Postelsia | MARINe

Postelsia (Sea palm)

Postelsia palmaeformis (Ruprecht 1852)

Last updated March, 2023

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

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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).



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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. In British Columbia, it is legal to harvest Postelsia with a wild aquatic plant harvest licence (more information is available here). In Washington, it is legal to harvest Postelsia with a sport fishing license. Harvest of the sea palm in Oregon is prohibited. In California, recreational harvesting is illegal, but commercial harvesting remains legal. 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).

Perhaps the biggest threat to the sea palm is the increased frequency of marine heatwave (MHW) events associated with climate change. Major declines of Postelsia at all MARINe sites where this seaweed is monitored have been observed during periods of elevated sea water temperatures. Declines were especially pronounced during the 2014-2016 MHW event. Postelsia is an annual species with very limited dispersal ability meaning that the size of each year’s population is dependent on the reproductive output of local adults from the previous year (Dayton 1973, Coyer et al 1997, Kusumo et al. 2006, Paine et al. 2017). Thus, post-MHW recovery has been slow in many areas, and at one site near the southern range limit of the species, the sea palm is now locally extinct. Because Postelsia appears to be particularly vulnerable to MHWs, and these events are predicted to increase in frequency and intensity in the future (Perkins et al. 2012, Oliver et al. 2018, Frolicher et al. 2018, Damaraki et al. 2019), harvesting regulations in California are currently under review.

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