*Article* **A New Approach to Integrated Multi-Trophic Aquaculture System of the Sea Cucumber** *Apostichopus japonicus* **and the Sea Urchin** *Strongylocentrotus intermedius*

**Fangyuan Hu †, Huiyan Wang †, Ruihuan Tian, Jujie Gao, Guo Wu, Donghong Yin \* and Chong Zhao \***

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

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

† These authors contributed equally to this work.

**Abstract:** The sea cucumber *Apostichopus japonicus* and the sea urchin *Strongylocentrotus intermedius* are two commercially important species and are widely cultured in China. Here, a laboratory experiment was conducted for 34 days to assess whether the survival, growth and behavior performances are better in the new commercially valuable integrated multi-trophic aquaculture (IMTA) system (group M, 90 *S*. *intermedius* and 37 *A*. *japonicus*/10,638 cm<sup>3</sup> of stocking density) than those in the control group for sea urchins (group U, 90 *S*. *intermedius*/10,638 cm<sup>3</sup> of stocking density) and the control group for sea cucumbers (group C, 37 *A*. *japonicus*/10,638 cm<sup>3</sup> of stocking density). We found that feeding behavior, crawling behavior, body length and body weight of sea cucumbers were significantly greater in group M than those in group C. These results suggest that the new IMTA system improves fitness-related behaviors and consequently leads to a better growth in *A*. *japonicus* while maintaining a high biomass. We further found that group M showed significantly larger body size and Aristotle's lantern reflex as well as significantly lower mortality and morbidity in sea urchins, compared to those in group U. This suggests that the new IMTA system greatly improves feeding behavior and body growth, and survival of cultured *S*. *intermedius*. This IMTA system is a promising candidate to promote the production efficiency of juvenile *A*. *japonicus* (as primary species) and *S*. *intermedius* (as subsidiary species) in China.

**Keywords:** echinoderm; IMTA; feeding behavior; growth; survival
