*2.2. Experiment I*

This experiment was carried out to investigate whether artificial reefs improve the survival and fitness-related behaviors (crawling, feeding, and adhesion) of sea cucumbers exposed to the air at 25 ◦C. The presence or absence of artificial reefs was the experimental factor. An artificial reef (length × width × height: 50 mm × 60 mm × 55 mm, Figure 1A) consists of seven plastic cylinders (diameter × length: 15 mm × 50 mm, Figure 1A), in which only one sea cucumber is accessible. The plastic boxes (top length × bottom length × height: 20 cm × 16.5 cm × 14 cm) with and without artificial reefs were named group O and group C, respectively (*n* = 6). Twelve plastics boxes were placed in two temperaturecontrolled tanks (length × width × height: 115 cm × 75 cm × 60 cm) to maintain the water temperature at 25 ◦C. Two groups of sixteen sea cucumbers (~1 g of wet body weight) without ulcerated body walls were randomly selected and placed in each box with aeration (Figure 1B). Groups C and O each had six parallel groups. Sea cucumbers in each box were fed 5 g of a mixture of sea mud and commercial feed (Anyuan Industrial Co., Ltd. Yantai, China) at a 1:4 ratio every day. Sand-filtered seawater was changed daily in plastic boxes. Sea cucumbers were exposed to the air for 30 min after the seawater was removed from each box. Sea cucumbers were cultured in the above experimental conditions for 5 days.

We identified the sea cucumber with ulcerated skin as a diseased individual (Figure 1D). The numbers of dead and diseased sea cucumbers in each group were counted after the experiment. Sea cucumbers without ulcerated body walls (Figure 1D) were placed in plastics boxes (top length × bottom length × height: 20 cm × 16.5 cm × 14 cm) in group C. Consistently, individuals without ulcerated body walls in group O were divided into two groups, among which sea cucumbers in one group were inside the artificial reefs and sea cucumbers in the other group were outside the artificial reefs. Then, the two groups were placed in two different cubic plastic boxes (top length × bottom length × height: 20 cm × 16.5 cm × 14 cm). Finally, we obtained three groups of sea cucumbers without ulcerated body walls, which were the ones from group C, the ones inside the artificial reefs in group O (group O—In), and the ones outside the artificial reefs in group O (group O—Out). Three sea cucumbers were randomly selected from each of the three groups for the measurement of fitness-related behaviors (feeding, crawling, and adhesion). Feeding behavior and crawling behavior of the sea cucumbers were recorded using a digital camera (FDR-AXP55, Shanghai Suoguang Electronics Co., Ltd., Shanghai, China) fixed above the plastic cubic boxes (length × width × height: 60 mm × 47 mm × 45 mm) within 1 h. Adhesion behavior was measured in cubic plastic boxes (length × width × height: 180 mm × 140 mm × 45 mm) with 18 compartments. We repeated this experiment 6 times using different sea cucumbers for each group (*n* = 6).

**Figure 1.** The diagrams show an artificial reef (**A**), the experiment designs for experiments I (**B**) and II (**C**), diseased and individuals without ulcerated skin (**D**), adhesion behavior (**E**), feeding behavior (**F**), and crawling behavior (**G**) of sea cucumbers.

#### *2.3. Experiment II*

We further investigated whether artificial reefs improve the survival and fitnessrelated behaviors (crawling, feeding, and adhesion) of sea cucumbers at 25 ◦C when disease outbreaks. The same artificial reefs as experiment I were consistently used in experiment II. Sixteen sea cucumbers without ulcerated body walls were placed in each box (Figure 1C). Diseased sea cucumbers were subsequently put in to create a disease outbreak. The plastic boxes (top length × bottom length × height: 20 cm × 16.5 cm × 14 cm) with and without artificial reefs were named as group O and group C, respectively (*n* = 6). Twelve plastic boxes (top length × bottom length × height: 20 cm × 16.5 cm × 14 cm) were placed in two temperature-controlled tanks (length × width × height: 115 cm × 75 cm × 60 cm). Each group was daily fed ad libitum with 5 g of a mixture of sea mud and commercial feed (Anyuan Industrial Co., Ltd., Yantai, China) at a 1:4 ratio. Sea cucumbers were cultured for three days without the seawater replacement in the same experimental conditions.

The numbers of dead and diseased sea cucumbers were counted after the experiment. Consistently, we obtained three groups of sea cucumbers without ulcerated body walls, which were the ones from group C, the ones inside the artificial reefs in group O (group O—In), and the ones outside the artificial reefs in group O (group O—Out). Crawling, feeding, and adhesion behaviors of sea cucumbers were recorded following the same procedures as described above for each group.
