In a large pot, combine the reserved shrimp shells with the seafood stock and bring the mixture to a gentle boil. Once boiling, reduce the heat to medium-low and allow it to simmer uncovered for about 20 minutes. Stir occasionally and skim off any foam or impurities that rise to the surface. When done, strain the broth into a large bowl, pressing the shells with the back of a spoon to extract every drop of flavor. Return the strained broth to the pot and keep it warm over low heat.

Meanwhile, heat the olive oil in a heavy-bottomed skillet over medium-high heat. Add the diced onion and sauté until soft and translucent, about 5 minutes. Stir in the carrot, celery, and bell pepper, and continue cooking for another 7 minutes, or until the vegetables begin to tenderize. Mix in the diced tomato and chopped chiles de arbol, cooking for an additional 5 minutes to let the flavors meld.

Add the tomato paste and, if using, the dried shrimp powder. Stir continuously for about 3 minutes to coat the vegetables and deepen the color and aroma. Transfer the sautéed mixture into the pot of warm broth, stirring to combine. Cover the pot and let it gently simmer over medium-low heat for 1 hour and 30 minutes, allowing the flavors to develop into a rich, spicy base.

Finally, add the prepared shrimp to the broth. Turn off the heat and let the shrimp poach in the hot liquid for about 3 minutes, or until just cooked through and pink. Serve the broth hot with lime wedges on the side for a fresh, citrusy finish.