They are not fish. they are not even vertebrates. they are echinoderms. You saythey don’t have brains – it is true that the nervous system is not organized in a central organ or ‘brain,’ but they do have a system of nerves that control sensory sytems, movement, balance, orientation, etc.
Starfish are Inverterbrites, nature has made it so they dont require a brain to survive. In effect this means that all the processes that happen inside are starfish are very simple and interconnected