Refresh the wakame seaweed in a bowl of boiling water for five minutes, then drain well and slice if necessary.
Cut the cucumber in half lengthways and scoop out the seeds with a teaspoon. Cut across into thin slices.
For the wakame dressing, in a bowl dissolve the dashi granules in the warm water and mix with the rest of the dressing ingredients. Add the wakame seaweed and the cucumber and mix together well with the dressing. Set aside in the fridge to chill.
To make the mayonnaise, mix the egg yolks, white wine vinegar and wasabi powder together in a bowl. Using a wire whisk gradually beat in the oil a few drops at a time, until you have incorporated it all and have achieved a mayonnaise-like texture. Season to taste with salt.
To serve, divide the crabmeat equally between four small ramekins and gently press down. (Or serve piled up.) Turn the crabmeat out onto four plates and spoon a tablespoon of the wasabi mayonnaise alongside. Take a large handful of the cucumber and wakame seaweed and place next to the crabmeat, sprinkle the bonito flakes on top of the seaweed, and serve.