Palm trees in the ocean? Surely you jest! Nope-just a different type of palm-a seaweed one. Postelsia palmaeformis, the sea palm, is a dead ringer for a land-type palm tree. It lives in the mid to low tide area, on rocky shores in areas where the waves are breaking almost continuously, so it isn’t as obvious as some of the seaweeds in the tide pools and along the shoreline. In fact, San Luis Obispo County is the farthest south this seaweed is recorded-out between Montana de Oro and Diablo Cove-because it likes cold water and wave action.
Postelsia is a brown alga, a kelp, which means it contains certain pigments that give it a greenish, brownish, or even golden color depending on where it is situated. Figure 1 shows a grouping of many individuals in the habitat where this seaweed is found. The individuals tend to live grouped very tightly together, so it looks like a palm forest. This is because they drop their spores directly down onto the holdfast and mussel bed below them where the young will grow up.
If you are looking at an individual alga, you will see that there is a small holdfast of convoluted “haptera” that is glued to the rock or mussel shells. These haptera are not roots, they do not take up nutrients like land plants do; they merely act a base to hold the alga to the substate. Then there is a hollow stipe, not a stem, which is very flexible, and can withstand the high stress, wave-crashing, environment. At the top are the blades, up to 100, where most of the photosynthesis is done and where the reproductive structures are formed. In Fig 2 you can barely see the holdfasts as these individuals were torn from the rocks during a storm and I found them on the beach.
The blades have longitudinal grooves in them, giving them more surface area to collect nutrients directly from the sea water; the grooves also act as valleys for the spores to flow down and be released when the time comes. As most of the larger kelp, Postelsia has a reproductive cycle called an “alternation of generations”. The spores are released from the blades in the late spring/early summer during a low tide. They drip down into the haptera or mussel bed below and grow into “gametophytes” (think ovaries and testicles) which are microscopic. The gametophytes will produce “gametes” (think eggs and sperm) that will join in reproduction to produce the visible alga, the “sporophyte”, which over the winter will grow rapidly to the adult form. These gametophytes and sporophytes alternate in time sequence and if one form or the other is removed from the cycle, the population crashes. Fig 3 shows a simple example of this process.
Back to Postelsia. The blades are highly nutritious, eaten raw or used dried, and very sought after by collectors. If the blades are taken, or the whole alga is taken, then there is nothing to reproduce and keep the population going. In British Columbia, Washington and Oregon no one can collect the alga. In California, recreational collecting is illegal, but commercial harvesting is legal with a permit. This is controlled by California Fish and Wildlife and there is a stiff penalty for being caught with the contraband. To find a specimen or a grouping of Postelsia on the beach is a rare sight and a delight. To see the alga in its habitat, out there where the waves are breaking, is even better. I challenge you to find them in your walks along the coast from SLO County northward.