This sea pen species is known as the the phosphorescent sea pen (Pennatula phosphorea). It is an animal related to tropical soft corals and is made up of lots of tiny polyps living together in a colony. They are most easily seen in the calm still water of many Scottish sea lochs where they can be found living on silty muddy sea beds. They are filter feeders and this species is one of three living in British waters. As the name suggests they are capable of bioluminescence and although i have not seen it, light can be emitted by the polyps
This photo was from a dark murky site on the north shore of Loch Leven, at a depth of about 18 metres.
Nikon D850, Sigma 15mm, dual Inon S2000 flashguns