1 pound cooked shrimp, chopped
1 pound jumbo lump crabmeat, picked through for bits of shell
1/2 cup mayonnaise
2 tablespoons chopped fresh parsley, plus 1/4 cup whole leaves
6 to 8 radishes, finely chopped, plus 2 thinly sliced
2 stalks celery, finely chopped, plus 1/4 cup thinly sliced
2 small shallots, finely chopped
Juice of 2 lemons, plus wedges for serving
Pinch cayenne
Kosher salt and freshly ground black pepper
4 large avocados, well washed
8 cups baby arugula
2 tablespoons olive oil