Crustaceans like crabs, crayfish and lobsters eat seaweed, as do plankton and humans. No seaweed species is known to be poisonous.
There are roughly 1,500 species of green seaweed, 6,500 species of red seaweed and 1,800 species of brown seaweed in the world today. Many creatures depend on seaweed to survive and others live in seaweed as a method of protection from predators.
Seaweed is used by humans in things like cosmetics, toothpaste and salad dressings, as well as in dishes like sushi. Most seaweed has to be anchored to rocks in order to survive, but a handful of species will only grow if it is floating in the ocean.