I don't know "exactly" how that jellyfish was done but I understand the technique involved... pretty darn COOOL!
I would have to experiment considerably to replicate it...
Anyway here is what is going on:
Each layer has an option for "Layer Blending" at the bottom of the layer properties (double click the layer. At the bottom is a menu).
This is just like the blending mode for layers in Photoshop; Multiply, screen, Add, etc etc. These options change how one layer effects layers below it. For instance a white filled shape with a black border set to multiply, will basically make the white "invisible" on layers below it. Muliply blending mode multiplies the RGB values of the layer TIMES the RGB values of the layers below.
Screen would be the OPPOSITE of multiply. The black stroke would be invisible and the white would overlay the layers.
By blurring and layering different vector layers in a specific way, and applying different blending modes, you can create AMAZING effects.
You will not be able to see ANY OF IT until you render though. You pretty much are working blind. But the results can be wonderful.
It takes some experimentation... and a lot of test renders.
Play around with layer blending modes... they are cool as heck.
-vern