Why don't fish crash into the side of the fishbowl?
Dear Straight Dope:
I just bought my first pet fish--a betta, more commonly known as a "Siamese fighting fish," and I put him next to my computer to aid me with his fishy wisdom and cavalier attitude toward the affairs of the world. We transferred him into a larger bowl, and he seems happier about things in general.
Having a pet fish means frequent trips to my local supply store, and the question popped into my mind--how is it that a fish whose eyes are on opposite sides of its head (and I assume is therefore incapable of binocular vision and depth perception as I know it) avoids bumping into the sides of the transparent glass bowl/aquarium? My betta can move extremely quickly, but he always manages to avoid a fishbone-crunching collision with the bowl. His safety is not in question, but my mental health is definitely on the line.
The fish aren't using their eyes to "see" the glass, but rather a special pressure-sensing system called the lateral line. This system is made up of sensory units called neuromasts, consisting of cells on the body surface that have a projecting hair encased in a gelatinous cap. When pressure waves in the water move the gelatinous caps and bend the hairs, the firing rate of nerve impulses sent to the brain by the neuromasts changes, enabling the fish to detect the waves. In fish these neuromasts are mainly found in a groove running along the side of the body, hence the term lateral line. Branches of the lateral line system extend onto the head as well. Besides fish, the system is found in some aquatic amphibians.
Fish set up pressure waves as they move through the water, and are able to detect the reflection and distortion of these waves from objects and thereby avoid them. They are also able to detect the pressure waves of other fish. The lateral line system can also pick up low frequency sound (a kind of pressure wave) at 100 Hz or less. Certain species of blind cave fish are able to navigate perfectly well in their lightless environment by using their lateral line system alone.
In some species neuromasts have evolved into other kinds of sensory organs. Sharks and some bony fish have modified neuromasts called ampules of Lorenzini that can sense electrical fields, such as those given off by other fish as they move about. The lateral line system is also thought to have given rise to several sensory systems found in the ears of terrestrial vertebrates like you and me. For example, our sense of hearing is based on hair-bearing neuromasts that detect pressure waves in the cochlear fluid within the inner ear set up by sound waves in the air. Likewise our sense of balance depends on neuromasts that detect changes in pressure within the semicircular canals due to acceleration in different directions.