Step 1 Bring a large pot of liberally salted water to a rolling boil. Stir in the rice and return the water to a boil; reduce the heat to maintain a rapid simmer. Cook until the rice is tender, 15 minutes. Drain the rice in a strainer and rinse quickly in cool running water. Line a large mixing bowl with a tea towel and empty the rice into it. Fold the tea towel over the top of the rice and set aside to cool, about 5 minutes.
Step 2 When the rice has cooled slightly, add the olive oil, 2 tablespoons lemon juice and salt to taste, stirring lightly with a fork to avoid crushing the tender rice grains. (The rice can be prepared to this point a day ahead and refrigerated in a tightly covered container. Bring to room temperature before proceeding.)
Step 3When almost ready to serve, combine the shrimp, red onion and tomato in a medium bowl and season with red wine vinegar and one-fourth teaspoon salt, or to taste.
Step 4Add the shrimp mixture to the rice along with the arugula and pine nuts. Stir to combine well, taste and add more salt or lemon juice if necessary. (The recipe can be prepared to this point up to 30 minutes in advance.)
Step 5Transfer to a serving bowl and serve at room temperature.