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.