Ultrastructure of otolith increments and checks in the teleost fish Oreochromis niloticus.

An otolith increment is composed of an incremental zone and a discontinous zone. Observation of the otoliths of Oreochromis niloticus with transmission and scanning electron microscopy reveals that organic fibers are concentrated in the discontinuous zone and are relatively scarce in the incremental zone. On the other hand, calcium carbonate crystals are chiefly packed in the incremental zone and are less dense in the discontinuous zone. Both fibers and crystals are oriented perpendicular to the growth increments. Otolith checks, or discontinuities, contain even denser fibers and fewer fine crystals than does the discontinuous zone. A higher proportion of individual fibers is prominently stained in the discontinuous zone and check than in the incremental zone. Other features of individual fibers appear to be the same among the three zones. The crystals on either side of a check or a discontinuous zone resemble each other both in size and orientation. Zonation of protein and mineral components of otoliths can be understood in terms of both incremental growth and the daily cycle of deposition.