Incredible plants: the sea palm

The sea palm, a rocky intertidal kelp found along western North American shores is one of the most distinctive seaweeds in the world. In shape it is remarkably similar to terrestrial palm trees; it has a knobby holdfast for firm attachment to the rocks, an elastic stipe for flexibility, and a tuft of drooping rugose blades at the top. The species name is Postelsia palmaeformis and it was first described by western science in 1852 based on collections made during a Russian expedition to western North America in 1839 (Abbott and Hollenberg 1976).

 

Individual sea palm (left) and grove (middle) at Carmel, Monterey Co, CA. Close-up of the holdfast on a plant at Gleason Beach, Sonoma County, CA. (right).

Postelsia is distributed on rocky shores from central California to British Columbia. It occurs at about the middle intertidal zone, so only a moderately low tide is necessary to see it exposed. However, it is not among the safer seaweeds to hunt for along the coast since it grows on highly-wave exposed shores where it is beaten by the surf. Research by Paul Dayton and Robert Paine in Washington State has shown that this wave energy is necessary for persistence of this annual species in the rocky intertidal (Dayton 1973, Paine 1988). Sufficient wave action removes carpets of sessile mussels, giving the sea-palm bare space to colonize. Or, in some cases, juvenile palms settle on mussels or algae and eventually cause both species to get removed by waves from the rock and open more space – a sort of suicide for the good of the species (Dayton 1973). Without the assistance of physical disturbance, mussels would dominate the rocks leaving no space for Postelsia.



A grove of Postelsia at Glass Beach in Mendocino County, CA takes an incoming wave.



Newly recruited sporophytes, Gleason
Beach, Sonoma Co., CA.

Sea-palms occur in gregarious patches because their spores typically only disperse short distances (Dayton 1973). Perhaps during rising tides, spores drip from the hanging fronds of the plant onto adjacent rocks and settle rapidly (Paine 1988). There they germinate into a microscopic stage of the kelp life cycle known as the gametophyte. Sperm from male gametophytes fertilize eggs attached to female gametophytes, and there start a new generation of adult plants (the sporophytes).



Sampling some "sea crunchies"
(dried sea palm blades) made by a
company in northern California.

Like some other seaweeds, Postelsia is edible. Seaweeds tend to be rich in minerals like iodine, but they may also have high levels of more toxic metals, so they should probably be eaten only in moderation.










References
Abbott, I.A. and G.J. Hollenberg. 1976. Marine Algae of California. Stanford University Press, Stanford, CA.
Dayton, P.K. 1973. Dispersion, dispersal, and persistence of the annual intertidal alga, Postelsia palmaeformis Ruprecht. Ecology 54:433-438.
Paine, R.T. 1988. Habitat suitability and local population persistence of the sea palm Postelsia palmaeformis. Ecology 69:1787-1794.