Aonori 青海苔, or green laver, is a type of edible green seaweed which includes species from the genus Monostroma and Ulva. It is referred to as aosa アオサ in some parts of Japan. The color of the aonori is intense, beautiful green. It has a distinctive fragrant green flavor. This type of seaweed is rich in calcium and is a moderate source of iodine.
Aonori is sold dried, and is used in soups and tempura. It is also sprinkled atop a variety of Japanese dishes like yakisoba and okonomiyaki.
There are 3 main groups of edible seaweed: Red algea, green algea and brown algea. Most edible seaweeds are marine algae whereas most freshwater algae are toxic. Seaweed contains high levels of iodine and calcium. It is possibly a source of vitamin B12 (see ref), but the amount is variable and therefore, not dependable.
Because it comes from the sea, seaweed contains sodium. It should be avoided by anyone on a sodium-restricted diet. Wakame has the highest sodium content, with kelp and laver having significantly less.