Cook the sea lettuce, gigartina and sugar kelp in half a liter of water for 10 minutes, then drain and chill.
Place the clams in a steamer and cook in the seaweed cooking water. When they have opened, carefully remove the flesh from the shells and set aside. Strain the water used for cooking the seaweed and clams and reduce to 300 g. Set using the gelatin.
Set aside one grapefruit for grating and another for removing the segments. Carefully peel the remaining two, removing any pith from the skin, then squeeze and strain the juice. Mix with the glucose and reduce to one quarter. Add the grapefruit skins and grated rind, cover and chill. Strain.
Wash the cardoon and place in iced water to curl. Cut into small pieces.