Postelsia palmaeformis Ruprecht
Meerespalme (German), Sea Palm (English)
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.
Postelsia palmaeformis, is the only described species in the genus. Like most brown algae, it has two phases in its lifecycle, each with a distinct form. The visible palm-tree-shaped sporophyte is composed of a holdfast, a single stipe, and a canopy of grooved “lanceolate” blades on which spores are produced. The spores attach to the rock (or other hard surface) then germinate and develop into the microscopic gametophyte phase. Male gametophytes produce sperm, which fertilize the eggs made by female gametophytes. Fertilized eggs grow into new sporophytes. (See chapters on Life Cycle and Reproduction)
See labeled illustration above.
SPOROPHYTE (diploid): “Sporangial thalli with relatively small holdfast of stout, branched haptera. Stipe erect, cylindrical and hollow, tapering slightly from base to apex. Apex of stipe with many short, radially disposed, simple branches, each terminating in single blade. Branch and blade splitting longitudinally into 2 blades and branches, usually of equal size, the splitting beginning at junction of branch and blade. Blades sharply pointed, narrowly linear, the margins dentate. Both flattened surfaces of blade with deep, parallel, longitudinal grooves, the grooves of 1 surface alternating with those of other. Sporangia in linear sori, these lining grooves of blade.…Sporangial thalli to 60 cm tall, usually growing in extensive stands; stipes erect and blades pendant when plants are exposed by recession of tide; mature plants golden brown, with 100 or more blades, these to 25 cm long; sporangia first produced in late spring; blades becoming eroded after fruiting; spores released during low tide and remaining in grooves of blades, dripping off the slender tips onto holdfast or nearby rock. Locally abundant in areas exposed to surf, saxicolous, high to midtidal.” (Abbott and Hollenberg 1976, p. 251)
GAMETOPHYTE (haploid): “…dimorphic, dioecious, oogamous, branched uniseriate filaments; growing to a small size and producing gametes, even in the absence of blue light or added iron in the medium, which are 2 conditions found to be necessary for gametogenesis in gametophytes of other members of the order Laminariales.” (Algaebase) See also Lewis 1995.
Male gametophytes are smaller than females, and their cells are relatively pale. Males produce antheridia (sperm-producing organs), and females produce eggs. (Lewis 1995)
Conservation and Management
The fronds (aka blades) are edible by humans and have become increasingly popular as additions to soups or salads. Blades can regenerate if removed during the spring and summer growing season, but the alga cannot regenerate if cut through the stipe. Blades produce the spore phase of the lifecycle, so if too many are removed before spores are released, a local population can be endangered or exterminated (Report to Fish & Game Commission 2004).
Recent research suggests that some of the harvesting practices claimed as “sustainable” may not be (Thompson et al. in press). The authors recommend the following limits on harvest: allow the trimming of fronds only, never the stipe, and allow only one harvest per year, before reproduction begins.
Harvest of Postelsia palmaeformis is illegal throughout most of its range – Oregon, Washington, and British Columbia. Recreational harvest is prohibited in California, but the California Fish and Game Commission issues licenses to commercial seaweed harvesters who pay a royalty to the state per wet-ton collected (Report to Fish & Game Commission 2004).
Over-harvest by humans. Though this is regulated throughout its range.
[Refer to life-cycle diagram among images above]
Postelsia has the typical two-phase kelp life-cycle, with a visible diploid sporophyte and a microscopic haploid gametophyte. The grooves on the blades of the sporophyte are lined with sori, in which haploid (1N) spores are produced via meiosis (cell division that results in the halving of the number of chromosomes). Mature spores drip from the hanging blades to the rock below (or possibly mussel valves or barnacle plates) during low tide, i.e., when the alga is exposed to the air. These heterokont spores (with two flagella – one long and mature and the other short and immature) can swim only about 1-3 meters before they attach to the substrate. Within as soon as 1 day after release, a spore can germinate, growing rapidly into a branched filamentous several-celled gametophyte. There is a 1:1 ratio of male to female gametophytes, both of which can be fertile within about 8 days of release. Swimming sperm fertilize the eggs extruded by females. The diploid (2N) zygote over-grows the female gametophyte to become a new palm-tree-shaped sporophyte.
(Dayton 1973, Paine 1988, Lewis 1995, Anderson 2004, Kusumo et al. 2006)
In laboratory studies, Lewis (1995) found a diploid number of 26-34 chromosomes in sporophytes (the macroscopic “palm-tree” stage) and a haploid number of 14-17 chromosomes in sporangia (the location of meiosis to produce spores) and in gametophytes (the microscopic stage that produces eggs and sperm).
A marine species. It occurs in the upper intertidal of rocky shores with high wave energy.
Heavy wave action is necessary for the persistence of populations of Postelsia palmaeformis. Sea palms will be overgrown by the mussel Mytilus californianus unless patches of the mussel are periodically removed by strong waves, either directly or via the crushing-force of drift logs. In the absence of bare rock (the best attachment site for Postelsia), sea palms will attach to mussel shells. This increases drag on the mussels, thus increasing the probability that patches of overgrown mussels will be sucked from the rock, leaving space for the next generation of Postelsia to attach and grow. (R. T. Paine, personal communication; Paine 1979)
Mussels might also facilitate the recruitment of new generations of Postelsia by providing protection for gametophytes and young sporophytes (Blanchette 1996).
From Hope Island, British Columbia south to San Luis Obispo County, California (Algaebase).
Produces its own food via photosynthesis and absorbs nutrients from the water.
Postelsia competes for space in the rocky intertidal with the mussel Mytilus californianus, which can eventually crowd it out (Paine 1988).
P. palmaeformis is harvested by humans for food. This can lead to the permanent extinction of a patch. Because of this, all regions where it occurs have regulations that limit or outlaw its collection.
Disperses as spores, with very short dispersal distances of 1-3 meters (R. T. Paine, personal communication; Kusumo et al. 2006).
Postelsia palmaeformis is an annual. Sporophytes become visible in February or March, become reproductive during the spring, and post-reproductive individuals are abraded and torn from the substrate in the fall (Dayton 1973, Paine 1988). The next generation of sporophytes probably begins developing within a few weeks of spore release (see Lewis 1995) but are too small to see until late the following winter.
[Refer to life-cycle diagram among images above]
Sea palm sporophytes produce zoospores (motile spores) in structures aligned in the grooves on their blades. Spores have half the number of chromosomes that the parent sporophyte has. After release from the parent, spores grow into tiny gametophytes of only a few-several cells in size. Sperm produced by male gametophytes fertilize eggs extruded by female gametophytes. Most females produce 1 egg, but some produce 2, and a few produce 3. All three can be fertilized to grow into 3 related (at least half-sibling) sporophytes. (Lewis 1995)
There’s no evidence that Postelsia can reproduce vegetatively, though some self-fertilization probably occurs (Lewis 1995; Kusumo et al. 2006).
Evolution and Systematics
Systematics and Taxonomy
Nereocystis luetkeana (bull kelp) is the closest living relative of Postelsia palmaeformis (Lane et al. 2006).
Humans eat the fronds of Postelsia, but unless proper harvesting techniques are used, this can lead to the extinction of a patch (Thompson et al. in press; Report to Fish & Game Commission 2004).