Fluid Pressure Provides Support

Biological Strategy

Blue crab

AskNature Team

Image: Jeremy Thorpe - EOL Images / Flickr / CC BY - Creative Commons Attribution alone

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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Crustaceans

Subphylum Crustacea (“crust”): Crabs, barnacles, shrimp, pillbugs

Some crustaceans, like pillpugs and woodlice, live on land, but most are found in the ocean. Their heads are distinctive among the arthropods: they have two pairs of antennae and three pairs of feeding appendages that help them hold and crunch food. Crustaceans also have a unique larval stage called the nauplius (named after the son of Poseidon) which has three pairs of legs and one eye. Most crustaceans are in class Malacostraca, which contains a wide range of species that live in mostly marine environments, like krill, lobster, and mantis shrimp.

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Nature’s Innovations: Animals as Engineers

This lesson, with eight short videos, examines how animals have solved engineering problems and how humans have mimicked those solutions. This lesson addresses middle and high school Next Generation Science Standards in Life Sciences and Engineering Design

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The body of the blue crab functions during exoskeletal molt using hydrostatic pressure.

“The aquatic blue crab Callinectes sapidus maintains mobility by switching to a hydrostatic skeleton ¹⁰ — a fluid-based skeleton that is common in soft-bodied invertebrates ¹¹. Hydrostatic skeletons are arranged so that the force of muscle contraction is transmitted by an essentially incompressible aqueous fluid ^11–13. Muscle contraction increases the pressure in the fluid, causing the deformations or stiffening required for support, movement and locomotion.” (Taylor and Kier 2006:1005)

“Like vertebrates, crustaceans usually move their limbs using muscles attached to a hard skeleton–albeit one on the outside of the body rather than the inside. But when a crab sheds its skeleton to grow a bigger shell, the muscles are left without any rigid surface to pull against. How do they do it? William Kier and Jennifer Taylor of UNC-Chapel Hill investigated, and found that, during molting, crabs use hydrostatic pressure to create a stiff structure against which muscles can pull. Fluid pressure in the claw goes up as the muscles contract; if you remove a claw during molting, it deflates like a flat tire. Once the shell has hardened, however, pressure does not change during muscle use. Soft-shelled crabs are the first animals known to use both a skeleton and hydrostatic pressure for support.” (Jones 2003:17)

Blue Crab Molting

Arthropods, like the blue crab, have an exoskeleton that must be shed—or molted—when they grow. Starting at about 1:00, this video from Shape of Life shows the molting process of a blue crab and how it pulls its legs and body out of its old shell.