Soil-dwelling nematodes 

Nematodes, otherwise known as roundworms and in the phylum Nematoda, are everywhere. It’s been estimated that around 80% of multicellular animals globally are nematodes. Their name ‘nematode’ comes from the Greek words ‘nēma’ meaning thread, and ‘eidos’, meaning ‘resembling’. Which they really do.

Nathan Cobb is still renowned as the father of nematology in the United States, a title that was very well deserved. Apart from his vast knowledge of nematodes, he was also an accomplished artist and writer, as seen from the famous and beautiful quote above.

Cobb coined the name for the study of nematodes as ‘nematology’ and because of this, suggested that nematodes be renamed nemata and put them into their own phylum, Nemata. Later, nematodes were demoted into a class of the now obsolete phylum Aschelminthes, and called Nematoda. When the class was subsequently elevated once more into its own phylum, the term ‘Nematoda’ was retained instead of more correctly deferring to Cobb’s original phylum naming of Nemata. Because of this, some biologists use this term rather than Nematoda.

Nearly half of the world’s nematodes are parasitic, with many capable of causing diseases and problems in vertebrates and invertebrates alike. Most animals and plants have their own specific nematodes living within them.

Nematodes range from the microscopic, at around 0.08mm, all the way up to well over a few metres long for some parasitic species, with the largest known nematode so far discovered being Placentonema gigantissima, a monster at a reported 8.4 metres-long for one specimen, that was found living in the reproductive tract and placenta of a sperm whale.

They are the deepest dwelling animals on earth so far found, having been recorded at a depth of 3.2 kilometres, or just over 2 miles down, in the hot, subterranean ground water at the base of an exploratory borehole in Tau Tona, a South African gold mine. At those depths, there is scant oxygen or food and the temperature is a scalding 48 degrees centigrade. It was thought for many years that only microbes were capable of existing in such a nightmarish environment. But life always finds a way. In such an ‘adapt or die’ scenario, Halicephalobus mephisto, named after Faust’s demon, Mephistopheles, showed every sign of being uniquely adapted for that specific environment, having developed an unusually large amount of heat-shock proteins (Hsp70, that protects against heat damage) encoded into its genome. A remarkable animal.

Some nematode families are terrestrial and free-living, and are found in their highest amounts where organic matter is highest. They usually range from around 0.3mm to up to 5mm long, although a few species can reach up to 10mm. In our arbitrary groupings of soil life, they bridge between microfauna, mesofauna and macrofauna but I include them here in soil mesofauna.

Soil dwelling nematodes have become able to take advantage of practically any food source and environmental condition. While not common, some nematodes can be predatory, feeding on other similar-sized soil animals, other nematodes and protozoa. Many more feed on plants and roots, decaying organic matter, substrate, fungal hyphae and bacteria. Some can also be omnivorous. Even terrestrial free-living nematodes remain reliant on water, surviving due to the water film present in soils, and becoming more mobile as the water increases. It must also be pointed out that as might be expected from such an ubiquitous animal, while they are very much adapted to their terrestrial environment, many species cannot cope with too much water, such as from excessive rainfall and flooding, while others are able to thrive. Soil nematodes also require sufficiently large soil pores in order to move, as, like their mesofauna compatriots, they are too small to influence or change their soil surroundings.

In ‘Feeding Habits in Soil Nematode Families and genera– An Outline for Soil Ecologists’, Yeates et al suggested that soil nematodes could be split into eight basic feeding types- plant feeder, hyphal feeder, bacterial feeder, substrate ingester, predator of animals, unicellular eucaryote feeder, dispersal or infective stage of parasites, and omnivore. These groupings are also known as trophic groups. The different feeding groups are usually easy to identify from their different ‘mouth’ morphologies.

Nematodes are a major plant pest in agriculture around the world, as well as a much-used organic pest control, but farming aside, the non-parasitic, bacteria and fungal-feeding free-living nematodes also play a hugely important part in the nutrient cycling in soils. This is due to their presence in huge amounts in fertile soils, where they are able to recycle large amounts of minerals and other nutrients tied up in bacteria, fungi and other things, and make them available for plant roots to take up.

In soil biology, nematodes are an important bioindicator due to their spread across all trophic levels on the planet, a short lifespan, and their quick response to any environmental change, as well as the ease of cultivating them in a laboratory environment.

Biodiversity generally decreases rapidly as one moves away from the equatorial regions. This pretty much holds true across plants, vertebrates and invertebrates alike. However, in an article published in Nature in 2019, entitled Soil nematode abundance and functional group composition at a global scale, Van den Hoogen et al detail an unexpected finding, in which nematodes were found to actually increase in their biodiversity, being at their most abundant in tundra and boreal forest regions, but present in every soil across the globe. Even poor soils are capable of sustaining millions of nematodes in a single square metre of soil.

Phoresy and nictation

Nematodes are often seen travelling relatively harmlessly on Collembola and other micro-arthropods, in a process called phoresy, though the unwilling transportation vehicles do sometimes seem to exhibit some discomfort.

Phoresy is also practiced by other invertebrates like pseudoscorpions and mites. However, in soil-dwelling nematodes, it seems to be only done by juveniles of certain species, and as a reaction to overcrowding, lack of food, moisture, or temperature changes. The juveniles then experience an alternative developmental stage, known as dauer, during which they thin down and stop eating. This stage impels them to stand up on end and gently flex and wave into the air, questing for a host, as seen below, with a springtail, Sminthurinus aureus. In nematodes, this ability is known as nictation. Dauer can be seen as a survival technique, in the same way that tardigrades are able to turn into tuns, a switched off stage, until better conditions are experienced.

Nematode biology

Nematodes basically consist of an outer tube of cuticle, containing two other smaller tubes, the pharynx/ intestine and the reproductive system. Below, you can see the pharynx leading from the head part to the start of the intestine. The majority of the body is dominated by the reproductive system, seen here by the defined area of darker yellow.  This is in stark contrast to the Enchytraeids, whose gut and digestive contents can be tracked from head to tail. Nematodes have teeth or a piercing stylet with which to feed and ingest prey or plant juice.

Soil nematodes use ascarosides, pheromones unique to nematodes to achieve a huge amount of complex tasks. The pheromones trigger the entry and exit stages of dauer, as mentioned above, sex attraction, aggregation and other social behaviours, depending on each acaroside, their combination and the concentration being exhibited by the pheromone. It’s also entirely possible that other information may also be passed on and read and have an impact, such as life history and metabolic state.

They also have an interesting muscle structure which means, apart from the head, which has more mobility, the body can only flex dorsally and ventrally, which looks a little like mindless thrashing in the soil.