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Effects of Hypoxia on Buoyancy Control and the Development of Lordosis in Physostomous and Physoclistous Fish Species

Author(s): Bagowski CP, Bertola LD, Schoonheere E, Wilms I, Kabli S, Alia A and Groot HJM

The swimbladder is a well-known adaptation with whichmany fish species create upward hydrodynamic forces to prevent them from sinking. Development and survival rates are adversely affected by deflated swim bladders. Based on morphology, an open swimbladder system (physostomous) and a closed swimbladder system (physoclistous) are distinguished. In this study two physostomous species, zebrafish (Danio rerio) and goldfish (Carassius auratus), and two physoclistous species, tilapia (hybrid of Oreochromis mossambicus and Oreochromis niloticus ) and the cichlid (Haplochromis piceatus) were exposed to severe chronic hypoxia. The zebrafish showed reduced buoyancy. X-ray pictures andMRI scans showed that all individuals exposed to severe hypoxia suffered from deflated swimbladders after three weeks. To maintain their position in the water column under hypoxic conditions, zebrafish redirect their swimming movements and swim at an angle of around 45 degrees, which ultimately leads to the development of lordosis. The other three tested species were able to keep their swimbladders inflated and maintained their buoyancy.As a result, none of them changed their swimming movements or developed lordosis. Our results demonstrate that coping with low oxygen levels is done in a species specific manner and that severe chronic hypoxia effects zebrafish on the long term.

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