By Samuel Bolton
What does your esophagus have in common with a type of mite with an extremely elongated body? The answer is, both of them employ a peristaltic mode of motion. Peristalsis occurs when the many muscles that line a body or food channel contract and relax in wave-like succession. When you swallow, you use peristalsis to conduct food through your esophagus and into your stomach. A number of different elongated invertebrates, including earthworms and insect larvae, also use peristalsis as a form of locomotion.
I have been collaborating with Gary Bauchan, Ronald Ochoa, and Chris Pooley at the USDA’s Beltsville Agricultural Research Center in order to research the locomotion of the Nematalycidae, a worm-like family of mites. These mites live in deep mineral soil or sand. Until we began our research, the only genus of this family to have been studied with respect to its locomotion was Gordialycus, which uses peristalsis to move its extremely elongated body through the sand.
Unlike earthworms, Gordialycus does not have circular muscles. Therefore, it cannot constrict its body diameter in order to extend itself forward. Instead, Gordialycus has a form of peristaltic locomotion that uses longitudinal muscles to pull a short region of the body forward. At the same time, this region is pushed forwards by the relaxation and extension of the area behind it. Locomotion requires that contraction and relaxation together spread up the length of a body as a peristaltic pulse. In this way, the body can move bit by bit, with little overall change in body length.
In a recent article in Entomology Today, I summarized our discovery of a new genus and species of Nematalycidae, which I collected across the road from the museum where I work. That new genus, which we named Osperalycus, has an adult body length of over half a millimeter, making it the second longest genus within the Nematalycidae — yet it is only about half the length of Gordialycus. The combination of close proximity and suitable soil meant that we were able to extract live specimens of Osperalycus and compare its locomotion to that described from observations of Gordialycus. Because Osperalycus is much more elongated than a number of other invertebrates that move via peristalsis, including many insect larvae, we expected to observe peristaltic motion. To our surprise, we found that this mite uses a very different mode of locomotion.
The movement of Osperalycus involves the constriction of the central body region, which generates much of the hydraulic pressure needed to extend the anterior or posterior body region. The absence of peristalsis means that waves of contraction are not coupled with waves of relaxation (see video below).
It is possible that the body length of this mite is within a limit that does not require peristalsis for locomotion. In contrast, Gordialycus uses peristalsis for the likely reason that any other mode of locomotion would be unworkable for a mite with such a highly elongated body.
As previously mentioned, peristalsis involves moving a body bit by bit. Without it, extending or contracting a long body region requires good anchorage. In Osperalycus, anchorage is facilitated by the two rear pairs of legs, which can hold the central body region in place during the extension or contraction of another body region. These legs are dramatically reduced in Gordialycus, which is probably because they impede peristalsis.
The anus, which protrudes out strongly in all nematalycids, is also used for anchorage. We discovered that Operalycus will often push off against it when extending forwards (see video below). It seems likely that it is used in a similar way in other nematalycids. In the case of Gordialycus, the anus is especially prominent and, therefore, appears to be especially useful for the initiation of peristalsis, which for the purpose of forward locomotion begins at the anus.
Even differences in the ultrastructure of the integument seem to be adaptive for locomotion. We were able to determine this by using a cryo-scanning electron microscope housed at the Beltsville Agricultural Research Center in Maryland. We already knew that Osperalycus and Gordialycus, the two longest bodied genera, have integuments that are covered with flat circular structures that help their elongated bodies grip the sand or soil. What we did not know was that the nematalycid genera with adult body lengths below 0.5 millimeters instead have integuments that are covered with pointed versions of the same structures. These pointed structures probably provide additional grip, which would appear to be useful to nematalycids with comparatively short bodies.
This is the same basic principle as the tread on a tire. A more pointed surface improves the body’s grip so that one part of the mite can hold still while another part pushes off or pulls up against it. The longer nematalycids get much of their grip through their longer bodies. In a not so very different way, the road grip of an automobile can be improved by wider tires (the equivalent of extra body length) or tires with finer tread elements (the equivalent of a body surface that is covered in pointed gripping structures).
More information is available from our open-access article, which was recently published online in Experimental and Applied Acarology.
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Samuel Bolton is currently a PhD student at the Acarology Lab at Ohio State University. He has just completed a pre-doctoral Smithsonian Fellowship based at the Beltsville Agricultural Research Center in Maryland. His research is largely based on the evolution and systematics of the Endeostigmata (including the Nematalycidae) — an extremely ancient and basal group of mites that dates back to the Devonian.