An otolith, also called statoconium or otoconium,
is a structure in the saccule or utricle of the inner ear, specifically in the vestibular
labyrinth of vertebrates. The saccule and utricle, in turn, together make the otolith
organs. They are sensitive to gravity and linear acceleration. Because of their orientation
in the head, the utricle is sensitive to a change in horizontal movement, and the saccule
gives information about vertical acceleration. Description
Endolymphatic infillings such as otoliths or statoconia are structures in the saccule
and utricle of the inner ear, specifically in the vestibular labyrinth of all vertebrates.
In vertebrates, the saccule and utricle together make the otolith organs. Both statoconia and
otoliths are used as gravity, balance, movement, and directional indicators in all vertebrates
and have a secondary function in sound detection in higher aquatic and terrestrial vertebrates.
They are sensitive to gravity and linear acceleration. Because of their orientation in the head,
the utricle is sensitive to a change in horizontal movement, and the saccule gives information
about vertical acceleration. Statoliths can be found in many invertebrate
groups but are not contained in the structure of an inner ear. Mollusk statoliths are of
a similar morphology to the displacement-sensitive organs of vertebrates; however, the function
of the mollusk statocyst is restricted to gravity detection and possibly some detection
of angular momentum. These are analogous structures, with similar form and function but not descended
from a common structure. Statoconia are numerous grains, often spherical
in shape, between 1 and 50 µm; collectively. Statoconia are also sometimes termed a statocyst.
Otoliths are agglutinated crystals or crystals precipitated around a nucleus, with well defined
morphology and together all may be termed endolymphatic infillings.
Mechanism The semicircular canals and sacs in all vertebrates
are attached to endolymphatic ducts, which in some groups end in small openings, called
endolymphatic pores, on the dorsal surface of the head. Extrinsic grains may enter through
these openings, typically less than a millimeter in diameter. The size of material that enters
is limited to sand-sized particles and in the case of sharks is bound together with
endogenous organic matrix that the animal secretes.
In mammals, otoliths are small particles, composed of a combination of a gelatinous
matrix and calcium carbonate in the viscous fluid of the saccule and utricle. The inertia
of these small particles causes them to stimulate hair cells when the head moves. The hair cells
are made up of 40 to 70 stereocilia and one hair cell, called the kinocilium, which is
connected to an afferent nerve. When the body changes position or begins a movement the
weight of the membrane bends the stereocilia and stimulates the hair cells. Hair cells
send signals down sensory nerve fibers, which are interpreted by the brain as motion. The
brain interprets the orientation of the head by comparing the input from the utricules
and saccules from both ears to the input from the eyes, allowing the brain to discriminate
a tilted head from movement of the entire body. When the head is in a normal upright
position, the otolith presses on the sensory hair cell receptors. This pushes the hair
cell processes down and prevents them from moving side to side. However, when the head
is tilted, the pull of gravity on statoconia shift the hair cell processes to the side,
distorting them and sending a message to the central nervous system that the head is no
longer level but now tilted. This theory may have to be reevaluated because of an experiment
in which a blindfolded owl in zero gravity was able to keep its head level while a handler
was rocking its body back and forth. There is evidence that the vestibular system
of mammals has retained some of its ancestral acoustic sensitivity and that this sensitivity
is mediated by the otolithic organs. In mice lacking the otoconia of the utricle and saccule,
this retained acoustic sensitivity is lost. In humans vestibular evoked myogenic potentials
occur in response to loud, low frequency acoustic stimulation in patients with sensioneural
hearing loss. Vestibular sensitivity to ultrasonic sounds has also been hypothesised to be involved
in the perception of speech presented at artificially high frequencies, above the range of the human
cochlea. In mice sensation of acoustic information via the vestibular system has been demonstrated
to have a behaviourally relevant effect; response to an elicited acoustic startle reflex is
larger in the presence of loud, low frequency sounds that are below the threshold for the
mouse cochlea, raising the possibility that the acoustic sensitivity of the vestibular
system may extend the hearing range of small mammals.
Paleontology After the death and decomposition of a fish,
otoliths and statoconia may be preserved within the body of an organism or be dispersed before
burial and fossilization. Dispersed otoliths are one of the many microfossils which can
be found through a micropalaeontological analysis of a fine sediment. Their stratigraphic significance
is minimal, but can still be used to characterize a level or interval. Fossil otoliths are rarely
found in situ, likely because they are not recognized separately from the surrounding
rock matrix. In some cases, due to differences in colour, grain size, or a distinctive shape,
they can be identified. These rare cases are of special significance, since the presence,
composition, and morphology of the material can clarify the relationship of species and
groups, as in the case of lineages of primitive fish which shows that endolymphatic infillings
were similar in elemental composition to the rock matrix but were restricted to coarse
grained material, which presumably is better for the detection of gravity, displacement,
and sound. The presence of these extrinsic grains, in osteostracans, chondrichthyans,
and acanthodians indicates a common inner ear physiology and presence of open endolymphatic
ducts. Ecology
Composition The composition of fish otoliths is also proving
useful to fisheries scientists. The calcium carbonate that the otolith is composed of
is primarily derived from the water. As the otolith grows, new calcium carbonate, usually
aragonite but sometimes vaterite, crystals form. As with any crystal structure, lattice
vacancies will exist during crystal formation allowing trace elements from the water to
bind with the otolith. Studying the trace elemental composition or isotopic signatures
of trace elements within a fish otolith gives insight to the water bodies fish have previously
occupied. The most studied trace and isotopic signatures are strontium due to the same charge
and similar ionic radius to calcium; however, scientists can study multiple trace elements
within an otolith to discriminate more specific signatures. A common tool used to measure
trace elements in an otolith is a laser ablation inductively coupled plasma mass spectrometer.
This tool can measure a variety of trace elements simultaneously. A secondary ion mass spectrometer
can also be used. This instrument can allow for greater chemical resolution but can only
measure one trace element at a time. The hope of this research is to provide scientists
with valuable information on where fish have traveled. Combined with otolith annuli, scientists
can add how old fish were when they traveled through different water bodies. All this information
can be used to determine fish life cycles so that fisheries scientists can make better
informed decisions about fish stocks. Growth rate and age Finfish have three pairs of otoliths – the
sagittae, lapilli, and asterisci. The sagittae are largest, found just behind the eyes and
approximately level with them vertically. The lapilli and asterisci are located within
the semicircular canals. The shapes and proportional sizes of the otoliths
vary with fish species. In general, fish from highly structured habitats such as reefs or
rocky bottoms will have larger otoliths than fish that spend most of their time swimming
at high speed in straight lines in the open ocean. Flying fish have unusually large otoliths,
possibly due to their need for balance when launching themselves out of the water to “fly”
in the air. Often, the fish species can be identified from distinct morphological characteristics
of an isolated otolith. Fish otoliths accrete layers of calcium carbonate
and gelatinous matrix throughout their lives. The accretion rate varies with growth of the
fish – often less growth in winter and more in summer – which results in the appearance
of rings that resemble tree rings. By counting the rings, it is possible to determine the
age of the fish in years. Typically the sagitta is used, as it is largest, but sometimes lapilli
are used if they have a more convenient shape. The asteriscus, which is smallest of the three,
is rarely used in age and growth studies. In addition, in most species the accretion
of calcium carbonate and gelatinous matrix alternates on a daily cycle. It is therefore
also possible to determine fish age in days. This latter information is often obtained
under a microscope, and provides significant data to early life history studies.
By measuring the thickness of individual rings, it has been assumed to estimate fish growth
because fish growth is directly proportional to otolith growth. However, some studies disprove
a direct link between body growth and otolith growth. At times of lower or zero body growth
the otolith continues to accrete leading some researchers to believe the direct link is
to metabolism, not growth per se. Otoliths, unlike scales, do not reabsorb during times
of decreased energy making it even more useful tool to age a fish. Fish never stop growing
entirely, though growth rate in mature fish is reduced. Rings corresponding to later parts
of the life cycle tend to be closer together as a result.
Age and growth studies of fish are important for understanding such things as timing and
magnitude of spawning, recruitment and habitat use, larval and juvenile duration, and population
age structure. Such knowledge is in turn important for designing appropriate fisheries management
policies. Diet research
Since the compounds in fish otoliths are resistant to digestion, they are found in the digestive
tracts and scats of piscivorous marine mammals, such as dolphins, seals, sea lions and walruses.
Many fish can be identified to genus and species by their sagittal otoliths. Otoliths can therefore,
to some extent, be used to reconstruct the prey composition of marine mammal diets.
Sagittal otoliths are bilaterally symmetrical, with each fish having one right and one left.
Separating recovered otoliths into right and left, therefore, allows one to infer a minimum
number of prey individuals ingested for a given fish species. Otolith size is also proportional
to the length and weight of a fish. They can therefore be used to back-calculate prey size
and biomass, useful when trying to estimate marine mammal prey consumption, and potential
impacts on fish stocks. Otoliths cannot be used alone to reliably
estimate cetacean or pinniped diets, however. They may suffer partial or complete erosion
in the digestive tract, skewing measurements of prey number and biomass. Species with fragile,
easily digested otoliths may be underestimated in the diet. To address these biases, otolith
correction factors have been developed through captive feeding experiments, in which seals
are fed fish of known size, and the degree of otolith erosion is quantified for different
prey taxa. The inclusion of fish vertebrae, jaw bones,
teeth, and other informative skeletal elements improves prey identification and quantification
over otolith analysis alone. This is especially true for fish species with fragile otoliths,
but other distinctive bones, such as Atlantic mackerel, and Atlantic herring.
See also Ossicles
Otolithic membrane Otolith microchemical analysis
References External links
Otolith Research Lab – Bedford Institute of Oceanography.

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