Carbonic anhydrase is not the only factor regulating otolith mineralization in fish in dependence of the gravitational environment Original Research Article

Earlier experiments have shown, that fish otolith growth and mineralization is slowed down by hypergravity (hg). The enzyme carbonic anhydrase (CAH) provides carbonate and, thus, plays a major role in otolith calcification. Indeed, CAH reactivity in inner ear maculae is downregulated by hg. The following experiment was designed in order to elucidate as of whether CAH is the only factor regulating otolith mineralization in dependence of the gravity vector: A first group of larval cichlid fish (Oreochromis mossambicus) was reared in normal aquarium water at 1g (1g-Aq). A second group received hg (3g, 7 days) as a physical factor to decrease CAH reactivity (3g-Aq). A third group (1g-AZ) was (at 1g) treated with azetazolamide (AZ; 1 g/l), an inhibitor of CAH (the AZ-concentration used resulted in a complete inhibition of CAH as had been proven by a biochemical assessment of enzyme activity). The last group was maintained both in AZ and at hg (3g-AZ). Both the saccular and utricular otoliths (sagittae and lapilli, respectively) of the 1g-AZ group showed a decrease in otolith growth (surface area) as compared to the 1g-Aq animals (1g-AZ < 1g-Aq). Similar results were obtained when comparing 3g-Aq with 1g-Aq samples (3g-Aq < 1g-Aq). Regarding sagittae, AZ treatment had no significant additional effect on otolith mineralization under hg (3g-AZ = 1g-AZ). In case of lapilli, however, growth received a further reduction when reared in 3g-AZ (i.e., 3g-AZ < 1g-AZ). Thus, in lapilli, hg and AZ added their effects on otolith growth. This finding clearly indicates that hg does not only act on otolith growth via a regulation of CAH activity.