Influence of hypergravity on fish inner ear otoliths: I. Developmental growth profile.

Inner ear stones (otoliths) of larval cichlid fish Oreochromis mossambicus were marked with the calcium-tracer alizarin-complexone (AC) at 1 g earth gravity before and after a 3, 7, 14 or 21 days stay of the animals at hypergravity conditions (hg; 3 g, centrifuge). After the experiment, the otoliths' area between the two AC-labellings was measured with regard to size and asymmetry (size difference between the left and the right stones). Both utricular and saccular otoliths (lapilli and sagittae, respectively) continued growing in a linear way at hg, but growth was significantly slowed down as compared to parallely raised 1 g-control specimens. In case of bilateral asymmetry between the corresponding otoliths its formation in hg-animals became reduced as compared to the 1 g controls. The reduction of asymmetry was much more pronounced in the sagittae than in the lapilli. The latter result supports an earlier hypothesis, according to which especially a low sagittal asymmetry has a functional advantage. In general, the results strongly suggest that otolith growth is continuously regulated in dependence of the environmental gravity vector.