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Histopathological alterations in triangle sail mussel (Hyriopsis cumingii) exposed to toxic cyanobacteria (Microcystis aeruginosa) under hypoxia

Wu, Fangli, Kong, Hui, Shang, Yueyong, Zhou, Zuoqiang, Gul, Yasmeen, Liu, Qigen, Hu, Menghong
Aquaculture 2017 v.467 pp. 182-189
Chlorella vulgaris, Hyriopsis cumingii, Microcystis aeruginosa, anaerobic conditions, aquaculture, biomass, cilia, diet, dissolved oxygen, epithelium, eutrophication, freshwater, gills, histopathology, hyperplasia, hypoxia, lakes, microalgae, microcystins, mussels, normoxia, stomach, toxicity, China
Bloom-forming microalgae and hypoxia are considered as the two main co-occurring stressors associated with eutrophication. How these two stressors might alter the sail mussel exposed to toxic cyanobacteria and hypoxia has not been well evaluated histophysiologically. Therefore, the present study was designed to evaluate the combined effects of the harmful cyanobacteria (Microcystis aeruginosa) and hypoxia on the histopathological alterations in the gill, digestive gland, and stomach of an ecologically and economically important mussel species inhabiting lakes and reservoirs, the triangle sail mussel (Hyriopsis cumingii). To evaluate the histophysiological changes due to the stressors, a 2×2 factorial design with four treatments was adopted: two dissolved oxygen levels (hypoxia: 1mgO2l−1; normoxia: 6mgO2l−1) and two treatments, one with 0% and another with 100% (1.8×107cellml−1 ca. 50mgl−1) cell biomass of toxic M. aeroginosa. The non-toxic green alga (Chlorella vulgaris) was supplemented to maintain the same algal biomass in the four treatments. Samples from the treatments were collected on days 0, 3, 5, 7, 14, and 21. The microcystin content in sail mussels exposed to M. aeruginosa increased over the entire experimental span, under both normoxia and hypoxia, but was significantly lower in mussels under hypoxic conditions. Pathological alterations were observed in the gills, digestive diverticula and stomach of mussels under hypoxia, when fed 100% M. aeruginosa diet, and when intoxicated with M. aeruginosa under hypoxia. Hyperplasia, exfoliation of latero-frontal cilia, filament fusion, and epithelial desquamation of filaments in the gills were observed following 7days of exposure to M. aeruginosa and hypoxia. After 7days of exposure, the damage in the gills, digestive gland, and stomach increased. Hypoxia induced more severe effects in the gill structures than the harmful alga, whereas the digestive gland was more affected by exposure to M. aeruginosa. The stomach, however, was equally affected by both stressors. Results of this preliminary study highlight the need for further in depth studies on the combined effect of toxic cyanobacteria and hypoxia on the triangle sail mussel, and the potential implications for the freshwater pearl production in China.Hyriopsis cumingii has been considered as an aquaculture species for pearl production in China. In recent years, the mussels have been often suffered from hypoxia and toxic algae, but the effects of Microcystis aeruginosa associated with hypoxia on the morphological changes of selected organs such as gill, digestive gland and stomach are unclear. This study just clarified that the selected tissues of H. cumingii were more severely affected by the both stressors, which may clarify how the toxic algae and hypoxia cause some diseases in the mussel H. cumingii.