Fig. 3. Microstructure and morphology of otoliths from tilapia juveniles immersed in 300 p.p.m. thiourea (TU) from 15 to 27 days post-hatching (d.p.h.), followed by a recovery period in fresh water from 28 to 41 d.p.h. (C,D,F). Otoliths of normal fish are shown in A,B,E for comparison. The innermost fluorescent ring indicates the beginning of TU treatment. The outer three fluorescent rings were laid down on the second, fourth and sixth day after ending TU treatment. A–D are left sagittae; E and F are right sagittae. AR, antirostrum; PR, postrostrum; R, rostrum. Scale bars, 100 µm.