“Middle Eastern” or “Israeli” large-grain couscous is about the size of tapioca pearls and is available at many grocery stores and natural-foods stores.
In a covered saucepan over medium-high heat, bring water, couscous, saffron, 1⁄2 teaspoon salt, 1⁄4 teaspoon pepper, and butter to a simmer. Turn heat down to medium-low; cook approximately 20 minutes. Remove from heat; let stand, covered, 10 minutes or until couscous is tender and separates easily. Chill in refrigerator.
In a medium bowl, add remaining ingredients, minus the arugula. Season with remaining salt and pepper; mix gently. Let marinate 30 minutes in refrigerator. Drain, reserving marinade; toss with couscous.
Mound four separate piles of arugula on plates. Lightly dress with reserved marinade. Place couscous-shrimp mixture atop arugula; garnish and serve.