*3.3. Aristotle's Lantern Reflex of S. intermedius*

Significantly higher Aristotle's lantern reflex was observed in group M (4.0 ± 1.2 times·min<sup>−</sup>1) than that in group U (3.1 ± 1.5 times·min<sup>−</sup>1) (Mann–Whitney *<sup>U</sup>* = 611.5, *<sup>p</sup>* = 0.017, Figure 4B).

#### *3.4. Feeding and Crawling Behaviors of A. japonicus*

The tentacles activity frequency (6.1 ± 3.1 times) and crawling frequency (10.5 ± 8.1 times) of group M were both significantly higher than those of group C (4.0 ± 2.8 times, Mann–Whitney *U* = 176, *p* = 0.020 for tentacles activity frequency, Figure 4C) (5.2 ± 2.6 times, *t* = 3.045, *p* = 0.005 for crawling frequency, Figure 4D).

#### **4. Discussion**

#### *4.1. Growth Performance of A. japonicus at a High Biomass*

The sea cucumber *A*. *japonicus* (>1 g of body weight) are seeded into the bottom of the sea in the pond culture and stock enhancement in China [33]. This suggests that juvenile *A*. *japonicus* require an intermediate culture in land-based nursery tanks. However, slow growth and serious food wastage occur in the production of juvenile *A*. *japonicus* (<1 g of body weight), which greatly hampers the development of the aquaculture industry. Significantly better body size and body weight were found in group M (90 *S*. *intermedius* and 37 *A*. *japonicus*/10,638 cm3) than those in group C (37 *A*. *japonicus*/10,638 cm3). High stocking density is generally considered to display negative impacts on the growth of cultured animals [34,35]. Reducing density is therefore a general practice for the trade-off between animal welfare and economic benefits [36,37]. This study suggests an effective approach that greatly improves body growth while maintaining a high biomass. More research should be conducted to support the potential extension to large-scale aquaculture using this new IMTA system.

#### *4.2. Fitness-Related Behaviors and Growth in A. japonicus*

Feeding and crawling are essential fitness-related behaviors, displaying a strong relationship with the growth performance of *A*. *japonicus* [38]. Feeding behavior is defined as the process of sea cucumbers collecting diets using the tentacles around their mouth [39,40]. The tentacles activity commonly reflects food consumption in the sea cucumber *Cucumaria frondosa* [28]. Crawling behavior is the movement pattern that sea cucumbers crawl to a place where is suitable for their survival [30,31]. Significantly greater tentacles activity frequency and crawling frequency occurred in group M than those in group C, which probably leads to better body growth of *A*. *japonicus*. It has been well documented that juvenile *A*. *japonicus* lose their balance when they move to the silt or sand [41,42], because their ambulacral feet are adapted for attaching to a large enough surface [43]. The surface of the plastic box used in group M was smooth and covered with powdered diets, which probably provided an optimal shelter and habitat for *A*. *japonicus*, and thus improved their fitness-related behaviors. This indicates that the production efficiency of *A*. *japonicus* can be further improved in the existing nursery tanks. For example, adding artificial reefs to the bottom of nursery tanks could represent a promising candidate to promote the growth performance of *A*. *japonicus*.
