The blood belly comb jelly.
It is the only species in its genus, and its genus is the only one in its family. In other words, this species is so special it has a family of its own. It also doesn't have Hox genes. Hox genes determine the position of body subparts (for example, first comes head, then torax, then legs. if humans didn't have Hox genes, you could have been born with your head in the place of your butt and vice versa). The lights aren't the result of a chemical reaction but of the simple physical phenomenon of refraction: cillia vibrate so fast they refract light. And the main function of the cillia is actually swimming.
Like the rest of Ctenophora, they also don't have microRNA, unlike all other animals. They also have colloblasts, which are used to stick to prey. The entire organism is basically just a big mouth that floats through the oceans and eats anything it attracts with its lights, from microorganisms to other comb jellies. I probably have share some genes with abyssal creatures because I'm deeply attracted by these predatorial lights.
how big is that thing
As with most abyssal creatures, size greatly varies. From 15mm to 50cm.