Microplastics come directly or indirectly from human activities. The bay and coastal areas are constantly under pressure from human activities, including tourism, fishing, and aquaculture. Microplastic pollution is now recognized as a great threat to bay environments. In this study, we assessed microplastics in the Xincun Lagoon Bay, which had multiple human activities, to understand whether human activities could directly reflect the pollution level of microplastics. The results showed the dominant microplastics were small (100–500 μm, 45.2%) and transparent (57.0%), mainly consisting of fibers (95.7%), and cellophane (54.4%) was the dominant polymer. The color, type, and component indicated that wastes from aquaculture, laundry wastewater in the Tanka fishing raft area, and domestic wastes from tourism were the primary sources of microplastics in Xincun Bay. During the rainy season, microplastic abundances in surface water and surface sediment at the outer bay were found to be significantly higher (108.8 ± 37.1 item/L and 250.4 ± 92.0 item/kg, respectively) than those found at the inner bay (34.3 ± 12.9 item/L and 167.6 ± 71.7 item/kg, respectively), which presented a reverse trend in the dry season (p < .01). Although Xincun bay has a relatively closed environment and diverse human activities, concentrated rainfall and strong winds could cause microplastic abundance inside the bay during the rainy season to be lower than that outside the bay. Furthermore, the comparison of microplastic abundances in the areas with intensive human activities and non-human activity areas indicated that the intensity of human activities could not directly reflect the pollution of microplastics in the lagoon bay. It is also necessary to consider the geographical and seasonal characteristics in order to explain the pollution degree of microplastics in a bay.

Based on 16S rRNA gene high-throughput sequencing technology, the microbial community structure in the gut of three tropical sea cucumber species, Holothuria atra, Stichopus chloronotus, and S. monotuberculatus, and their habitat sediments were analyzed. The OTUs in the gut contents of S. monotuberculatus, S. chloronotus, H. atra, and their sediments were 2489 ± 447, 1912 ± 235, 1628 ± 150, and 4162 ± 94, respectively. According to alpha diversity analysis (Ace, Chao1, Shannon, Simpson), the richness and diversity of microflora in the gut of all three sea cucumber species were significantly lower than that in sediments (p < 0.01). Anosim analysis showed that the intra-group differences were less than the inter-group differences (R > 0), and the microbial community composition of the sediment was distinctly different from those of three sea cucumber species (p < 0.05). UPGMA tree and Anosim analysis also revealed that the gut microbial communities of H. atra and S. chloronotus were more similar than that of S. monotuberculatus. Proteobacteria was the predominant phylum in all samples, and there was no significant difference in relative abundance among all groups. Actinobacteria was also a dominant phylum, and the relative abundance in S. chloronotus was significantly higher than that in other samples (p < 0.05). Potential probiotics and sequences related to Lactobacillus and Pseudomonas that may be developed for sea cucumber culture were also found in the study. It is speculated that the main reason for the difference in microbial communities between gut microbiota and environmental sediments may be the unique and quite different environment in the digestive tract of sea cucumbers. Whereas, the differences in gut microbiota among the three sea cucumber species were caused by selective feeding. These findings may provide basic data for tropical sea cucumber gut microflora studies and assist in the sea ranching and aquaculture development of the tropical sea cucumber species.

In this study, the eukaryotic composition of gut contents in three tropical sea cucumber species, Stichopus monotuberculatus, S. chloronotus and Holothuria atra were surveyed and compared by metabarcoding analysis based on 18S rRNA gene V4 region. The sequences were assigned to 21.80 ± 1.07, 22.60 ± 0.68 and 22.40 ± 0.25 different phyla from the gut contents of S. monotuberculatus, S. chloronotus and H. atra, respectively, and those in sediment samples were assigned to 21.00 ± 1.67 phyla. The results of α-diversity showed that surface sediments had a greater eukaryotic diversity than gut contents, yet the guts of sea cucumbers had an enrichment effect on some microorganisms, including Diatomea and Apicomplex. A comparison of the gut eukaryotic community among the three species suggested that the feeding preference was different: S. monotuberculatus fed mainly on Diatomea and Arthropoda, and the other two species had higher Apicomplexa concentrations, which may be due to differences in the morphology of the tentacles and habitat preferences. Moreover, obvious different eukaryotic community composition in the gut contents of the three sea cucumber species and the surrounding sediments also might result from the animals’ selective feeding for sediment patches. The current study filled in gaps about feeding mechanisms of tropical sea cucumbers and provided a basis for further exploring the mechanism about selective feeding and sea cucumber–sediment interaction in the future.

Sea cucumber Stichopus monotuberculatus is one species of tropical sea cucumbers with high recognition and economic value. While advances have been made in the nursery rearing of the sea cucumber, influence of environmental factors on its locomotion behavior remains less understood, which restricts the establishment of mariculture technologies. In the present study, locomotion behavior of S. monotuberculatus under different temperatures and flow velocities were examined through controlled simulation experiments. Results showed that the creeping activities were obviously affected by temperature, and the most active movement and feeding behavior were recorded at relatively high temperatures. Diurnal variation of locomotion and feeding activities indicated that S. monotuberculatus displayed an evident nocturnal activity pattern, being the most active at night, exhibiting intermediate activity at dusk, and minimal activity during the daytime. The movement velocity decreased with the increasing flow speed and was only 1.65 ± 1.35cm·min^-1 in the highest flow rate group (20.8 ± 3.4cm/s). Moreover, the sea cucumber S. monotuberculatus displayed positive rheotaxis behavior of moving downstream at all flow velocities. Overall, the sea cucumber S. monotuberculatus exhibited high locomotor and feeding activities at night of relatively high temperature, and its favoured flow regime was downstream and low water velocity area. These findings may assist the sea ranching and aquaculture development of the tropical commercial sea cucumber species.

Sea ranching of tropical edible sea cucumbers is an effective way to relieve the overfishing stress on their natural resources and protect the coral reef ecosystem, yet only a few species have been applied in the sea ranching practice based on hatchery-reared juveniles around the world. In this study, an 8-month (April to December) sea ranching study for hatchery-reared edible sea cucumber Stichopus monotuberculatus juveniles was carried out at a tropical coral reef island area in Sanya, China. Several growth performance indexes and basal nutritional components were monitored. Results revealed that the sea cucumbers had a growth rate of 0.35~0.78 mm day^-1 during the experimental period, reaching 15.9 cm long before winter. The weight gain reached 491.13% at the end, and most sea cucumbers were able to grow to the commercial size (over 150 g WW) in the first year of sea ranching. The overall specific growth rate (SGR) and survival rates were 0.73 and 27.5%. Most of the death occurred in the first month after release (25.0%–37.5%), and this is probably due to inadaptation to the sudden change of the environment from the hatchery to the wild, which is proved by the remarkable decrease in nutritional indexes (amino acids, total lipid, and crude protein). Stable isotope and lipid biomarkers revealed that the food source mainly comes from water deposits (with microbes), Sargassum sanyaense seaweed debris, phytoplankton, and coral mucus-derived organics. The study proved the feasibility of the sea ranching of the hatchery-reared S. monotuberculatus juveniles in the tropical coral reef island area. Also, it is highly recommended that appropriate acclimation operation before release should be carried out to improve the survival rate of this species.