*Article* **Artificial Reefs Reduce Morbidity and Mortality of Small Cultured Sea Cucumbers** *Apostichopus japonicus* **at High Temperature**

**Huiyan Wang †, Guo Wu †, Fangyuan Hu, Ruihuan Tian, Jun Ding, Yaqing Chang, Yanming Su \* and Chong Zhao \***

Key Laboratory of Mariculture and Stock Enhancement in North China s Sea, Ministry of Agriculture and Rural Affairs, Dalian Ocean University, Dalian 116023, China

**\*** Correspondence: suym@dlou.edu.cn (Y.S.); chongzhao@dlou.edu.cn (C.Z.)

† These authors contributed equally to this work.

**Abstract:** Summer mortality and morbidity are serious environment-related problems in cultured sea cucumbers (*Apostichopus japonicus*). Air exposure probably worsens the impact of high temperature on cultured sea cucumbers. In this present study, two laboratory experiments were designed to investigate the effects of artificial reefs on mortality, morbidity, crawling, feeding, and adhesion behaviors of small sea cucumbers (~1 g of wet body weight) after air exposure and disease outbreaks at 25 ◦C, respectively. Significantly lower mortality and morbidity occurred in the group with artificial reefs compared with those in the group without artificial reefs in the two experiments. This present study found that the stressed sea cucumbers cultured inside artificial reefs showed a significantly higher adhesion index, feeding behavior, and crawling frequency than those cultured without artificial reefs. In disease challenge assays, small sea cucumbers cultured inside the artificial reefs showed a significantly higher adhesion index and crawling frequency than those cultured without artificial reefs at 25 ◦C. Feeding, crawling, and adhesion behaviors of sea cucumbers cultured outside artificial reefs were not significantly different from those cultured without artificial reefs. The experimental results indicate that sea cucumbers with good fitness-related behaviors may be less affected by the disease and more likely to move into the crevices of artificial reefs. Fitness-related behaviors were poor in sea cucumbers cultured outside artificial reefs, so we considered them as affected individuals. Thus, artificial reefs provide a place to reduce the physical contact between unaffected and diseased/affected individuals, showing a potential to reduce disease transmission. Our present study establishes a cost-effective approach to increasing the survival of small sea cucumbers in seed production at high temperatures.

**Keywords:** sea cucumbers; morbidity; mortality; artificial reef; high temperature
