Combine the capers and the brine from the jar, tomatoes, oil, salt, and pepper in a large bowl. Let stand until the tomatoes release their juices, about 10 minutes.
Add the bread and toss to coat. Let the salad stand, tossing occasionally, until the bread softens slightly and soaks up some of the tomato liquid, about 20 minutes. Toss with the arugula just before serving.