**4. Conclusions**

This study provides the first complex analysis of application of LbL-assembled polyelectrolyte microcapsules in the crustacean organism on the example of *E. verrucosus*. It is also the first study assessing the invertebrate immune response to PEG-covered implants. According to our results, the central hemolymph vessel should be proposed as the most convenient organ for visualization of fluorescent microsensors in most small crustaceans with translucent integument. The tested number of microcapsules (that was su fficient for stable visualization in the central hemolymph vessel) did not show lethal or sublethal toxic e ffects. Although the cellular immune response to the PEG-coated microcapsules was delayed in comparison to microorganisms, it also was the case. Nevertheless, during the first hours the proportion of involved hemocytes was low and could not a ffect a significant number of microcapsules. This makes the application of such microcapsules for physiological sensing possible, at least for the first several hours after injection.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/8/1246/s1, Figure S1: Checkup of *E. verrucosus* exoskeleton integrity after injection between the sixth and seventh segments of pereon (area of the central hemolymph vessel), Figure S2: Melanization (dark area in the center) inside an aggregate of hemocytes with microcapsules two days post introduction of microcapsules into the culture media, Figure S3: Example of hemocyte accumulation around an aggregate of microcapsules one week post injection into the central hemolymph vessel of *E. verrucosus*.

**Author Contributions:** Conceptualization, A.G., E.S., E.B., A.N. and M.T.; Data curation, E.S., A.G., A.N. and E.B.; Formal analysis, E.S., A.G. and A.N.; Funding acquisition, M.T.; Investigation, E.S., A.N., I.D. and Y.R.; Methodology, E.S., A.G., A.N. and I.M.; Project administration, M.T. and I.M.; Supervision, A.G., E.B., M.T. and I.M.; Validation, E.S., A.G. and A.N.; Visualization, E.S., A.G. and A.N.; Writing—original draft, E.S., A.G. and A.N.; Writing—review & editing, E.S., A.G., E.B., I.D., Y.R., M.T. and I.M.

**Funding:** The study was supported by the Russian Science Foundation (#17-14-01063), the Russian Foundation for Basic Research (#19-54-04008) and Lake Baikal Foundation (#02-3/14). E.S. also acknowledges the partial support from the Ministry of Science and Higher Education of Russia and Deutscher Akademischer Austauschdienst ("Goszadanie": #6.13468.2019/13.2).

**Acknowledgments:** We acknowledge Lidia Yu. Raevskaya and Irina A. Zaravnyaeva (Irkutsk Regional Diagnostic Center, Pathology Department) for help in the preparation of the histological section, as well as Polina Drozdova (Irkutsk State University) for the photo of *E. verrucosus* and help in the preparation of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
