**4. Conclusions**

Our results present the first evidence of the microbial origin of 6-*epi*-monanchorin (**2**), previously isolated from the secreted mucus trapping net of the marine polychaete *C. variopedatus*. Using the 16S rRNA gene analysis, it was revealed that diverse *Vibrio* species are dominant bacteria cultured from the *C. variopedatus* mucus net. In addition, it was shown that these bacteria belong to several different species of the genus *Vibrio*. The strain CB1-14, producing alkaloid **2**, was recognized as a new species in the genus *Vibrio* by phylogenetic reconstruction using eight protein-coding genes. Our results suggest that filter-feeding polychaetes should be considered as a novel source of alkaloid-producing bacteria.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/17/4/213/s1. HRESIMS spectra of 6-*epi*-monanchorin (**2**) isolated from polychaete *Chaetopterus variopedatus* and marine bacterium Vibrio sp. CB1-14; 1H NMR spectra for 6-*epi*-monanchorin (**2**) isolated from polychaete *Chaetopterus variopedatus* and marine bacterium *Vibrio* sp. CB1-14; HPLC chromatograms of culture medium extract and cells extracts of CB1-14 and Table S1. Accession numbers and Chimera identification of 16S rRNA of bacterial isolates. The GenBank accession numbers for all 16S rRNA nucleotide sequences will be available on online on the NCBI server after 20 July 2019.

**Author Contributions:** T.M., V.S., M.I., L.S., and V.K. wrote the paper; L.S. isolated and purified 6-*epi*-monanchorin; E.B. performed 16S rDNA study; N.C. and M.I. performed genome sequencing of CB1-14 and MLSA study; R.P. and P.D. performed MS study.

**Funding:** This work was supported by the Grant No. 17-14-01065 from the RSF (Russian Science Foundation).

**Acknowledgments:** The authors are grateful to Guzev K.V. for nucleotide sequencing of 16S rDNA sequences and Dr. Grebnev B.B. for the animal identification. The spectral data were obtained on the equipment of the Collective Facilities Center (The Far Eastern Center for Structural Molecular Research (NMR/MS) PIBOC FEB RAS).

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