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Postelsia palmaeformis is an intertidal brown alga that occurs on rocky shores of the Pacific coast of North America, usually in patches from a few to 100s of individuals. As an autotroph, it produces its own food via photosynthesis. P. palmaeformis is an annual and like many brown algae has two distinct morphologies during its lifecycle – the microscopic gametophyte and the macroscopic sporophyte, which resembles a small palm tree. The sporophyte’s maximum height is variable, but can reach 60-75 cm. The dispersal distance for P. palmaeformis is unusually low for an annual alga – only about 1-3 m. Because of this, the extinction of a patch (via over-harvesting by humans, for example) can be permanent.
The sea palm is an interesting, well-studied, and charismatic organism:
- It’s the only kelp that can stand erect in air – without the support of the water. Yet the strong stipe is also flexible enough to allow the alga to bend with the motion of powerful waves.
- Not only does it live in areas with waves so strong that most other organisms can’t survive, but it actually requires heavy surf and can't establish in areas of calmer water. (Remember this as you explore the intertidal, and be careful – if there’s Postelsia, there will be strong waves.)
- Postelsia has a complex relationship with the mussel, Mytilus californianus, that covers much of its rocky habitat. Bare rock is the best place for sea palms to settle, but if there isn’t any nearby, it will settle on the mussels. The mussels will still eventually crowd it out, unless something removes a clump, leaving room for Postelsia. It’s usually strong waves that suck the mussels from the rocks. And mussels with Postelsia growing on their shells are more likely to be removed. So sea palms can “help” make space for themselves in the mussel-bed. But the mussels may actually help Postelsia survive during parts of its life-cycle, the tiny gametophyte (egg or sperm-making) stage and the young sporophyte (palm-tree-shaped) stage, by providing protection from harsh sun and drying air.
- Postelsia doesn’t disperse very far from its parent. That’s unusual for an annual plant or alga, especially one living in an area with such intense competition for space from other organisms. Annuals don’t tend to be very good at holding onto space, so they rely on being able to send their propagules (parts that can be used for making new individuals) to colonize newly opened spaces.
- People like to eat Postelsia fronds. However, in many places it's illegal to collect it, because of the risk of driving local populations/patches to extinction.