Histological Analysis of the Mechanisms of <i>Chattonella</i>-Induced Hypoxemia in Yellowtail

The effect of Chattonella exposure on gill histology of yellowtail was examined to clarify the following questions: (1) Is the branchial edema, previously reported as the main branchial lesion in response to Chattonella exposure, truly caused by Chattonella cells or a secondary histological alteration by fatal hypoxemia during the exposure? (2) Is the branchial edema responsible for the rapid decline of blood oxygen levels during the early stages of Chattonella exposure? Fish dying from Chattonella exposure showed many types of lesions, especially severe epithelial separation in the lamellae and filaments. In contrast, the gills of fish dying from environmental hypoxia showed very few lesions and were histologically hardly discernible from the control gills. This demonstrated that the branchial edema was induced by Chattonella and not by hypoxemia. On the other hand, the epithelial separation had not developed when the tissues were sampled immediately after the onset of hypoxemia. Therefore, the edema cannot be the cause of hypoxemia. The only histological change observed in the gills sampled at blood oxygen drop was blockade of interfilamental spaces by mucus. Based on these results and the previous findings of oxygen radical production by Chattonella, we hypothesized that oxygen radicals, released from Chattonella cells, stimulated mucus cells in the gills, and the secreted mucus, possibly plus Chattonella cells trapped within the mucus, destroyed the gas exchange capacity of the gills by shunting respiratory water current away from the lamellae.