FWIW, among vegans, there is a bit of controversy over certain bivalves. Oysters and Mussels both lack a sophisticated central nervous system and likely are closer to plants than even higher invertebrates in terms of anything like pain perception. There's a reasonable case to be made that ethical vegans should be able to eat them, especially considering that oyster and mussel farming actually removes pollution from the water and has a positive environmental impact (unsurprisingly, there is plenty of disagreement though).

I don't know much about sea sponges, starfish, or jellyfish so I won't comment on those. Octopuses though are actually very intelligent compared to most other invertebrates. There are examples of complex problem solving, observational learning, and even tool use. Their intelligence is very "different" than what we are used to and hard to compare apples to apples, but is probably closer to the level of mice/rats/dogs than to insects or fish.