The amount of harissa to add depends entirely on just how spicy your harissa is and just how spicy you want the final sauce.
Stir it into the sauce a little at a time, tasting until just the right level of heat is achieved.
20 mussels, cleaned (see NOTE)
1 1/2 cups water
2 tablespoons olive oil
12 large whole large shrimp, peeled with tails on
340 g cleaned calamari (squid) or cuttlefish, cut into 1-inch pieces
1 small sweet Italian green pepper (also called Italian frying pepper), or 1/2 medium green bell pepper, cored, de-ribbed, seeded and cut into 1/2-inch pieces
1 plum tomato, seeded and cut into 1/2-inch pieces
225 g small peeled shrimp, defrosted if frozen
3 tablespoons tomato paste
Homemade or store-bought harissa
395 g dried spaghetti
Combine the mussels and 1/2 cup of the water in a small saucepan over medium-high heat.
Cover; once the water's boiling, cook/steam for about 2 minutes, shaking the pan from time to time, until the mussels have opened.
Discard any that do not open. Let cool.
Heat the oil in a large saute pan over medium-high heat.
Once the oil shimmers, add the large shrimp and cook, turning them over until pink and opaque, 3 to 4 minutes. Transfer to a platter.
Add the calamari and cook until it is just tender, 5 to 8 minutes.
Add the green pepper and cook until it begins to soften, about 5 minutes.
Stir in the tomato, and cook for 1 minute, then add the small shrimp and cook, stirring, for 1 minute.
Stir in the 3 tablespoons of tomato paste and some harissa (to taste; start with 1 tablespoon).
Add the remaining 1 cup water; once the liquid is barely bubbling, cook for 5 minutes.
Meanwhile, drain off the liquid from the mussels, strain and reserve.
Twist off the empty half of each shell and discard.
Taste the sauce and add more harissa, as needed (in testing, we added 1/2 tablespoon more).
If the calamari are not completely tender, add a touch of reserved mussel liquid and cook them a bit longer.
Taste the sauce and add salt, as needed.
Meanwhile, bring 4 quarts of water to a boil in a large pot over high heat.
Add a generous pinch or two of salt, then add the pasta.
Cook, stirring from time to time to keep the pasta from clumping together, until al dente following the package directions.
Drain, but do not rinse, shaking off any water that clings to the pasta.
Toss the pasta with the sauce.
Divide among individual warmed bowls. Arrange the reserved shrimp and mussels on the top of the pasta.
The mussels you buy should be glistening.
Immerse them in a bowl of water and ice cubes for 30 minutes before you cook them; lift them out of the water to avoid reintroducing any grit.
If you're not going to cook them right away, rinse them in very cold water and refrigerate in a loosely covered bowl.
Chilled mussels should be shut tightly; if any of them are not, or they have broken or cracked shells, discard them.