Freshwater

88
Part II: Fish and How to Care for Them
Figure 7-2:
The lateral
line system
is a different
kind of
sensing
system from
your own.
The Lateral Line System
The lateral line is located on both sides of their body and runs from the back
of the eye to the base of the tail fin. These lines are composed of small neuromasts
(receptors) which contain cilia (very fine hairs) in fluid-filled canals.
These canals detect vibrations in the water, and the vibrations form an
“image” inside the fish’s brain.
The eyes
A fish’s eyes are often large to compensate for the poor lighting conditions
that exist under water. Usually the eyes are located on the sides of the head,
and some species can rotate them 360 degrees. In certain species that live in
areas of total or semi-darkness (such as the blindcave fish), the eyes are
absent altogether. Over time, the eyes have been selectively removed
through the evolutionary process.
Some fish do have the ability to see a few colors at various depths, but they
have great difficulty adjusting to rapid light changes because their iris works
slowly. For this reason, fish act “shocked” and may panic when an aquarium
light is suddenly turned on or off without warning in a room that is still dark.
So if you turn on your aquarium light right after you get up in the morning
and then notice that your fish are stuck to the ceiling, you probably frightened
them a little bit.