Analysis of the alternating opaque and translucent zones in fish otoliths is a widely used method to determine age in fish. The mechanisms underlying the annual periodicities in the formation of these zones remain unknown, although various interpretations and explanations have been presented to explain the appearance of the opaque zones. Here I review the biological and structural characteristics of the opaque zones in otoliths from a number of fish species. The results of this review suggest that the opaque zones can be classified into four different types. Type A is a dark opaque zone, displaying minute, dense crystals, and is typically formed at a young life-history stage. Type B comprises an aggregation of grooves and discontinuous crystals that are formed during growth-stagnant periods. Type C can be described as a washy black zone that has the appearance of an object that is smeared with ink; it is formed during seasons of active growth. Type D has deep grooves, appears luminous in transmitted light in etched otolith sections, and is formed during the spawning season. Types A and C otolith opaque zones are typical of younger fish and are formed during growing periods. They are complementary to Types B and D otolith opaque zones which are formed during periods of stagnant growth and/or during the spawning seasons and which are typical of older fish, which generally lack Type A zones. To ensure precise age determination in fish, it is therefore necessary to understand the structural and biological characteristics that produce these four distinct types of opaque zones.
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