The group of vertebrates known as fish is one of the most diversified on the whole planet. They are able to survive in a wide variety of ecosystems, including freshwater, saltwater, and even the trenches found deep inside the ocean. There are many alterations that fish have developed in order to be able to survive in the many environments they inhabit; but, can all fish see equally well? Because fish spend most of their lives below and only surface to breathe at night, there is a school of thought that suggests they do not need sharp eyesight because there is so little light at the depths of the ocean.
There are certain fish that have excellent night vision, while others have an enhanced sense of smell. Still other fish will use their fins to “sense” their surroundings in order to locate potential sources of food and potential dangers. Because so little study has been conducted on this subject, it is difficult to determine what it is like to have different forms of eyesight when underwater.
The environment in which an individual species spends most of its time, either on land or in water, is the most reliable indicator of the quality of its vision. However, just because you are unable to see something does not imply that it does not exist in the world! Some animals, including humans, are able to see motion with just one eye open, even when there is no light present whatsoever, provided that the moving object is moving swiftly enough.