Wakame
Dried or salted seaweed that swells into soft, dark green leaves when soaked — tender, slightly slippery, mildly of the sea.
Wakame is a brown seaweed that in Japan is sold mostly dried and cut into small pieces, less often salted. Once soaked it turns soft and silky with a slight bite and a somewhat slippery surface; the flavour is mildly salty and of the sea, far gentler than nori or kombu. Because it softens so quickly, it goes into soups only just before serving, and for salads a few minutes in cold water is enough. Dried wakame is available in Asian grocery stores, often also in health food and organic shops or by mail order; kept dry and tightly sealed it lasts a very long time.
Substitute:
The closest match is alaria, an Atlantic relative sometimes sold in organic shops as Atlantic wakame; it is a little firmer and needs longer soaking. Other seaweeds such as nori or kombu taste noticeably different and have a quite different texture, and leafy greens like spinach add colour but neither the taste of the sea nor the tender, slippery quality — where wakame is the point of the dish, as in seaweed salad, there is no real substitute.