Nematodes
Overview
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
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]
Feeding Styles
Herbivores: Herbivoral nematodes are the majority of nematodes known with about 50% of all nematodes being herbivoral. The mouthparts of an herbivoral nematode consist of a small needle that is used to puncture the plant while feeding. The two main herbivores are Ectoparisites, which feed on the outside of the root and Endoparisites, which feed on the inside of the root structure.
Bacterivores: These are the nematodes that are primarily responsible for being beneficial in decomposition and nutrient cycling. The mouthpart of a bacterivoral nematode is a hollow opening that allows the bacteria to be ingested. Bacteria are everywhere in the soil, which is why this is the 2nd most populous group of nematodes.
Fungivores: These nematodes are also very important in decomposition and nutrient cycling in the soil. The mouthpart is similar to the herbivores mouthparts, it is a stylet that is designed to pierce the fungal hyphae during feeding.
Predators: These nematodes are great for keeping certain populations at bay because of their predation preferences. Most predatores eat other plant-parasitic nematodes, and other free living nematodes; although they can eat protozoans and even some insects. They mouthpart is an opening that is designed for attaching to it's host, and a stylet that is designed to scrape away at the tissue of the host.
Omnivores: Omnivoral nematodes typically prefer to eat on plants, bacteria, and fungi; although if their food source is scarce they will eat other nematodes and insects. Their mouthpart is similar to the predatory nematodes.
Unknown: There are many species of nematodes that have not been studied a lot, therefore their food source is unknown to us. [7]
Importance In Soil
Nematodes are a critical component in the soil ecosystem in many different areas. Primarily, certain nematodes are very beneficial to agricultural soils, and some are detrimental to agricultural soils. Bacterial and fungal feeding nematodes are great mineralizers and create optimal soil conditions for crops or other plants nearby. Mineralization is the process of taking organic compounds and creating inorganic compounds that are more available to plants nearby. Some bacterial feeding nematodes are able to take up Nitrogen in the form of proteins and release them as readily available ammonium. This mineralization process is great for maintaining a healthy farm. Not only do these nematodes help fix nitrogen, they also are vital in nutrient mobility and decomposition in soil. Nematodes can breakdown fungus or bacteria and put the nutrients in a cycle that allows new plants to thrive. [6]
Predatory and insect-parasitic nematodes are important for controlling populations in the soil. The predatory nematodes tend to eat other nematodes and thus keep a population check on different nematode species, maintaining a healthy biodiversity in the soil. Insect-parasitic nematodes are especially important to farmers because they can keep a harmful pest at bay in a certain crop. The insect-parasitic nematode can enter it's host and kill the host while taking in nutrients from the host. This can assist in crop health by removing pests that can destroy a crop. [6]
However, there are plant-parasitic nematodes that can destroy crops and overall plant biodiversity in an ecosystem. This can be an issue to farmers because their entire crop can be destroyed by these nematodes. They will enter the root system of a plant and will divert the nutrients from the plant to the nematode. This can often kill the plant if the infestation is large enough. Farmers are finding better techniques to keep their crops safe from these detrimental nematodes by rotating their crops more often. [6]
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.
[6] Ugarte, Carmen, and Ed Zaborski. “Soil Nematodes in Organic Farming Systems.” EXtension, articles.extension.org/pages/24726/soil-nematodes-in-organic-farming-systems.
[7] McSorely, Robert. “Soil Inhabiting Nematodes.” Featured Creatures, July 1997, entnemdept.ufl.edu/creatures/nematode/soil_nematode.htm.