Interesting marine life?

Not just necessarily vertebrates/invertebrates, but also plant life, rock types, etc, extinct or extant. When I say interesting, I mean pretty much anything. Interesting features, behaviour, shapes, colours, textures, so on. Would be really nice if they add to a horror/creep factor, but not necessary. To give you an idea, some of the things I've found interesting are the strange eyes of Ipnops; the hunting habits and generally ugly appearance of the stargazer fish; the pharyngeal jaw of moray eels; the eldritch dentition of the extinct Atopodentatus before a non-crushed skull was found and showed it to be a prehistoric vacuum cleaner. Really, though, don't let that limit any suggestions you might have!

I would probably restrict this to saltwater, anywhere in the world, but I'm not averse to looking at freshwater stuff either. 

As for why I'm asking, I'm currently gathering inspiration for creature designs for Rogue Magic, my personal project. It's a story about a gang of thieves who steal a magical artefact and accidentally unleash a sealed-away power which now very much wants them. While, yes, I'm designing creatures, plants/rocks/etc can be extremely inspirational for shape language and other features, so I don't want to exclude or overlook anything. I'd be extremely grateful if anyone's got stuff to share!