These were drawn for my mum for a school project she's doing involving underwater civilizations. She asked me to invent up with one as inspiration for her class and this is what I came up with.

I've decided to call these guys Kessin. They used to live happily in shallower water until their food sources started running out, forcing them to move ever deeper in search of supplies. Eventually they found their way into the pitch-black thermal trenches deep beneath the surface, where their prey was plentiful. However they weren't the only large creatures down there; something nasty lurked in the darkness, preying on any Kessin that found themselves separated from their group.
Eventually, using their ingenuity, they were able to build colonies that floated above the seafloor using the thermals from the vents to keep themselves aloft. They were able to produce light sources by growing luminous algae in clear containers and spacing them around to see by.