Studies on a bacterial disease of sea urchin Strongylocentrotus intermedius occurring at low water temperatures

Mass mortalities of cultured sea urchin Stronglyocentrotus intermedius occu rred at Date, Shiriuchi and Shikabe Fisheries Breeding Centers in Hokkaido, from the end of May to the beginning of June (seawater temperature ranged 11 similar to 13 degrees C) in 1995. A short rod psychrophilic bacterium, w hich has a single polar flagellum was isolated from coelomic fluid and body surface of diseased animals at the three Centers. The isolates were Gram-n egative, oxidase positive, and acid production from glucose anaerobically w ithout gas production. The G + C contents of DNAs from the representative s trains Da-2 (Date), Sr-3 (Shiriuchi) and Sk-1 (Shikabe) were 42.7, 43.5 and 44.3 mol%, respectively. On the basis of these characteristics, it was pre sumed that the isolates belong to the genus Vibrio. Experimental infections with sea urchin were performed by immersion method using 10(7), 10(6) and 10(5) CFU/ml of the representative strains Da-2, Sr-3 and Sk-1 at 11 degree s C. Groups, which were injured mechanically on the body surface showed hig her mortalities than that of the uninjured group. Especially, the group inj ured on the connective layers around the teeth showed the highest mortality . These results suggested that the isolated bacterium is the causative agen t of the disease of sea urchin Strongylocentrotus intermedius occurring at low water temperatures.