The word "otolith" is formed from two Greek roots:
otos (ear) and lithos (stone). An otolith
("earstone") is a small bone-like piece of calcium
carbonate found in the ears of many vertebrates. It rests like a
pebble in a small cavity lined with sensitive hairs, and its pressure
on the hairs tells the brain which way is down. This information is
crucial for a sense of balance.
Otoliths are found in the heads of all fish other than sharks,
rays and lampreys, and are important tools for understanding the
lives of fish and fish populations. Microscopic growth rings, like
the rings found in trees, mark the otoliths and record the age and
growth of a fish from hatching until death. These growth rings can be
affected by many factors, including migration patterns and water temperatures.
Otoliths found in and around shipwrecks have been used to gather
data about the species of fish that inhabited the area of the
shipwreck two hundred (or more) years ago, enabling biologists to form
conclusions about changes in oceanic environments.
Since the vestibular system of fish is similar to that of humans,
otoliths from fish are also being used to study the effects of gravity
removal on the vestibular system (which registers sensations of
motion and balance). By recording data from the otolithic nerves of
fish sent into space, researchers hope to gain a better
understanding of the changes in signals sent by the inner ears of
astronauts as they adapt to microgravity.