Violet bioluminescent Polycirrus sp. (Annelida: Terebelliformia) discovered in the shallow coastal waters of the Noto Peninsula in Japan.

Terebellidae worms have large numbers of tentacles responsible for various biological functions. Some Terebellidae worms whose tentacles emit light are found around the world, including exceptional violet-light-emitting Polycirrus spp. found in Europe and North America. However, there is no video-recorded observation of the luminous behavior of such unique species in nature, and the genetic information related to their ecology are lacking. Here, for the first time, we video-recorded the violet-light-emitting behavior of an undescribed Japanese worm in its natural habitat. The worm was designated as Polycirrus sp. ISK based on morphological observations, and the luminescence spectrum showed a peak at 444 nm, which is an exceptionally short wavelength for bioluminescence in a shallow coastal water environment. An analysis of differentially expressing genes based on separate RNA-Seq analysis for the tentacles and the rest of body revealed the specific expression of genes that are probably involved in innate immunity in the tentacles exposed to predators. We also found a Renilla luciferase homologous gene, but coelenterazine was not detected in the worm extract by analyses using a liquid chromatography and a recombinant Renilla luciferase. These results will promote an understanding of the ecology and luminescence mechanisms of luminous Polycirrus spp.