Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Postelsia (Sea palm)

Postelsia palmaeformis (Ruprecht 1852)

Kingdom Chromista, phylum Ochrophyta, class Phaeophyceae, order Laminariales, Family Laminariaceae

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Description

Resembles a small palm tree, up to 60 cm tall with a thick, flexible, cylindrical stipe and small hapterous holdfast. Plants can have as many as 100+ grooved blades that reach 25 cm long and hang down when plants are exposed at low tide. Mature plants turn from green to golden brown. Usually found growing in extensive stands (Abbott and Hollenberg 1976).

Habitat and Geographic Range

Found in mid-intertidal zone in areas of high wave exposure; distribution patchy, but typically abundant where present. Vancover I., Br. Columbia, to San Luis Obispo Co., Calif. (Abbott and Hollenberg 1976).

Synonyms

None

Similar species

None

Natural History

This annual brown alga exhibits heteromorphic alternation of generations, with two distinct phenotypic phases: the macroscopic diploid (2N) sporophyte and the microscopic haploid gametophyte (1N) (Blanchette 1996). Postelsia sporophytes generally first appear in winter, grow rapidly in spring, become reproductive in late spring/early summer and are typically ripped out by large winter storms (Blanchette 1996). Spores are released during low tide and remain in grooves of blades, dripping off the slender tips onto the surrounding substrate, which results in very limited dispersal (Abbott and Hollenberg 1976). These spores grow into haploid gametophytes, which release gametes that fuse and grow into the visible sporophytes.

Postelsia appears to have a complex relationship with the mussel, Mytilus californianus. The microscopic female and male gametophytes establish themselves within mussel beds, which may be ideal for germination and protection from wave exposure. When mussels are cleared from the rocks by harsh waves or predation, the diploid sporophytes resulting from gamete release can begin to grow (Blanchette 1996). Without a disturbance to open up space within the mussel beds, the juvenile Postelsia sporophytes would be excluded by the competitively dominant mussels (Paine 1988). Postelsia can recruit to areas other than gaps in mussel beds (e.g. on mussels, barnacled, turf algae), but populations are more stable and densities are highest within these bare patches (Paine 1988). Range of this alga is limited by physical (light/dessication) and biological (competition with mussels) factors (Nielsen et al. 2006). 

Edible seaweed harvesting has been a cottage industry since the late 1970s, which has historically included the collection of Postelsia. However, the sea palm is now a protected species and illegal to harvest in British Columbia, Washington and Oregon. In California, recreational harvesting is illegal, but commercial harvesting remains legal. Between 2000 and 2001, it is estimated that between 2 and 3 tons of Postelsia were harvested in California. The blades are eaten raw or are dried, and dried blades sell for up to US$45 per pound. Commercial harvesters of Postelsia must purchase a US$100 license, pay a royalty to the State of California (US$24 per wet ton of algae harvested), and submit a monthly harvest log (Miller 2002). Common practice is to clip blades above the meristem which allows for regeneration of new blades. However, removing blades can limit a sporophyte's ability to produce spores and contribute to subsequent populations. Recovery from harvesting depends greatly on the season of collection, suggesting that additional regulation of the timing of harvest could help to protect Postelsia from overharvesting (Thompson et al. 2007).

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