Macrocystis pyrifera


Macrocystis pyrifera is the common name for the giant kelp, which is the largest of all seaweeds. This species is the major canopy-forming seaweed and it can be up to 55 m (180 ft) in length.

When Charles Darwin saw this seaweed in 1834, he was very fascinated by the kelp, and after he first observed this kelp, he noted: “I can only compare these great aquatic forests…with the terrestrial ones in the intertropical regions. Yet if in any country a forest was destroyed, I do not believe so nearly so many species of animals would perish as would here, from the destruction of kelp.” Even though this is a long time ago and there was not much knowledge about the role of kelp in the ecosystem, he was really onto something. All kelps are considered foundation species, which means they have a strong role in structuring the community in which they live. Many other species are dependent on them as they need the habitat they provide.

The giant kelp has a very good ability of adapting to the habitat in which they live. Earlier we thought there were several species in the genus Macrocystis, however, now we know there is only one. The reason why we thought there were multiple species within this genus was because their holdfast was so differently looking. The holdfast is what seaweeds use to hold on to were they live, and brown seaweeds can have very large holdfasts. Now we now that the shape of the holdfast is determined by the depth in which the giant kelp lives, and geography is usually a better indicator of genetic similarity than morphology.

The giant kelp is one of the species in my herbarium that I made in the class Marine Botany in Moss Landing Marine Labs. Since the species is so large, I could only fit one blade from the kelp on the paper. You can see that this species has some distinct parts; the floaters (pneumatocysts).


The giant kelp can grow up to 60 cm vertically per day. One reason why this kelp can grow this quick, is because it has  sieve elements. These act like thick straws, and move sugar in a velocity up to 1m/h. This picture is taken with the foldscope, and it is a cross-section of the stipe showing the sieve elements.


Giant kelp can be found from Alaska to Baja California. The next time you are close to the coast-line, you can look for the floating canopies, and think back to the times when Darwin was fascinated by this huge kelp.


Written by Therese Meyer

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