Different Oxygen Stresses on the Responses of Branchial Morphology and Protein Expression in the Gills and Labyrinth Organ in the Aquatic Air-breathing Fish, Trichogaster microlepis.

Chun-Yen Huang and Hui-Chen Lin (2016) Organisms face direct challenges from a variety of abiotic factors in the environment. Changes in dissolved oxygen are one of the most common types of these challenges. The gills of some fish species can compensate for ambient oxygen changes by exhibiting morphological and functional plasticity that give the gill the ability to modify its structure. In the previous studies of aquatic air- breathing fish with an accessory air-breathing organ (the labyrinth organ), we found morphological and functional specializations between the 1st and 4th gills in the genus Trichogaster. This study investigated morphological and functional changes in the gills and labyrinth organ of the aquatic air-breathing fish T. microlepis over a 28- day period of oxygen uptake stresses. The experimental design was as follows: (1) a control group (held under normoxia with air-breathing respiration (ABR) allowed); (2) a hypoxic group (held under hypoxia with ABR); and (3) a restricted group (held under normoxia without ABR). We recorded the cumulative mortality of the fish and the frequency of ABR between the control and hypoxic groups, conducted morphological examinations of the lengths of gill filaments and lamellae of gills and determined the relative abundance of carbonic anhydrase II (CAII) and Na+/K+-ATPase (NKA). Mortality in the restricted group was higher than those in the control and the hypoxic groups. The frequency of ABR in the hypoxic condition was higher than that in the control. The lengths of the lamellae in the 1st, 3rd and 4th gills in the restricted group were significantly longer than those in the control group after 14 and 28 days. In addition, the relative abundance of CAII was significantly increased only in the labyrinth organ in the hypoxic group compared to the control group on day 3. The relative abundance of proliferating cell nuclear antigen also was significantly increased in the 1st gill, 4th gill and the labyrinth organ in the restricted group compared to the control group on day 14. This study showed, for the first time, that the 4th gill in T. microlepis, which is generally much-reduced compared to the other gills, can have elongated lamellae when fish are subjected to the restricted group for 28 day. The relative abundance of CAII in the labyrinth organ was significantly higher under hypoxic group than under control group on day 3.