Review of the growth regulation processes of otolith daily increment formation

Otolith growth is a complex phenomenon integrating various factors that can be considered either as endogenous or exogenous, although they are always regulated by the physiology of the fish. Both types of factors may operate upon the anabolism and catabolism of the fish, and these processes are refl ected in the rhythmic deposition of the two main constituents of the otolit h: the organic matrix and the aragonite layers, Both components reflect the endogenous process in their periodicity, and the exogenous process in the amount of material laid down in the otolith, resulting in how and where the increments are formed. At the endogenous level, several morphological and functional aspects are r elevant. The main one is the role of the otolith as a mechano-receptor in t he inner ear. Thus, inner-ear anatomy and function regulate otolith growth and morphology, which in turn determine the daily growth rates. Environment al conditions, transmitted through the physiology of fish, affect the otoli th growth rate (increment width) but increment periodicity may be disrupted only in extreme cases of physiological stress. The current state of the art is reviewed and the otolith growth paradigm is summarized. Relevant subjects not yet studied are pointed out for future r esearch. (C) 2000 Elsevier Science B.V. All rights reserved.