**5. Conclusions**

In this study, we have explored the stress-induced mucus secretion and its constituent composition in the jellyfish *A. coerulea* by a combination of proteomics and metabolomics. Our first conclusion is that two different but successive phases have been drawn from the initial stress to the final autolysis and death, with an obvious inflection point occurring at 30 min after removing the jellyfish from seawater. The results of proteomics using GO and KEGG analyses drew our second conclusion that the proteins in stress-induced mucus are independent (i.e., different) to those in tissue homogenate. We also identified that the mucus-enriched proteins are mainly located in the extracellular or membrane-associated region, while the tissue-enriched proteins are located in the intracellular compartment. The results of metabolomics are of particular interest; the potential neurotransmitter or neuromodulator, tryptamine, displays the maximal FC value of 7.8, a significantly elevated value among 16 other different metabolites in stress-induced mucus compared to those of tissue homogenate. This supports the hypothesis that a drastic nerve stress response, as well as a tempestuous release of neurotransmitters, occurs upon stress initiation. Finally, 11 metalloproteinases, four serine proteinase inhibitors, three SODs, three complements, and four toxin-related proteins were successfully assigned to function as self-protective components. In summary, our results provide a constituent profile of proteins and metabolites in stress-induced mucus and tissue homogenate of *A. coerulea*. This profile is, we believe, particularly important in equipping us with a better understanding of the process of stress-induced mucus secretion, as well as signaling the important role these self-protective components play in the innate immunity of jellyfish and in the ongoing discovery of novel bioactive compounds.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/16/9/341/s1, Table S1: Sequences of self-protective proteins.

**Author Contributions:** Investigation, W.L., F.M. and G.J.; Methodology, L.X.; Project administration, L.Z.; Software, H.L., C.M. and T.L.; Writing—original draft, W.L. and L.X.; Writing—review & editing, G.L.M., J.Z. and L.X.

**Funding:** This work was supported by the Pujiang talent plan project (16PJ1411000) from Shanghai science and Technology Commission, Excellent youth talent program from Shanghai Municipal Commission of Health and Family Planning (2017YQ007), and the general program from the National Natural Science Foundation of China (81470518, 81770329).

**Acknowledgments:** The authors acknowledge the assistance of MeiGui Zhu, Na Yi and Ning Chen for determination of proteomics from the company Bangfei Bioscience in China, and the help of language refining by I. Darren Grice from Institute for Glycomics of Griffith University, Australia. We also thank Xinshu Li who generously provided us with the jellyfish *A. coerulea*.

**Conflicts of Interest:** The authors declare no conflict of interest.
