Remove the shell from the shrimps and put them into the water: this will add extra flavor to your pasta. Salt it and start heating it.
Chop the cherry tomatoes.
Put the oil into the pan, at medium-high heat. Add the garlic and (optional) chilli pepper. After few minutes, add the tomatoes and some basil.
When the tomatoes have released their water (help this gently squeezing them with a wooden spoon) add the tuna and lower the heat. Salt and pepper according to your taste.
In the meanwhile, salted water should be boiling so that you can drop the paccheri into it.
After about 6-7 minutes, the sauce is ready and the paccheri are half-cooked.
Time to drain the pasta into the pan. Don't waste the water, you will need some to finish cooking the paccheri.
Finish cooking the paccheri in the pan, gently mixing them with the sauce and adding some pasta water (about 10 minutes). When paccheri are almost ready, add the shrimps: they need just 1 minute to be ready
Your shrimps and tuna paccheri are ready. Serve them with that delicious sauce and enjoy!