**2. Materials and Methods**

#### *2.1. Experimental Animals*

Juvenile *S*. *intermedius* (7.4 ± 1.0 mm of test diameter, 0.2 ± 0.1 g of wet body weight, mean ± SD) (*n* = 20) and *A*. *japonicus* (green type, 30.1 ± 3.5 mm of relaxed body length, 1.1 ± 0.4 g of wet body weight, mean ± SD) (*n* = 10) were randomly chosen from an aquafarm of Lvshun, Dalian (121◦13 E, 38◦88 N) on 13 April 2021 and Dalian Zhuang Yuanhai Ecological Seedling Industry Co., Ltd. (122◦69 E, 39◦27 N) on 16 April 2021, respectively. They were subsequently maintained in fiberglass tanks (length × width × height: 1150 × 750 × 600 mm) with aeration in the Key Laboratory of Mariculture & Stock Enhancement in north China's Sea, Ministry of Agriculture and Rural Affairs at Dalian Ocean University (121◦56 E, 38◦87 N). The incandescent light intensity was ~30 lx with the photoperiod (12 light: 12 dark), according to the culture management commonly used for seed production in China. Sea urchins and sea cucumbers were fed the leaf blade of fresh kelp *S*. *japonica* and a commercial sea cucumber powdered diet (mainly composed of algal powder, the grain size of 0.125 mm) (Anyuan Industrial Co., Yantai, China.), respectively. Two-thirds of seawater was changed daily. Water temperature and salinity were monitored daily using a portable water quality monitoring meter (Xylem Co., OH, USA). They were 15.4 ± 0.2 ◦C and 30.8 ± 0.1‰, respectively.

Experimental animals were fasted for three days to standardize their nutritional status, and their initial body sizes were subsequently assessed at the beginning of the trial.
