**7. Conclusions**

Comparative immunobiology studies have led to the abandonment of the idea that invertebrates do not possess immune capacity. Cnidarians possess components of the main routes of immunity of invertebrates. The receptors and pathways already identified indicate that these basal invertebrates are far from being "simple" in the range of methods they have to deal with potential germs and pathogens.

Cnidarian-associated microbial communities are probably a result of a functional cross-talking because cnidarian need to control the resident beneficial microbes, not as a response to invasive pathogens, but because, just as black can exist only if white is visible, so too the use of the same thrifty ways for distinguishing pathogens could be considered the possible origin of the first immunity arms.

In Cnidarians, the crucial activities in structuring tissue-associated microbial communities, useful to the animal's health, are related to the increasing evidence of the existing innate immune responses involved in the disruption of harmful microorganisms. The present review represents a contribution to reduce the gaps in the current knowledge, regarding the complex relationships established between cnidarians and microorganisms, as well as to provide an overview of the potential biotechnological applications of the defensive compounds present in these invertebrates.

**Author Contributions:** The first two authors contributed equally. L.S. reviewed all the aspects related to the Cnidarian-microorganisms relationships. L.S., M.G.P., D.P. and M.C. examined all the immunological aspects. L.S., M.G.P. and M.C. wrote the manuscript.

**Acknowledgments:** Financial support was provided by PJ\_RIC\_FFABR\_2017\_004312 incentivi alle attività base di Ricerca ANVUR and the Life Project REMEDIA Life "REmediation of Marine Environment and Development of Innovative Aquaculture: exploitaiton of edible/not edible biomasss" (LIFE Environment and Resource Efficiency 2016. LIFE16 ENV/IT/000343) funded by EU and by FFR-Cammarata from Scientific Research University of Palermo (2014) and MC RITMARE Project (CNR and CONISMA).

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