Viscoelastic Mucus Enables Locomotion

a yellow banana slug with large brown spots moves across a fallen tree trunk with moss on it

Biological Strategy

California banana slug

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Image: Ben Stanfield / Flickr / Some rights reserved

Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Gastropods

Class Gastropoda (“stomach-foot”): Snails and slugs

There are at least 40,000 known species of gastropod, with many more yet to be discovered, making it the largest class of mollusks. These animals exhibit torsion, meaning their internal organs are twisted 180° during development. The majority of gastropods have shells, and most open up on the right side. The largest gastropods are the size of a small child (3 feet, or 1 meter) while the smallest are barely visible to the naked eye.

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The foot of a banana slug generates a propulsive force against surfaces via mucus secretions that act as both a lubricant and an adhesive.

Snails and slugs only have one body part in contact with the ground at any one time, which raises the question: how do they develop a net force to produce propulsion? Most animals have multiple legs with some functioning as stationary supports while others move forward to act as future stationary supports. Initially, it may seem that using only one structure to both hold onto the ground and move over it is a paradox; however, snails and slugs evolved the answer long ago. Though the precise physical mechanism has yet to be revealed, snail and slug mucus is apparently used to both lubricate the snail’s movement over surfaces and to facilitate the transfer of adhesive force to the surface. Only a “viscoelastic” fluid could have both properties since ordinary fluids are either viscous or elastic. Snails and slugs move by producing a pedal wave that travels from one end of the body to the other (depending on the species of snail). The interwave regions are stationary (relative to the ground) and exert force against the mucus layer which is then transmitted to the ground; for the interwave region, the mucus acts as an elastic or adhesive material capable of transmitting mechanical force. On the other hand, wave regions travel over the ground and do not exert force on it; in this case, the mucus acts as a viscous lubricant. There is mounting evidence that the mucus is of great importance when the snail/slug is crawling across vertical or inverted surfaces, but that it may not be entirely necessary for motility on flat, level surfaces.

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