Cut the shellfish in bite-sized pieces. Grate the zest off of ½ orange and set aside. Section the oranges and grapefruits (see right).
No more than 1 hour before serving, halve, pit, and peel the avocados and cut each avocado into 12 wedges. Trim the base of each endive, and then, starting from the bottom, cut half of the head into ½-inch-wide slices. Leave the top half of the leaves wh