Transmission of black mouth disease shed light on the aquaculture management of the sea urchin Strongylocentrotus intermedius

Mass mortality of the sea urchin Strongylocentrotus intermedius and consequent economic losses occur in the aquaculture industry due to the outbreak of black mouth disease. Decreasing the physical contact between the healthy and diseased sea urchins is proposed to be an effective approach to preventing the disease transmission. Laboratory experiments were designed to investigate whether different levels of segregation can effectively prevent the disease transmission. The present study found that the number of diseased sea urchins in the segregation with two layers of PVC net (group T) was significantly lower than that without segregation (group E) on the first day. Further, there were no significant difference in the number of diseased sea urchins between the group T and the group with healthy sea urchins (group N) on the first day. These results indicate that the segregation with two layers of PVC net, which completely eliminates the physical contact between healthy and diseased sea urchin, is an effective approach to preventing the transmission of black mouth disease of S. intermedius. However, the number of diseased sea urchins in the group T showed no significant advantage on the third and seventh day. This indicates that the effectiveness of the complete segregation for disease transmission is greatly reduced if the diseased/dead sea urchins are not removed in time. Further, no significant difference was found in the number of diseased sea urchins in the group T on the first day after the air exposure, indicating that the complete segregation is not effective to prevent the disease transmission after air exposure. Surprisingly, the morbidities of groups in the experiment 2 on the seventh day (62.5 ± 3.2%) was significantly lower than that of groups in the experiment 3 on the fifth day (95.8 ± 1.4%). This clearly indicates that air exposure deteriorates the transmission of the black mouth disease. Collectively, the present study suggests that complete segregation, which eliminates the physical contact between healthy and diseased sea urchin, is an effective approach to preventing the transmission of the black mouth disease, provided that diseased/dead sea urchins are removed in time and air exposure is avoided. In addition, the present study indicates that the physical contact between healthy and diseased sea urchins is important route of the disease transmission, although other spread routes of pathogenic bacteria are not exclusive.