How do you get the clam on Mythology Island?

The big clam with the Giant Pearl is right outside the Hydra cave in the underwater realm of Poseidon. At the Poseidon altar, use the starfish from the statue's face in the museum, and cross the beach to take Aphrodite's test. Thn dive into the water and swim around the top and right side to reach the cave. Once you have the Hydra scale and the Pearl, use the Touchscreen Mirror from Aphrodite to take you back to the beach.