Abstract:Sagitta microstructure of young of the year Larimichthy polyactis collected in the Bohai Sea was studied, which is routinely used for age analyses. Results showed that there was usually one circular nucleus with one deeply black primordium in it, and only a few samples presented two nuclei or primordia. Average diameter of sagitta nucleus and primordia were (24.35±0.72) μm and (12.07± 0.58) μm, respectively. The first increment was observed on the second day, with a distance of (14.11± 1.08) μm to the central nucleus. The first feeding check appeared on the 4th increment, with a distance of (20.67±2.28) μm to the central nucleus, displaying larger width, deeper color, and higher clarity. From the otolith nucleus to approximately the 20th increment, increments gradually changed from a ring shape to an ellipse. Increment widths in the long axis ranged from 6.51 μm to 14.37 μm and tended to increase with age. Increment widths dramatically rose until approximately the 40th increment, reaching a value of (36.05±8.32) μm. Thereafter, increment mean width did not show any clear trend. The process of daily increment deposition of L. polyactis was accompanied by the formation of sub-daily increments. Sub-daily increment deposits displayed a lack of periodicity, irregularity, low clarity, and incompleteness compared with daily increments. Changing the depth of the field while observing these less defined increments resulted in a temporary elimination of the sub-daily increments from view. The analysis of otolith increment deposition has been widely applied and developed in age validation of larvae and juvenile fishes. The counting results directly determine analytical accuracy of the hatching date, spawning field distribution, and migration of wild individuals. The numerous presence of sub-daily increments will increase the difficulty of discriminating daily increments. L. polyactis has well-developed sagitta, which possesses greater width of daily increment and a greater number sub-daily increments. Therefore, distinguishing daily increments from sub-daily increments is the primary task of the research on the sagitta microstructure of L. polyactis. In the present study, the sagitta microstructure characteristics of L. polyactis were analyzed to provide a reference for daily increment judgment of wild L. polyactis and other fish species. Meanwhile, this study will supplement the biological information on L. polyactis and provide data for resource surveys of wild populations.
