To make the sauces, combine all the ingredients in separate bowls and set aside.
Place the coconut crab in a large saucepan with the second extract of coconut milk, chillies, lime juice and a pinch of salt and cook over high heat until the mixture comes to the boil. Cover and cook for 9 minutes, then, using tongs and a slotted spoon, remove the crab and set aside. Add the parrotfish and simmer over medium heat for 12 minutes, or until just cooked through.
Meanwhile, drop the land crabs into a large saucepan of boiling salted water over high heat. Once the water returns to the boil, cook for 6 minutes, then drain.
Cook the sea snails on a rack over an open fire for 4 minutes.
To serve, break the coconut crab into pieces and crack the claws, then do the same with the smaller land crabs. Serve the crab, fish and sea snails on a large platter with both sauces to the side, and the boiled cassava. You can serve some of the cooking broth to dip the cassava into if you like.
Note: To make second extract of coconut milk, crack a coconut with the back of a large cleaver. Reserve the watery liquid as it drains out, then scrape out the white flesh. Remove the brown membrane, then grate the flesh with a hand-grater or in a food processor. Place the grated flesh and the reserved coconut water in a blender and, using the pulse action, process until everything is well combined and the flesh is soft. Line a sieve with muslin and place it over a bowl. Pour in the blended coconut, then gather the sides of the muslin and squeeze over a bowl to extract as much liquid as possible from the flesh. This is known as the first extraction of coconut milk (or coconut cream). For the second extract, repeat the process using the coconut flesh that has already been blended and sieved once. Use filtered water if coconut water is not available.