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What is an Otolith?
Otoliths, or "earstones", are found in the head of all fishes other than
sharks, rays and lampreys. These pearly white stones are about the
size of a pea, and can be found in the fish's skull just below the rear
of the brain. They are not attached to the skull, but rather "float"
beneath the brain inside the soft, transparent inner ear canals.
| There
are 3
pairs of
otoliths
in each
fish; 1
large
pair
(the sagittae) and 2
small
pairs
(the lapilli and the asteriscii). It is the largest pair which is usually
used for determining age. The smaller pairs are about the size of the
tip of a pin. Despite their size however, the smallest pair (the lapilli)
is most often used for daily ring ageing. |
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Otoliths are rocks, not bones. This property makes them more
durable than bone. Indeed, the growth of the otolith is a one-way
process: new otolith material can be (and is) added to the outside
surface through time, but existing material cannot be removed. This
one-way growth process explains why otoliths can form and retain
such delicate structures as daily rings, whereas bone cannot.
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Otoliths have a very
distinct shape which is
characteristic of the
species of fish. That
is, different fish
species have
differently shaped otoliths. Indeed, the
shape is so distinctive
that biologists can use
the otoliths recovered
from seal and bird stomachs and droppings to determine the type of
fish they ate. Even the size of the otolith can be used to indicate the
size of the fish that was eaten. |
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