Moving at a Snail's Pace

This post is about slime. Well, it's about moving around in slime. *squish*

Mollusca, or mollusks, is a large and highly diverse phylum of invertebrate animals. Within this phylum is the Class Gastropoda, or gastropods, which include snails and slugs. Generally, gastropods, specifically snails, have an asymmetrically spiral (coiled on one side) shell that functions as a portable retreat. Slugs are almost identical to snails except they lack a shell. The snail and slug body consists of a head, foot, and visceral sac/hump, and mantle (pallium). The head includes a mouth surrounded by one or two pairs of tentacles which often carry eyes and a pharynx containing coarse or fine teeth on the radula (like a tongue). The foot is the main locomotive organ and is usually the part that is visible outside of the shell. On the sole of the foot are mucus glands that secrete the slime that the gastropods crawl on.The visceral sac contains most of the inner organs and the mantle is a tissue fold covering it.

Though snails and slugs have no external extremities they are quite capable of moving around in their environment. Understanding this movement has been of interest to scientists and engineers for some time, even inspiring new classes of robotic movement and adhesive locomotion. It is known that a series of pulses of muscle contraction and relaxation traveling along the central part of the foot's ventral surface allows the snail to move forward, and only forward in terrestrial gastropods. The pulses of muscles are called pedal waves, the regions of the foot between pedal waves are called interwaves, and the distance between the two is called the wavelength. When these waves interact with the mucus secreted by the gastropod propulsive forces are transmitted to the ground. In describing snail locomotion, the number of waves is classified according to their number and direction. They are classified as a single train of pedal waves (monotaxic) or as a two (ditaxic) or four (tetrataxic) series of waves. It is also known that the crawling speed is directly proportional to the speed and frequency of the pedal waves.

A 2010 study in the Journal of Experimental Biology takes a look at the mechanism by which the propulsive forces are generated during gastropod locomotion. To accomplish this the researchers used a newly developed force-cytometry method where they calculate the spatial and temporal distribution of pedal forces from measurements of the deformation produced by the snail when it contacts a surface of known elastic properties. This allows them to study the movement is great detail. They can measure the horizontal traction stresses to the surface underneath the snail/slug without any interference with the animal's body. Neat. The study also analyzes the kinematics (motion without reference to the forces causing it) of the pedal waves and its significance of the generation of traction force. They did this to find the relationship between speed/wavelength and velocity, to determine if the waves maintain a constant speed/wavelength, to find how the snail accelerates and decelerates, and to see if a change in speed is accomplished by increasing the number of waves or by varying the speed/wavelength.


Fig. 1. from the paper showing the ventral surface of the banana slug

That all sounds very...complicated. So how do you measure the pedal waves of a snail's foot? If you know any biomechanists then you know they like to do two things - put animals on treadmills and put animals on transparent surfaces. In this case they tested banana slugs (Ariolimax californicus and A. buttoni), grey field slugs (Deroceras reticulatum), and garden snails (Helix aspersa) by placing them on transparent surfaces, illuminating them, and then recording them crawling with digital cameras. Turns out that if you illuminate the body in different ways you can get different information about movement. Add all of those ways together and you get a 3D reconstruction of the snail's foot as it moves.
The researchers found that when a snail/slug moves forward there are alternating pedal wave and interwave regions propagating from the tail forward to the head, but the interwaves remain stationary with respect to the ground. That result wasn't all that surprising, and agreed with previous studies. When they looked a little closer at the high-resolution images they found that the organization of the waves was not symmetrical and did not move at a constant speed. They observed steady wave acceleration followed by abrupt deceleration, a variable speed of pedal waves which modulated the magnitude of stresses under each wave. This was unexpected and observed in more than one of their test species, suggesting that the pattern is mechanically relevant to locomotion. They also found that the net forward force was generated beneath each stationary interwave. This is where the animal is pressing the foot against the ground and then pulling it backwards, propelling the body forwards. The foot is actually lifted during the pedal waves. Another result showed that that the crawling speed increased with pedal wave frequency. Not all that surprising, have more waves then move faster. And the mucus, we can't forget the mucus. This study's experiments showed that the slugs were able to move themselves over very thin threads of mucus without changing the pedal wave pattern or frequency. This suggests that the amount of pressure applied by the foot doesn't really matter for propulsion. Considering the rugged surfaces that snails and slugs move on that is not all that hard to believe.

The take home message? Mucus is helpful but it is the muscle movements that allow snails/slugs to crawl.

Read the study here, and there are videos in the supplemental materials:
Lai, J. H., J. C. del Alamo, J. Rodriguez-Rodriguez, J. C. Lasheras (2010) The mechanics of the adhesive locomotion of terrestrial gastropods. Journal of Experimental Biology: 213(22), 3920. (DOI: 10.1242/jeb.046706)