Supportive Gel Enables Extreme Shape Change

Biological Strategy

Sea anemones

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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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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Coral and sea anemones

Class Anthozoa (“flower animals”): Corals and sea anemones

Most cnidarians are included in this class, which contains 6,000 species and 70% of all known cnidarians. These animals are exclusively found in marine environments. They have no jellyfish-like medusa stage, and so move very little or not at all throughout most of their life. They are carnivores and channel food down a groove lined with cilia, or hair-like appendages, to their mouths. Many also have a symbiotic relationship with photosynthetic algae, which are responsible for the vibrant colors associated with coral colonies.

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The supportive gel-like substance (mesoglea) of sea anemones allows extreme shape changing due to its viscoelasticity.

“Consider a solid material with properties and role about as distant from bone as a supportive, compression-resisting material can be. The body wall of a sea anemone–which can be quite substantial in size–consists of inner and outer surface layers separated by the thick mesoglea. One doesn’t go far wrong viewing the system as a tall can of seawater whose walls are mostly made of jelly…A typical anemone has a rare facility for changing shape, ranging from a low barrel to a tall cylinder with a few flourishes in between, over times ranging from seconds to hours…Obviously its mesogleal stuffing must participate in the process. Muscle drives some of the shape changes, in particular the sudden expulsion of water in the central cavity from its single apical opening. But tracts of cilia drive other changes, such as reinflation by pumping water back in. You may recall that…ciliary pumps produce exceedingly low pressures, and here we’re asking that they pump up creatures that may reach half a meter in height and live in moving water.

Alexander (1962) showed the crucial role of mesogleal viscoelasticity for anemones. In creep tests on samples, strain increased from an initial value of about 0.2 to a final level ten times that, achieved after around 10 hours. That means the mesoglea has a lot of viscosity relative to its elasticity–it’s hard to make it do anything fast but fairly easy to make it change shape slowly. It has a retardation time (calculated by Biggs; see Vincent [1990]) of a little under an hour. How nice! The pulsating or reversing flows of waves passing above won’t sweep it about very much, but after it has hunkered down, the low-pressure ciliary pump will be adequate to pump it back up again, albeit slowly. It can stand up to a single wave but deflect in a tidal current that imposes the same drag. Furthermore, the anemone’s body wall can resist the stresses of its own short-term muscle contractions, so it can bend or straighten without getting an aneurysm whenever its muscles aren’t active.” (Vogel 2003:360-361)

Last Updated September 14, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

17/06/2003 | Steven Vogel

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