Do prawns have poop in their head? That is actually a more popular question than whether or not prawns have nutritional value. We'll answer both questions.
Yes, prawns do store fecal matter in there head and therefore, "have poop in their head" for lack of better words. Shrimp are the same. Buying "de-veined" prawns (or shrimp) should address this.
As far as whether or not prawns have antioxidants or other nutritional value? They do have more antioxidants than most seafood, however their ORAC is still low relative to almost all plant-based foods.
Research Support, Non-U.S. Gov't: Carlsen MH, Halvorsen BL, Holte K, et al. Nutrition Journal NIH Jan 2010