Place the scallops in a large bowl with the fennel. Add the lemon juice, grapefruit zest and juice, jalapenos, olive oil, salt, and pepper. Gently toss until well combined.
Place 7 to 8 scallops on 4 chilled dinner plates. Divide the salad evenly among the 4 plates. To serve, top each salad with a fennel fronds and drizzle each plate with a little more olive oil.