Histopathological studies on the degenerative changes of somatic muscle in soft tunic syndrome of ascidian, Halocynthia roretzi

As a part of research to elucidate the pathogenesis of so called Soft Tunic Syndrome(STS), that caused mass mortalities in the cultured sea squirts, Halocynthia roretzi, the epidemiological and pathological analysis were done to both clinically normal and diseased groups of the farms of Tongyoung and Geoje coastal areas in southeast sea from February to July, 2008. In the histological finding of the tunic, most of individuals showed tunic softness syndromes that included the disarrangement and destruction of tunic fiber with the simultaneous presence of flagellates-like cells, recently suspected as main agents of tunic softness syndromes. Simultaneously, the intensive degenerative changes of the skeletal muscle of diseased sea squirts were recognized. The changes were characterized with the hyalinization and condensation of muscle fibril and hemocytic infiltration in the muscle fibers. Those were thought to be a kind of typical Zenker's necrosis as in the skeletal muscle of higher vertebrates. Besides of the diseased sea squirts, Zenker's necrosis of skeletal muscles were seen in the normal ones. Epidemiological inquiry for diseased groups revealed that the higher incidences of tunic softness syndrome were recorded in the fast growing groups and in the sites presuming the organic pollution. And Higher malondialadehyde(MDA) and glutathione peroxidase(GPx) activity were detected in the groups showing STS. Those results suggested that Zenker's necrosis of body muscles was a kind of"nutritional myopathy" by oxidative stress. Conclusively, it was considered that Zenker's necrosis of body muscles gives an important clue for elucidating pathogenesis of STS of cultured squirts. And it seems that the necrosis were caused by the oxidative stress to body muscle during abnormal rapid growth of sea squirts.

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