Seaweed

Seaweed (ワカメ, Wakame) is a type of deep sea creature (although they are plant-like algae) found in.

In New Leaf
Upon the players' donating the seaweed to the Museum, it can be found in the large pool in the first room of the ocean exhibit, along with many other species of deep sea creatures. It is found in the corner near the big rock close to the entrance of the large saltwater exhibit. The exhibit has this to say about the seaweed:

"Seaweed can be over three feet long. When its spores attach to rocks, they take root and grow. Seaweed is naturally brown, but turns to the dark-green color we associate it with when cooked. It tends to spoil easily when picked, so various preservation methods have been developed as a necessity. Such methods of preservation are sprinkling ash and drying it out, or boiling it and adding salt."

Further information
Although seaweed resemble plants, they are actually brown algae. The type of seaweed found on beaches and kelp in the ocean are included as brown algae. Worldwide, there are 1500–2000 known species of brown algae. Some species of brown algae, including kelp, can reach 60 meters in length and form underwater forests. Brown algae produce alginic acid, which is extracted for use versatile roles such as waterproofing, fireproofing, gelling agents, thickening agents for ice cream, smoothies, and cosmetics; making impressions for dentistry, prosthetics, and life-casting; and many more. Certain types of brown algae are edible, most notably, nori, which is to wrap rice and meat in sushi. Brown algae reproduce by spores and gametes, and they are most closely related to yellow-green algae.