Mertensia ovum - a ctenophore, also known as comb jelly.
From Claire Nouvian’s book, The Deep. Highly recommended. It’s a book that packs more than stunning photographs: the book details so much newly-gained knowledge of various deep-sea organisms, often photographed in situ.
Note that the iridescence often observed on a ctenophore’s ciliary rows are not an example of bioluminescence. It’s simply a refraction of light.