Soak wakame in at least 6 cups water for time specified on package, or until tender-crisp. Discard ribs, if any, and finely chop wakame. Strain soaking liquid through a fine-mesh strainer into a medium bowl and add shiitakes.
In a bowl, whisk together miso, 1/2 cup oil, the zest and juice, garlic, and parsley.
Heat remaining 3 tbsp. oil in a large pot over medium heat. Break up soba into roughly 1-in. pieces and toast, stirring often, until deep golden, about 10 minutes. Pour in the soaking liquid and stir in shiitakes. Cover and cook at a bare simmer about 15 minutes, or until soba is tender.
Stir miso dressing into soba to coat, then add wakame, dulse, and nori. Using a slotted spoon, divide among deep plates and top with baby radishes and/or carrots. Pour any remaining liquid around soba.
*Find the sea vegetables at Whole Foods Market or Japanese markets, or order online from the Ocean Harvest Sea Vegetable Company (ohsv.net). Nori is also in the Asian aisle of your grocery store. Look for miso in the refrigerated produce section, near the tofu.
Quick twist: Cook the soba in mushroom broth instead of sautéing. Spoon the miso dressing into bowls, top with soba, and pour in broth. Add sliced or diced fresh radishes and carrots, then cilantro sprigs.
Note: Nutritional analysis is per serving.