Otolith

An otolith, (oto-, ear + lithos, a stone), also called statoconium or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth. 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 (such as when in an elevator).

Mechanism
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 send signals down sensory nerve fibres, which are interpreted by the brain as motion.

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.

In 1991, Martin Lenhardt of the University of Virginia discovered that people can hear ultrasonic speech, perhaps using the saccule as a hearing organ.

Significance in Ichthyology
Finfish (class Osteichthyes) 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 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, many  drums 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 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 is possible (at least in some species) to estimate fish growth because fish growth is directly proportional to otolith growth. 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 much 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.

The composition of fish otoliths are proving useful to fisheries scientists. The calcium carbonate that composes the otolith is primarily derived from the water. As the otolith grows, new calcium carbonate, mainly aragonite, 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. Dr. Steven Campana is one of the leading researchers in the study of otolith trace elemental and isotopic composition. 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 informed decisions about fish stocks.