Studies on a Bacterial Disease of Sea Urchin Strongylocentrotus intermedius Occurring at Low Water Temperatures

Abstract

Mass mortalities of cultured sea urchin Strongylocentrotus intermedius occurred at Date, Shiriuchi and Shikabe Fisheries Breeding Centers in Hokkaido, from the end of May to the beginning of June (seawater temperature ranged 11-13°C) in 1995. A short rod psychrophilic bacterium, which has a single polar flagellum was isolated from coelomic fluid and body surface of diseased animals at the three Centers. The isolates were Gram-negative, oxidase positive, and acid production from glucose anaerobically without gas production. The G+C contents of DNAs from the representative strains 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 presumed that the isolates belong to the genus Vibrio. Experimental infections with sea urchin were performed by immersion method using 107, 106 and 105 CFU/ml of the representative strains Da-2, Sr-3 and Sk-1 at 11°C. Groups, which were injured mechanically on the body surface showed higher mortalities than that of the uninjured group. Especially, the group injured on the connective layers around the teeth showed the highest mortality. These results suggested that the isolated bacterium is the causative agent of the disease of sea urchin Strongylocentrotus intermedius occurring at low water temperatures.