*3.1. Mortality and Morbidity*

The mortality (53.3 ± 5.0%) and morbidity (54.4 ± 5.5%) of the group M were significantly lower than those in the group U (67.8 ± 12.1%, *t* = 7.349, *p* = 0.022 for mortality, Figure 2A) (68.0 ± 12.0%, *t* = 6.297, *p* = 0.031 for morbidity, Figure 2B). However, there were no significant differences in mortality (14.0 ± 13.0% for group M and 21.6 ± 11.8% for group C) and morbidity (15.3 ± 15.0% for group M and 23.9 ± 12.5% for group C) between groups M and C (*t* = 1.141, *p* = 0.310 for mortality, Figure 2C) (*t* = 1.156, *p* = 0.307 for morbidity, Figure 2D).

**Figure 2.** Mortality of *Strongylocentrotus intermedius* (**A**) and *Apostichopus japonicus* (**C**) as well as the morbidity of *S*. *intermedius* (**B**) and *A*. *japonicus* (**D**) between groups (mean ± SD, *n* = 6). Group C: the control group for sea cucumbers. Group U: the control group for sea urchins. Group M: the integrated multi-trophic aquaculture of sea cucumbers and sea urchins. The asterisk \* means *p* < 0.05.

#### *3.2. Growth*

Test diameter (12.8 ± 2.2 mm) and body weight (0.8 ± 0.3 g) of *S*. *intermedius* in group M were significantly higher than those in group U (10.4 ± 2.1 mm, *t* = 6.197, *p* < 0.001 for test diameter, Figure 3A) (0.5 ± 0.2 g, Mann–Whitney *U* = 669, *p* < 0.001 for body weight, Figure 3B). Further, body length (38.2 ± 3.1 mm) and body weight (1.4 ± 0.8 g) of *A*. *japonicus* in group M were significantly larger than those in group C (33.9 ± 1.3 mm, *t* = 9.500, *p* = 0.012 for body length, Figure 3C) (1.0 ± 0.4 g, Mann–Whitney *U* = 1260, *p* = 0.005 for body weight, Figure 3D).

**Figure 3.** Test diameter (**A**) and wet body weight (**B**) of *Strongylocentrotus intermedius* between groups U and M. Body length (**C**) and wet body weight (**D**) of *Apostichopus japonicus* between groups C and M (mean ± SD, *n* = 6). Group C: the control group for sea cucumbers. Group U: the control group for sea urchins. Group M: the integrated multi-trophic aquaculture of sea cucumbers and sea urchins. The asterisks \*, \*\* and \*\*\* mean *p* < 0.05, *p* < 0.01, *p* < 0.001, respectively.

No significant difference in lantern length/test diameter was found between groups U (0.033 ± 0.006) and M (0.037 ± 0.005) (*t* = 0.002, *p* = 0.999, Figure 4A)

**Figure 4.** Lantern length/test diameter (**A**) and Aristotle's lantern reflex (**B**) of *Strongylocentrotus intermedius* between groups U and M (mean ± SD, *n* = 6). Tentacles activity frequency (**C**) and crawling frequency (**D**) of *Apostichopus japonicus* between groups C and M (mean ± SD, *n* = 6). Group C: the control group for sea cucumbers. Group U: the control group for sea urchins. Group M: the integrated multi-trophic aquaculture of sea cucumbers and sea urchins. The asterisks \* and \*\* mean *p* < 0.05 and *p* < 0.01, respectively.
