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| Otolith | |
|---|---|
 Otolith organs showing detail of utricle, otoconia, endolymph, cupula, macula, hair cell filaments, and saccular nerve | |
 Juvenileherring. Length 30 mm; 3 months old; still transparent; the otoliths are visible left of the eye. | |
| Details | |
| Identifiers | |
| Latin | statoconium |
| TA98 | A15.3.03.086 |
| FMA | 77826 |
| Anatomical terminology | |
Anotolith ((Ancient Greek:ὠτο-,ōto- ear +λῐ́θος,líthos, a stone) is a small crystal inside a structure in the inner ear; this structure, consisting of three hollow, connected tubes (thevestibular system) allows humans and many animals to sense orientation (up/down) in space, movement in space (including direction), and acceleration/deceleration (including direction) in space. Otoliths specifically assist in the sensing of acceleration/deceleration. These otoliths are usually in a fixed position; when they lose their usually fixed position and start to float freely, their movement will trigger potentially violent vertigo (rotatory vertigo) in which the room surrounding the patient spins rapidly. (Other causes of rotatory vertigo exist, and could be serious enough to require a check and precise diagnosis from a medical professional).
Following is an excellent detailed description and discussion of the occurrence and function of otoliths in humans and animals.
Anotolith (Ancient Greek:ὠτο-,ōto- ear +λῐ́θος,líthos, a stone), also calledotoconium,statolith, orstatoconium, is acalcium carbonate structure in thesaccule orutricle of theinner ear, specifically in thevestibular system ofvertebrates. The saccule and utricle, in turn, together make theotolith organs. These organs are what allows an organism, including humans, to perceive linearacceleration, both horizontally and vertically (gravity). They have been identified in both extinct and extant vertebrates.[1]
Counting the annual growth rings on the otoliths is a common technique inestimating the age of fish.[2]
Endolymphatic infillings such as otoliths are structures in thesaccule andutricle of theinner ear, specifically in thevestibular labyrinth of allvertebrates (fish, amphibians, reptiles, mammals and birds). In vertebrates, the saccule and utricle together make theotolith 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.[3][4] They are sensitive togravity and linearacceleration. 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 (such as when in anelevator).
Similar balance receptors calledstatocysts can be found in manyinvertebrate groups but are not contained in the structure of an inner ear.Mollusk statocysts are of a similarmorphology to thedisplacement-sensitive organs of vertebrates;[5] however, the function of the mollusk statocyst is restricted to gravity detection and possibly some detection of angular momentum.[6] These areanalogous structures, with similar form and function but notdescended from a common structure.
Statoconia (also called otoconia) are numerous grains, oftenspherical in shape, between 1 and 50 μm; collectively.[citation needed] Statoconia are also sometimes termed a statocyst. Otoliths (also called statoliths) are agglutinated crystals or crystals precipitated around a nucleus, with well defined morphology and together all may be termed endolymphatic infillings.[1][7][8]
Thesemicircular canals and sacs in all vertebrates are attached to endolymphatic ducts, which in some groups (such assharks) end in small openings, called endolymphatic pores, on the dorsal surface of the head.[1] 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 an endogenous organic matrix that the animal secretes.
In mammals, otoliths are small particles, consisting of a combination of a gelatinous matrix andcalcium carbonate in the viscous fluid of the saccule and utricle. The weight andinertia of these small particles causes them to stimulatehair cells when the head moves. The hair cells are made up of 40 to 70stereocilia and onekinocilium, which is connected to an afferent nerve. Hair cells send signals downsensory nerve fibers which are interpreted by the brain as motion. In addition to sensing acceleration of the head, the otoliths can help to sense the orientation via gravity's effect on them. 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 otoliths shifts the hair cell processes to the side, distorting them and sending a message to thecentral nervous system that the head is tilted.
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 (most likely thesacculus, due to its anatomical location). In mice lacking the otoconia of the utricle and saccule, this retained acoustic sensitivity is lost.[4] In humansvestibular evoked myogenic potentials occur in response to loud, low-frequency acoustic stimulation in patients with the sensorineural hearing loss.[3] Vestibular sensitivity toultrasonic sounds has also been hypothesized to be involved in the perception of speech presented at artificially high frequencies, above the range of the humancochlea (~18 kHz).[10] In mice, sensation of acoustic information via the vestibular system has been demonstrated to have a behaviourally relevant effect; response to an elicitedacoustic startle reflex is larger in the presence of loud, low frequency sounds that are below the threshold for the mouse cochlea (~4 Hz), raising the possibility that the acoustic sensitivity of the vestibular system may extend the hearing range of small mammals.[4]
After the death and decomposition of a fish, otoliths may be preserved within the body of an organism or be dispersed before burial andfossilization. Dispersed otoliths are one of the manymicrofossils which can be found through a micropalaeontological analysis of a fine sediment. Theirstratigraphic significance is minimal, but can still be used to characterize a level or interval. Fossil otoliths are rarely foundin situ (on the remains of the animal), 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. In the case of primitive fish, various fossil material shows thatendolymphatic 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 inosteostracans,chondrichthyans, andacanthodians indicates a common inner ear physiology and presence of open endolymphatic ducts.[1]
An unclassified fossil namedGluteus minimus has been thought to be possible otoliths, but it is hitherto unknown to which animal they could belong to.
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 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 orisotopic signatures of trace elements within a fish otolith gives insight to the water bodies fish have previously occupied.[11] Fish otoliths as old as 172 million years have been used to study the environment in which the fish lived.[12] Robotic micromilling devices have also been used to recover very high resolution records of life history, including diet and temperatures throughout the life of the fish, as well as their natal origin.[13]
The most studied trace and isotopic signatures arestrontium due to the same charge and similarionic radius tocalcium; 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 alaser ablation inductively coupled plasma mass spectrometer. This tool can measure a variety of trace elements simultaneously. Asecondary 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 frequented. Combined with otolith annuli, scientists can add how old fish were when they traveled through different bodies of water. This information can be used to determine fish life cycles so that fisheries scientists can make better informed decisions about fish stocks.
Finfish (classOsteichthyes) have three pairs of otoliths – the sagittae (singular sagitta), lapilli (singular lapillus), and asterisci (singular asteriscus). The sagittae are largest, found just behind the eyes and approximately level with them vertically. The lapilli and asterisci (smallest of the three) are located within the semicircular canals. The sagittae are normally composed ofaragonite (althoughvaterite abnormalities can occur[14]), as are the lapilli, while the asterisci are normally composed of vaterite.
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 (e.g.snappers,groupers, manydrums and croakers) will have larger otoliths than fish that spend most of their time swimming at high speed in straight lines in the open ocean (e.g.tuna,mackerel,dolphinfish).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 ofcalcium 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 resembletree rings. By counting the rings, it is possible to determine the age of the fish in years.[15] Typically the sagitta is used, as it is largest,[16] 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.[17] 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 (at least in some species) to estimate fish growth because fish growth is directly proportional to otolith growth.[18] 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. Furthermore, a small percentage of otoliths in some species bear deformities over time.[19]
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, andpopulation age structure. Such knowledge is in turn important for designing appropriatefisheries management policies. Due to the amount of required human labour in otolith age reading, there is active research in automating that process.[20]
Since the compounds in fish otoliths are resistant todigestion, they are found in thedigestive tracts andscats of seabirds andpiscivorousmarine mammals, such asdolphins,seals,sea lions andwalruses. Many fish can be identified togenus andspecies by their otoliths. Otoliths can therefore, to some extent, be used to deduce and reconstruct the prey composition of marine mammal and seabird diets.
Otoliths (sagittae) arebilaterally 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 andbiomass, useful when trying to estimatemarine mammal prey consumption, and potential impacts onfish stocks.[21]
Otoliths cannot be used alone to reliably estimatecetacean orpinniped diets, however. They may suffer partial or completeerosion in the digestive tract, skewing measurements of prey number andbiomass.[22] 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 preytaxa.[23]
The inclusion of fishvertebrae, jaw bones, teeth, and other informative skeletal elements improves prey identification and quantification over otolith analysis alone.[24] This is especially true for fish species with fragile otoliths, but other distinctive bones, such asAtlantic mackerel (Scomber scombrus), andAtlantic herring (Clupea harengus).[25]
'Sea gems' ornaments from fish otoliths have been introduced in the market in India recently, with the efforts of a group of enthusiastic fisher women inVizhinjam. Scientists fromCentral Marine Fisheries Research Institute (CMFRI) have trained these fisher-women. Ornaments from fish otoliths, known to the Romans and Egyptians as lucky stones, are continued to be used in countries like Brazil and the Faeröer, and are being collected and sold in an organized and sustainable manner in India.[26]
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