Your appetizer will feel right at home served in a seashell, at least if you're serving ceviche -- a dish made with marinated raw fish (we added red grapefruit in this version).
Cut off top and bottom of grapefruit to make level; cut down along sides to remove peel and pith. Working over a nonreactive bowl to catch juices, cut along membranes to remove segments. Cut segments into 1/2-inch pieces; refrigerate. Measure 1/4 cup juice.
In the bowl, combine 1/4 cup grapefruit juice, lime juice, red onion, jalapeno, salt, and pepper. Add scallops, stir, and refrigerate, covered, 3 hours. Before serving, stir in mint and 1 cup reserved grapefruit segments. Using a slotted spoon, transfer to plates.