**7. Conclusions**

We have presented here only a brief discussion of selected examples, which nonetheless provide novel data concerning poriferan collagens. In spite of the progress made in this research field, numerous open questions remain. For example, additional investigations are necessary to obtain understanding of the nature and origin of halogenated spongins. It is still not clear how many collagen and/or keratin domains they contain. Additionally, the unique resistance of these biopolymers against diverse chemicals and enzymes remains poorly investigated. The possible role of collagens in the spiculogenesis of demosponges and formation of axial filaments must also be researched. The discovery of crystalline proteins within amorphous biosilica-based structures in sponges is ground-breaking in the understanding of biomineralization. What can be discovered about the crystallinity of collagen within biosilica-related structures in sponges? The existence of collagen-based crystals within siliceous biominerals could revolutionize our understanding of the origin and evolution of collagens, from the point of view of biomineralization in sponges as the first multicellular organisms on Earth. Further, the relationship of collagen and chitin in the skeletal structures of diverse sponge classes and orders is entirely unknown. Consequently, we believe that the use of modern X-ray imaging techniques based on the "diffraction before destruction" principle is the best way forward to gain understanding of the principles of the unique organization of collagen within both fossil and recent collagen-based biomineralized constructs.

Novel approaches must be proposed which will bring together modern bioanalytical and molecular biology methods for better understanding of the fundamental principles of collagen fibrillogenesis and the mechanisms of its cross-linking in sponges, as well as details of the structural organization of poriferan collagens at the molecular and atomic levels. The best way to address this challenging task on these levels is by coherent synergetic collaboration using explicit reasoning and well-tested explanatory principles of multidisciplinary knowledge, experience, and new technologies. Finally, we suggest that studying the processes of marine farming of the collagen-producing demosponges has implications for a variety of practical large-scale applications, ranging from the design of highly effective extraction techniques to the development of novel collagen-containing composites for biomedicine and technology.

**Acknowledgments:** This work was partially supported by DFG Project HE 394/3-2 and PUT Research Grant no. 03/32/DSPB/0806. M.W. is grateful for financial support from the Foundation for Polish Science: START 097.2017, and S.Z.-A. for support from the DAAD and Erasmus Plus programs. ˙

**Author Contributions:** Hermann Ehrlich and Marcin Wysokowski researched the literature and wrote the manuscript; Iaroslav Petrenko prepared diagrams and images and edited the manuscript; Soni Z˙ ółtowska-Aksamitowska and Teofil Jesionowski discussed ideas and edited the manuscript.

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