Product Code Database
An otolith (, ōto-]] ear + λῐ́θος]], líthos, a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of . The saccule and utricle, in turn, together make the otolith organs. These organs are what allows an organism, including humans, to perceive linear acceleration, both horizontally and vertically (gravity). They have been identified in both extinct and extant vertebrates.
Counting the annual growth rings on the otoliths is a common technique in estimating the age of fish.
Similar balance receptors called can be found in many invertebrate groups but are not contained in the structure of an inner ear. Mollusk statocysts 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 Common descent.
Statoconia (also called otoconia) are numerous grains, often spherical in shape, between 1 and 50 μm; collectively. 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.Nolf, D. 1985. Otolithi Piscium; in H.-P. Schultze (ed.), Handbook of Paleoichthyology, Vol. 10. Gustav Fischer Verlag, Stuttgart, 145pp.
In mammals, otoliths are small particles, consisting of a combination of a gelatinous matrix and calcium carbonate in the viscous fluid of the saccule and utricle. The weight and inertia of these small particles causes them to stimulate when the head moves. The hair cells are made up of 40 to 70 stereocilia and one kinocilium, which is connected to an afferent nerve. Hair cells send signals down sensory neuron 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 the central 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 the Saccule, due to its anatomical location). 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 the sensorineural hearing loss. Vestibular sensitivity to ultrasound has also been hypothesized to be involved in the perception of speech presented at artificially high frequencies, above the range of the human cochlea (~18 kHz). In mice, sensation of acoustic information via the vestibular system has been demonstrated to have a behaviourally relevant effect; response to an elicited Startle response 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.
An unclassified fossil named Gluteus minimus has been thought to be possible otoliths, but it is hitherto unknown to which animal they could belong to.
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 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.
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. Lutjanidae, , many Sciaenidae) 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, Coryphaenidae). 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, Fish Age and Growth with Otoliths Tennessee Wildlife Resources Agency. Retrieved 2007-04-07. 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 (at least in some species) 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. Furthermore, a small percentage of otoliths in some species bear deformities over time.
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 fishery management policies. Due to the amount of required human labour in otolith age reading, there is active research in automating that process.
Otoliths (sagittae) 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 ( Scomber scombrus), and Atlantic herring ( Clupea harengus).
|
|
