Melt butter in a large dutch oven over medium heat.
Add reserved shrimp shells, onion, celery, carrot, thyme and garlic. Saute 3 minutes.
Add water, wine and tomato paste. Bring to a boil.
Reduce heat and simmer, uncovered, 35 minutes.
Strain through a sieve into a bowl and discard solids.
Bring 2 cups of broth to a boil in a medium saucpan.
Add couccous, remove from heat, cover and let stand 5 minutes. Fluff.
Heat oil in a large skillet over medium-high heat.
Add bell peppers, squash and zucchini, and saute 5 minutes.
Add shrimp, and continue cooking until shrimp is done, about 2 minutes.
Put couscous in a large bowl, make a well, and pour shrimp into center. Serve with remaining broth, if desired