The decentralized, localized neural system of the jellyfish can theoretically generate reaction response far faster than a centrally mediated system. Could it be that the jellyfish perception/reaction frame exist on a time scale far too quick for centrally mediated neural beings to comprehend? Does localized neural system of the jellyfish allow for directed action, or would such localizes neural net be purely limited to reactions?
I like the concept of umwelt, as it suggests at the recognition of identity being tied to communal/cultural/biological matrix of a creature. If we can extrapolate the concept further, in essence, human identity is linked to the shared experience within the context of his environment. It reminds me of Thomas Sowell's observation of objective cultural superiority existing in relation to environmental and functional matrix. In destroying our world, man pollutes the matrix that defines his being, and ultimately self-annihilates not only his physical aspect, but the very conception of humanity itself.
Dolphins definitely live in a faster time scale than we do. Jellyfish do seem to have fairly fast reaction times, but they're not unbelievably fast- their nerves aren't particularly thick, which is a large component of the speed of reactions and actions. I think that the localized parts of the jellyfish nervous system largely perform reactions over actions as well.
Umwelt is right there on my short list of my most used/favorite philosophical concepts. Where exactly we draw the line of umwelt is one of the most important and useful questions it poses. With ants or bees many thinkers happily claim that a hill or hive shares a single umwelt, but where does that end? They almost universally claim we all have individual umwelts, but a town or nation having an umwelt seems entirely plausible. I'd entirely agree with your extrapolation linking human identity to shared experience. The tendency to compartmentalize and treat individuals as discreet really is a problematic facet of Western culture at times. By doing so, it's exactly as you say- we're polluting the matrix, the shared experience that is the cultural umwelt.
I've occasionally used Sowell's ideas to argue against Charles Murray's ideas, which is a particularly entertaining and somewhat perverse exercise, since I've definitely noticed some overlap in the followers of the two.