Giant Kelp, Macrocystis pyrifera
Giant Kelp, Macrocystis pyrifera. Underwater photograph taken in coastal waters off La Jolla, California, September 2021. Photograph and identification of Bob Hillis, Ivins, Utah.
Giant Kelp, Macrocystis pyrifera, stipes, laminae, and pneumatocysts. Underwater photograph taken in coastal waters off La Jolla, California, September 2021. Photograph and identification of Bob Hillis, Ivins, Utah.
Giant Kelp, Macrocystis pyrifera, showing holdfast. Underwater photograph taken in coastal waters off La Jolla, California, September 2021. Photograph and identification of Bob Hillis, Ivins, Utah.
Phylogeny: Giant Kelp, Macrocystis pyrifera (Linnaeus) C. Agardh, 1820, is a brown algae in the Laminariaceae Family of Kelp. The genus Macrocystis is one of fourteen genera in the Laminariacease Family and this species is the only species in the Macrocystis genus. The name Macrocystis comes from the Greek words meaning “large bladder”. This refers to the gas-filled bladders which float this algae toward the surface. The species name Pyrifera comes from Latin meaning “bearing pears”, again referring to the gas-filled bladders. Algae in this family are not from the plant kingdom, but are from the Chromista kingdom. Chromists are photosynthesizing organisms that used to be known as Protista. This diverse kingdom includes brown algae, diatoms, mildew, and malarial parasites. This species is also commonly known as Bladder Kelp.
Morphology: Giant Kelp is the largest algae species in the world. It resembles a plant in appearance, but the resemblance stops there. The thallus (body of the organism) is comprised of three organs: the holdfast, the stipe, and the lamina. The thallus is held to the bottom substrate by a root-like holdfast. The intertwined and branched haptera tissue of the holdfast anchors the plant, but does not gather nutrients or water like a root does. The stem-like stipes provide structure and primarily connect the holdfast to the laminae. The stipes are smooth, round in cross section, and flexible. The leaf-like laminae are long and blade-like. The laminae are leathery and textured. The lamina margins have short narrow projections. At the base of each lamina is a gas-filled, pear-shaped, bladder, known as a pneumatocyst. The pneumatocysts lift the stipe and laminae toward the surface where the sunlight is most intense. Giant Kelp uses chlorophyll A and chlorophyll C for photosynthesis. The brown to amber color comes primarily from these chemicals and fucoxanthin. The laminae are often marked with white spots of the encrusting bryozoan, Membranipora membranacea. Giant Kelp can reach 45 m (148 feet) in length and has the ability to grow up to 50 cm (18.5 inches) per day, with growth limited only by sunlight and water nitrogen concentrations.
Habitat and Distribution: Giant Kelp is found attached to hard substrates such as rock. It may live along exposed coastlines, which are open to surge and currents, or in more protected bays. It often forms thick forests that can stretch for miles. It is generally found at depths between 3 m (10 feet) and 30 m (100 feet). Giant Kelp often grows longer than the depth of the water and lays on the surface, forming thick mats. Giant Kelp has a spotty distribution in subpolar to tropical oceans around the world. It is most common in temperate to subtropical waters. In Mexican waters it is found along the west coast of the Baja Peninsula from Punta San Hipolito, Baja California Sur and the central and northwest coasts of the Baja Peninsula. It is also found in the coastal waters of the Revillagigedo Islands.
Reproduction: Giant Kelp alternates between an asexual sporophyte generation and a sexual gametophyte generation. Adult sporophytes reproduce asexually through the production of spores, which produce gametophytes. Gametophytes are very small and produce either male (sperm) or female (eggs) gametes. The gametophytes release their gametes into the water. The eggs release a pheromone which attracts the motile sperm. The sperm swims to the egg and fertilization occurs and forms a zygote, which grows into a juvenile sporophyte. The sporophyte matures and completes the cycle.
Ecosystem Interactions: Giant Kelp is a critical part of many ecosystems. It provides food, structure, and camouflage for hundreds of species of fish, mollusks, crustaceans, and other invertebrates. As the lamina and stipes mature and slough off, or are torn lose by water movement, they continue to provide food and habitat whether they sink to the bottom, wash up on the beach, or drift into pelagic waters as a kelp paddy.
Human Interactions: Giant Kelp contains many chemicals which are used in the medical, cosmetic, food processing, aquacultural, manufacturing, and agricultural industries. It is harvested commercially by large kelp cutter ships which trim the top portion of the thallus, down to a depth of a meter or so below the water’s surface. The harvested portion grows back quickly. From a conservation perspective there is no global consensus on the status of Giant Kelp. It ranges from Endangered, to Threatened, to Not at Risk depending on the location. It’s conservation status has not been evaluated specific to Mexican waters.
Synonyms: Fucus giganteus, Fucus pyriferus, Laminaria pyrifera, Macrocystis angustifolia, Macrocystis communis, Macrocystis humboldtii, Macrocystis integrifolia, Macrocystis laevis, Macrocystis latifolius, Macrocystis luxurians, Macrocystis orbigniana, Macrocystis pelagica, Macrocystis planicaulis, Macrocystis pomifera, Macrocystis pyrifera var. humboldtii, Macrocystis pyrifera var. longibullata, Macrocystis pyrifera var. luxurians, and Macrocystis tenuifolia.