The edible ascidian, Halocynthia roretzi is a commercially important fisheries resource in Korea. However, there have been outbreaks of mass mortality due to soft tunic syndrome. It was discovered recently that the cause of death is infection by a protozoan parasite Azumiobodo hoyamushi. Because standard method for determining toxicity of this parasite is not well established, alamar blue assay and microscopic counting performed on the basis of morphological changes were used to estimate antiparasitic effects of 20 drugs having different action mechanisms. Through comparison of alamar blue assay and microscopic counting, 6 drugs were found to be potential in parasite-killing effects: amphotericin B, formalin, hydrogen peroxide, bithionol, benzalkonium chloride, bronopol (24hr-EC50≤20 ㎍/㎖). these assays were considered as methods to assess the antiparasitic effect against A. hoyamushi because potency ratios between the two methods were similar in all compounds except formalin.
Ascidians were artificially infected with A. hoyamushi treated using 40 ㎎/ℓ formalin, bronopol, chlorine dioxide or hydrogen peroxide for 1 hour, and then monitored for 24 hours for drug effects. Although very low mortality was observed, the number of surviving parasite cells in the ascidian tunic tissues was significantly reduced by treating with formalin or chlorine dioxide. Ascidians treated with chlorine dioxide underwent a visible whitish discoloration over the body surface. The preliminary data can be used as a basis to develop anti-protozoal agents against A. hoyamushi. *A thesis submitted to the Committee of the Graduate School, Kunsan National University in partial fulfillment of the requirements for degree of Master of Fisheries Science in February 2014