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How large can giant kelp grow?
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Algae
The macroscopic genus of algae known as Acetabularia is commonly called “mermaid's wine glass” because of the distinctive umbrella-like shape of the tips of its stalks.
Giant kelps of the genus Macrocystis are the largest known species of brown algae. Specifically, the giant bladder kelp (M. pyrifera) can reach lengths of 60 metres (197 feet) or more, with some sources noting it can measure up to 65 metres (215 feet) long.12
This species is recognized for having what is thought to be the fastest linear growth rate of any organism on Earth. While estimates of their maximum size can vary based on environmental conditions and measurement methods, they are consistently categorized as massive organisms that form extensive underwater forests in cool, coastal waters.213