Cabrillo Marine Aquarium

Giant Kelp

Macrocystis pyrifera


Fun Facts

    Under ideal conditions, giant kelp can grow about two feet a day.

    Giant kelp typically grows in nutrient-rich turbulent water, therefore allowing it to grow to a possible height of 175 feet.




The thallus (or body) of giant kelp consists of flat or leaf-like structures called blades. The blades originate from elongated stem-like structures, called stipes. The holdfast, a branching root-like structure, anchors the kelp to the ocean floor.  The holdfast is unlike the root system of a plant, is does not carry nutrients or water; it only anchors the kelp to a rock. Nutrients and water are absorbed across the body wall. Gas-filled bladders called pneumatocysts form at the base of the blades and help keep the thallus upright and close to the surface of the water near the sun, to aid in photosynthesis. Some plants are so tall that the upper blades spread across the top of the water to form a dense canopy. Often many kelp plants attach and grow close together, forming a unique habitat called the kelp forest.

Pieces of decomposing kelp (called detritus) can either sink to the depths of the ocean or get washed ashore, providing food for deep sea animals or intertidal marine life, respectively. Giant kelp has a multitude of inhabitants. Invertebrates graze on the blades, fish seek shelter in the dense forest and thousands of invertebrates live in the holdfast, such as brittle stars, sea stars, anemones, sponges and tunicates. Sea otters like to hunt in the kelp forest, where they find their favorite food and can wrap up in the kelp to keep from drifting away.

Giant kelp is harvested as a source of algin, a binding agent used in the production of many foods and cosmetics, like ice cream, toothpaste and many cereals.

To learn more about giant kelp and the kelp forest habitat, visit the Kelp Forest Room at the Aquarium.