Kelp

Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera.[3] Despite its appearance, kelp is not a plant; it is a heterokont. Kelp grows in "underwater forests" (kelp forests) in shallow oceans, and is thought to have appeared in the Miocene, 5 to 23 million years ago.[4] The organisms require nutrient-rich water with temperatures between 6 and 14 °C (43 and 57 °F). They are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 metres (100 to 260 ft). https://en.wikipedia.org/wiki/Kelp

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