some types of these creatures live in deep ocean waters, they mostly live in shallow waters where sunlight can reach:
"They occur in coastal waters but are sometimes found as deep as 1,300 meters"
Therefore their vision system is not useless. But what about those species (including but not limited to some types of mantis shrimps) that live in deep dark waters? Is their vision system considered a vestigial feature?