Since sunlight never reaches the deeper parts of the sea, these regions are always totally dark. Yet fish live here and swim about without bumping into each other.
How is this so? Well, fish actually “see” without their eyes; they use other sensory organs.
The next time you catch a fish, look for a line running along its skin, from the gills to the tail. All fish have this marking, called the lateral line. All along the lateral line are openings in the fish’s skin. These openings allow water vibrations to reach sensory organs which are located under the lateral line.
These sensory organs, in turn, are connected to the fish’s nervous system, and are very sensitive to slight vibrations in the water.
By reading the vibrations in the water around it, a fish can tell when another fish is approaching, what direction it’s coming from, and how far away it is. The fish can also tell when a rock or other object is close by, because these objects cause vibrations as water flows around them.
So a fish can navigate through the darkness without striking another fish or a rock, and can “see” even in total darkness.