engleski

Otolith morphology and growth of the endemic species Knipowitschia croatica (Gobiidae)

Fish otoliths are complex polycrystalline structures, composed of calcium carbonate and an organic template, being part of the stato-acoustical organ of advanced fish corresponding to the inner ear (labyrinth). They are responsible for the ability to hear and the sense of equilibrium. This study addressed the sagittae otoliths of the freshwater goby species Knipowitschia croatica to obtain data on morphology and growth of otoliths and age structure. Polished and etched otolith thin sections were used for morphological and mineralogical observations carried out using an optical polarization microscope and a scanning electron microscope (SEM). Initial results show sagittae otoliths, having a rounded, slightly convex appearance, and being composed of two polymorphs of calcium carbonate, calcite in the core (nucleus) region and aragonite on the outer part. This fact has not been recorded in the literature. The otolith morphology follows the crystal structure of the observed polymorphs with crystals having a rhomboedric habit typical for calcite in the nucleus, with layers extending to 90μ m. This boundary corresponds to approximately 21 days of age. From this point, a progressive replacement of calcite by aragonite was observed, with the appearance of needle-like aragonithic crystals forming thin crystalline layers to the outer edge of the otolith. Moreover larval otoliths showed only a calcite rhomboedric habit, with a single crystal approximately 1 μ m thick. The age structure of K. croatica, inferred from sagittal otolith readings, showed only two age categories: 0+ and 1+. Total length of males in the 0+ category ranged from 34 to 46 mm and from 45 to 52 mm in the 1+ category. For females, TL ranges from 37 to 45 mm in the 0+ category and from 44 to 53 mm in the 1+ category. No statistically significant difference in age was found for males and females based on their average total lengths. These preliminary results show a peculiar crystalline polymorph structure of the K. croatica otolith. Age structure is in accordance with previously reported data for the short life span of the sand gobies (seen in the genera Knipowitschia and Economidithys and most species of the genus Pomatoschistus), which rarely exceeds two full years, making this one of the main differences between these gobies and those from the genus Gobius.

otolith morphology; endemic; Knipowitschia croatica

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