Nematodes

Overview

A Hemicycliophora, a species of Nematoda. [1]

According to the Merriam-Webster Dictionary, Nematode is defined as, "Any of a phylum (Nematoda or Nemata) of elongated cylindrical worms parasitic in animals or plants or free-living soil or water".[2] Nematodes are the most diverse, and adapted phylum of metazoans and are found all over the earth in many different regions. There are expected to be around 50,000 species of Nematodes that exist today. Nematodes are one of many phyla that are classified as a protostome, although they do not follow with Annelids because of the Nematode's tubular design.

Anatomy

The body of a nematode is long and narrow; The term nema is a Greek word meaning thread, a close resemblance to the body shape of a nematode. A nematode is a very unique yet simple organism when you take notice of it's anatomy. The only muscles that a Nematode has all run longitudinally along the inside of it's body, therefore a nematode can only wiggle side to side and no other way. These muscles lie just below the epidermis and around the gut cavity. The epidermis of a nematode is very unique because it is a mass of cellular material and nuclei without separate membranes. This is unusual because most animals have an epidermis comprised of cells. A very important feature of the epidermis is that is secretes a thick outer cuticle that serves as a relative exoskeleton for the nematode, although this is far from being a true exoskeleton. Arthropods and Ecdysozoans are two other groups of animals that commonly shed their cuticle multiple times before adulthood.

There are only two nerves in a nematode, a ventral nerve, and a dorsal nerve. The ventral nerve runs along the bottom of the nematode and runs the entire length of the nematode. The ventral nerve has many "nerve centers" along its length. The dorsal nerve runs along the back(top) of the nematode for the entire length as well. Both nerves are connected by a nerve ring near the head of the nematode. Unlike most animals, the muscle cells of a nematode actually run towards the nerve cells.

The digestive system of a nematode is very simple; There is a head with some sensory organs, a mouth that leads to a throat that is designed to increase the surface area of the food, a gut cavity along the nematode, and leaving through the anus. The nutrients in the food are dispersed along the cavity of the nematode, where it is regulated by an excretory canal along its sides.

There is no specialized respiratory system in nematodes, nor is there a specialized vascular system. Nematodes rely on diffusion in order to obtain its oxygen and nutrients. Diffusion is a movement of molecules from a high concentration to a low concentration, which is how the molecules travel into the nematode when it needs it. This is helpful because a nematode doesn't need complex organs to breathe unlike humans.

The process that truly allows for nematodes to live in harsh conditions is called cryptobiosis; a feature that allows nematodes to completely put a halt to its life functions. Allowing nematodes to "turn-off" when conditions are unfavorable, and to "turn-on" when they are in more suitable conditions.[3]

Cryptobiosis

An example of a cryptobiosis in the "Sleeping Chironomid", or Polypedilum vanderplanki [5]

There are many states of cryptobiosis that have been identified and studied only slightly. The most studied state is called anhydrobiosis. This form of cryptobiosis undergoes in an organism when there is a lack of water. The organism undergoing anhydrobiosis will have an un-measurable amount of metabolic processes going on in it's body. anhydrobiosis is an extreme form of desiccation (drying out), but a form that still allows the organism to live. A nematode that undergoes Anhydrobiosis go from having a water content of 75-80% to a water content of 2-5%. If desiccation occurs quickly, the organism is likely to die; Therefore a slower dessication will lead to a more reliable form of anhydrobiosis. Nematodes that are in anhydrobiosis are able to survive in incredibly harsh conditions, not only for a few days or weeks. "Filenchus polyhypnus (Steiner & Albin) Meyl was revived from a dry herbarium specimen after 39 years" (McSorely). Not much is known exactly how anhydrobiosis works on nematodes, but the permeability of a nematodes cuticle changes, and so does the packing of tissues and organelles. [4]

Nematodes primarily undergo anhydrobiosis, but there are some species that undergo more than just anhydrobiosis. They can undergo anoxybiosis, which is a condition where a nematode will take in water and enter a stage of low productivity. Entering this stage is as close to dying without actually dying, and similar to anhydrobiosis, a nematode can survive for months to decades in a state of anoxybiosis. This state is a result of being in a place that is very low on oxygen, or anoxia. [4]

The other partially studied states of cryptobiosis are chemobiosis, cryobiosis, and osmobiosis. Chemobiosis is a state that occurs in response to high levels of toxins or other chemicals. This state has not been studied in nematodes, instead they are studied in Tardigrades. Cryobiosis is a state that occurs in response to very low temperatures and when the water molecules around an organisms cells are freezing. This is a way to keep an organism alive when facing freezing conditions. An organism that displays this state of cryptobiosis is a lobster. Osmobiosis is a state that an organism enters when in response to an increased solute of whatever solution it is in. This is the least studied state of them all, although it is clear that metabolism in some organisms can change due to an increased solute. [4]

References

[1] Warner, Fred. “Blueberries and Plant-Parasitic Nematodes in Michigan.” Michigan State University, pestid.msu.edu/blueberries-and-plant-parasitic-nematodes-in-michigan/.

[2] “Nematode.” Merriam-Webster, Merriam-Webster, www.merriam-webster.com/dictionary/nematode.

[3] Wagner, B. M. “Introduction to the Nematoda.” Basic Flight Physics, www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html

[4] McSorely, Robert. “ADAPTATIONS OF NEMATODES TO ENVIRONMENTAL EXTREMES.” BioOne, Florida Entomological Society, June 2003, www.bioone.org/doi/full/10.1653/0015-4040%282003%29086%5B0138%3AAONTEE%5D2.0.CO%3B2.

[5]“About Sleeping Chironomid.” Sleeping Chironomid, Sleeping Chironomid Research Group, www.naro.affrc.go.jp/archive/nias/anhydrobiosis/Sleeping%20Chironimid/e-about-yusurika.html.