Artificial reefs reduce the adverse effects of mud and transport stress on behaviors of the sea cucumber Apostichopus japonicus.

Poor survival of seeds reduces the production efficiency of the sea cucumber Apostichopus japonicus in pond culture. We investigated the effects of sea mud on the movement-related behaviors of A. japonicus with different body sizes. Mud significantly decreased crawling behavior and wall-reaching behavior in small seeds (~ 1 g of body weight), but not in the large ones (~ 2.5 g of body weight). These behaviors were significantly greater in the large seeds of A. japonicus than those in the small individuals when they were both on the mud. This clearly suggests that mud has negative effects on the movement-related behaviors of small seeds, but not on large individuals. We further assessed the effects of inevitable transport stress on the movement-related behaviors of A. japonicus on mud. Significantly poorer performances in crawling behavior, wall-reaching behavior and struggling behavior were observed in stressed A. japonicus (both sizes) than those in unstressed groups. These new findings indicate that transport stress further increases the adverse effects on the movement-related behaviors of A. japonicus on mud. Moreover, we investigated whether adverse effects can be reduced when individuals are directly seeded onto artificial reefs. Crawling behavior, wall-reaching behavior and struggling behavior in stressed A. japonicus (both sizes) seeded onto artificial reefs were significantly greater than those on mud, whereas artificial reefs did not significantly improve the crawling and struggling behaviors of unstressed small seeds. These results collectively indicate that mud and transport stress show negative impacts on the movement-related behaviors of sea cucumbers. Artificial reefs greatly reduce these adverse effects and probably contribute to improving the production efficiency of sea cucumbers in pond culture.

Transcriptomes reveal the involved genes in the sea urchin Mesocentrotus nudus exposed to high flow velocities.

Despite the importance of flow velocity in marine ecosystems, molecular mechanisms of the water flow induced behavioral and growth changes remain largely unknown in sea urchins. The present study compared the gene expressions of the sea urchin Mesocentrotus nudus at high flow velocities (10 cm/s and 20 cm/s) and low flow velocity (2 cm/s) using transcriptomes. A total of 490 and 470 differentially expressed genes (DEGs) were discovered at 10 cm/s and 20 cm/s, respectively. There were 235 up-regulated and 255 down-regulated genes at 10 cm/s, 213 up-regulated and 257 down-regulated genes at 20 cm/s, compared with sea urchins at 2 cm/s. Further, there were 72 overlapped DEGs involved in regulation at both 10 cm/s and 20 cm/s. Gene Ontology (GO) functional annotation showed that DEGs were mainly enriched to cellular process, cell part, binding, and metabolism process. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis found that DEGs were enriched in three pathways related to amino acid metabolism and lipid metabolism. A number of genes related to growth and metabolism of sea urchins were mobilized in high flow velocity environment. We further highlighted a muscle-associated gene ankyrin-1, which is correlated with the movement of tube feet at different flow velocities. The present study provides valuable information on the molecular mechanisms of changed behaviors and growth when sea urchins are exposed to high flow velocity.

Macroalgae and interspecific alarm cues regulate behavioral interactions between sea urchins and sea cucumbers.

Sea urchins and sea cucumbers are mutually beneficial organisms in kelp ecosystem. As herbivores, sea urchins process kelp through feeding and egestion, providing inaccessible food for benthic consumers such as sea cucumbers. Sea urchins in turn profit from the sediment cleaned by sea cucumbers. However, behavioral interactions between them remain poorly understood, which greatly hampers our understanding on the relationship between ecologically important benthic species in marine ecosystems and the regulating mechanism. The present study investigated behavioral interactions between sea urchins Strongylocentrotus intermedius and sea cucumbers Apostichopus japonicus in laboratory conditions. We revealed that the presence of sea urchins caused significant higher speed movement of A. japonicus. Interestingly, the negative effects of S. intermedius on A. japonicus were significantly reduced in the shared macroalgal area. For the first time, we found the interspecific responses to alarm cues between sea cucumbers and sea urchins. Conspecific responses were significantly larger than the interspecific responses in both sea urchins and sea cucumbers. This indicates that interspecific response to alarm cues is an efficient approach to anti-predation and coexistence in mutually beneficial organisms. The present study shed light on the interspecific relationships and coexistence between sea urchins and sea cucumbers in kelp ecosystem.