Southern Sea Palm, Eisenia arborea

Southern Sea Palm, Eisenia arborea. Collected from a tidal pool of Bahía Santa Rosalillita, Baja California, August 2019. Collection, photograph and identification courtesy of Barry Mastro, Escondido, California.

Phylogeny:  Southern Sea Palm, Eisenia arborea (J.E. Areschoug 1876),  is a member of the  Lessoniaceae Family of Kelp. The genus Eisenia is one of five genera in this family, and there are eight species in this genus. The Eisenia Genus was named after the Swedish-American scientist Gustav Eisen, who was known for his work with algae and earthworms. Eisenia is also a genus of earthworms, which complicates researching species by name. The species name arborea comes from the Latin meaning “tree-like.”

Morphology: The Southern Sea Palm attaches to the seafloor with a holdfast (root-like anchor). It has a stipe (stalk) reach a maximum of 1.5 m (4 feet 10 inches) in height. The top of the stipe splits into two bunches of blades, giving this kelp a palm tree-like appearance. The blades often have a rippled texture, though those growing in more turbulent water tend to be smoother. The margins of the blades are strongly toothed. The blades reach a maximum of 40 cm (16 inches) in length.

Habitat and Distribution: Southern Sea Palms are found attached to rocks, usually in areas exposed to significant water movement, though sometimes they are found in protected bays. They live intertidally, and to a depth of 20 m (67 feet). Southern Sea Palms often form an understory beneath the taller Giant Kelp and Bull Kelp plants. This kelp has a discontinuous range in the temperate to subtropical eastern Pacific. In Mexican waters they are found along the west coast of the Baja Peninsula, from Magdalena Bay, Baja California Sur, northward along the central and northwest coasts of the Baja Peninsula. They are the second most abundant brown algae in Baja California.

Predators: Southern Sea Palms are eaten by various gastropods and crustaceans, though not sufficiently to consume a whole adult plant. Water movement may tear the blades lose so that they fall to the bottom and are eventually consumed by  detritivores.

Reproduction: Southern Sea Palms are a perennial plant. They reproduce sexually by releasing spores from specialized blades (sporophylls) that are located near the hold fast. The spores germinate into very small, thread-like, male and female gametophytes (haploid). The male gametophytes release sperm, which fertilize the eggs from the female gametophytes. This forms a zygote, which grows into a  sporophyte (diploid). They can also be made to reproduce asexually when cultured for aquaculture.

Ecosystem Interactions: Southern Sea Palms provides important shelter and habitat for various species of crustaceans, fish, mollusks, and other invertebrates.

Human Interactions: Southern Sea Palms are cultivated in Asia for use as a food source, for folk medicines, and for cosmetics. In Mexico it is harvested as food for abalone farming. This species is of interest to science because it is a source of a phlorotannin which may have anti-diabetic, anti-cancer, anti-oxidation, antibacterial, and radioprotective properties. From a conservation perspective the Southern Sea Palm has not been evaluated, however, they are very common and widespread in distribution making them, most likely, a species of Least Concern.

Synonyms: Ecklonia arborea.