Otoliths are complex polycrystalline bodies composed of needle shaped crystals of calcium carbonate in the form of aragonite radiating outwards in three dimensions from a centrally located nucleus and passing through a network of fibrous collagen-like protein called otolin. Otolin resembles keratin in its amino acid composition and is necessary as a "blueprint" in the mineralisation process ( 57, 8). The otolith grows as more material in the form of new crystals is deposited on the outer surface, a cyclical process dependent on internal calcium metabolism rates and on amino acid synthesis. Mugiyia et al. ( 73) noted that the rate of accretion on various parts of the otolith was proportional to the concentration of adjacent macular cells and that the deposition of calcium was under endocrinological control. (79 ,85 ,78 ,90 ,61 ,62 )
Elemental analyses of otoliths has shown that they are relatively pure compounds with total inorganic impurities of less than 1% of otolith weight. In the labrinth, the otolith is surrounded and impregnated with the endolymph fluid. This fluid may be of major importance in determining the ultimate composition of the otolith. Endolymph is rich in K+, relatively low in NA+ , has a low protein content and a . pH that is more alkaline than blood plasma. The endolymph proteins are thought to play a central role in calcium carbonate precipitation and inhibition in the otolith. However, the mechanism of of otolith formation (biomineralisation and organomineralisation) is still not well understood. ( 85 ,90 ).
The structure and composition of the fish otolith is influenced by many factors operating at many levels. The most important factors influencing biomineralisation include:
- the rate of uptake of different elements in the surrounding water which is influenced by fish metabolism and the method of entry into the fish (e.g., via gills or via gut);
- the physical and chemical properties of calcium carbonate / aragonite which influence the incorporation of other elements into the otolith;
- the chemical composition of the water which may vary in terms of geographic location, salinity and temperature regimes;
- the condition of the fish (e.g., gonad maturation, growth cycle) which will influence the the composition of tissue and blood and effect the incorporation of different elements into the otolith.
Otolin contains water soluble proteins and water insoluble proteins. The water soluble contain a high content of acidic amino acids and have a high capacity for calcium binding. The role of the water soluble proteins is thought to be structural.(78 ,85 ,90 ,63 )