Variable Postures Aid Intertidal Zone Survival

small green sea palm plants are seen on a marine surface with rough sea waters in the background

Biological Strategy

Sea palm

AskNature Team

Image: Dr. Sophie McCoy/Florida State Univ./NOAA Photo Library / Flickr / Some rights reserved

Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Sea palms survive changing intertidal conditions by adopting various postures, thanks to a unique set of mechanical properties.

“Postelsia palmaeformis–the scientific name makes the same allusion as the common name, ‘sea palm’–lives in the lower intertidal zones of rocky, wave-swept shore on the west coast of North America. The plant, shown in figure 21.4, never reaches a meter in height, so it’s not exactly a big tree. But, like trees, it has three parts, an attachment (holdfast), a column (stipe), and photosynthetic laminae (fronds). It stands against gravity, getting higher in dense stands, and it bends in response to lateral force, for it, waves. Unlike any tree, though, it responds to lateral force by bending until almost prostrate and then springing back upright. Concomitant with that untreelike behavior are a set of most untreelike material properties, set out in a lovely paper by Holbrook, Denny, and Koehl (1991). Table 21.1 lists these, along with typical values for wood.

“Postelsia‘s ‘trunk’ certainly looks wimpy next to wood. It gives rather than standing tall in the face of fierce force, getting from a high second moment of area just enough flexural stiffness to stand erect at all. Only in work of extension does it play in the same league as wood, although it gets its high value by quite a different tactic–high stretchiness instead of high strength. Regaining its erect posture depends on reinvesting that work of extension, which requires a high resilience, which it has. But, as Holbrook, Denny, and Koehl emphasized, the energy storage underlying that resilience undermines its toughness. Postelsia is notably brittle and sensitive to scratches–its stipe may be soft, but cracks propagate all too readily. Before dismissing this alga as just a lowly tree wannabe, bear in mind that it has extraordinarily high photosynthetic productivity and that it invests far more, proportionately, in photosynthetic organs than any ordinary tree. Its fronds represent no less than 38 percent of its energy content or 35 percent of its weight (Lawrence and McClintock 1988)–compare this with the trivial weight and burning yield of the annual leaf-fall of a tree that yields a cord or so (perhaps 2 metric tons) of dry firewood.” (Vogel 2003:434-435)

“Sea-palms Postelsia palmaeformis Ruprecht are annual brown algae that grow on wave-swept rocky shores, often forming dense stands. Unlike most macroalgae, Postelsia stands upright in air–like trees. The stipe flexibility that permits Postelsia to withstand waves is provided by the low elastic modulus
(5-10 MPa) of stipe tissue; in spite of the weakness (low breaking stress, ~ 1 MPa) of this tissue, a large amount of energy ( ~ 100 kJ/m 3) is required to break a stipe because they can be extended by 20-25 % before breaking. Although made of such easily deformed tissue, Postelsia can stand upright in air due to the width (high second moment of area) and resilience of their stipes, but the brittleness (low work of fracture, 400-900 J/m 2) that accompanies this resilience renders them susceptible to breakage if they sustain deep scratches. Although wave-induced stresses experienced by individuals in aggregations are not lower than those experienced by isolated sea-palms, photon flux densities of photosynthetically active radiation within these dense groves are less than 10% of those above Postelsia canopies. A number of morphological features differ between canopy, understory, and isolated individuals. Canopy plants in dense aggregations are taller than isolated individuals and may exceed limiting proportions for elastic stability. Postelsia shows photosynthetic characteristics of “shade-adapted” plants, understory individuals being especially effective at using low light. Despite this, blade growth rates of understory plants are lower than those of either canopy or isolated individuals.” (Holbrook et al. 1991:39)

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Last Updated August 18, 2016

References

“Postelsia palmaeformis–the scientific name makes the same allusion as the common name, ‘sea palm’–lives in the lower intertidal zones of rocky, wave-swept shore on the west coast of North America. The plant, shown in figure 21.4, never reaches a meter in height, so it’s not exactly a big tree. But, like trees, it has three parts, an attachment (holdfast), a column (stipe), and photosynthetic laminae (fronds). It stands against gravity, getting higher in dense stands, and it bends in response to lateral force, for it, waves. Unlike any tree, though, it responds to lateral force by bending until almost prostrate and then springing back upright. Concomitantwith that untreelike behavior are a set of most untreelike material properties, set out in a lovely paper by Holbrook, Denny, and Koehl (1991). Table 21.1 lists these, along with typical values for wood.

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