A Chaos of Delight

Nematodes

 
 
Nematode.jpeg
 
 

 
 

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.

Nearly half are parasitic, many capable of causing diseases and problems in vertebrates and invertebrates alike. However, some of the nematode families are free-living in soil, and often being a little larger than others, are the easiest to photograph, most only reaching around 2mm, end to end, although some species can reach up to 10mm. They bridge the area between both arbitrary groupings of mesofauna and microfauna 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. They can be predatory, feeding on both alive and dead animals, other nematodes and protozoa, or feeding on plants and roots, soil substrate, fungal hyphae and bacteria. Some can also be omnivorous.
They can be a major plant pest in agriculture around the world, but farming aside, they’re also a hugely important part of nutrient cycling in soils due to their ubiquity.

coiled nematode.jpeg

A famous quote by Nathan Cobb suggests that if all the matter in the world was swept away, leaving only nematodes, then ghostly, nematode-formed shapes would remain of every single thing. Oceans, humans, mites, trees... That's how amazing and numerous they are.

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, the authors- van den Hoogen, Geisen and Crowther 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.

Nematode Somerset

Nematode Somerset

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.

 
 

Mesostigmatan mite with passengers, East Pennard, Somerset, March 2015

Dicyrtoma fusca var. 1 with a face full. Wookey Hole woods, Somerset UK, Jan 2017

 
 

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 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.

Sminthurinus aureus with host-finding nematodes

Sminthurinus aureus with host-finding nematodes

Close up of Sminthurinus aureus with host-finding nematodes

Close up of Sminthurinus aureus with host-finding nematodes

 
Nictation in juvenile nematodes, questing for a host. Somerset, UK

Nictation in juvenile nematodes, questing for a host. Somerset, UK


 

Nematode from grassland, East Portlemouth, South Devon, UK Aug 2016.

 

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 of dauer, as mentioned above, sex attraction, aggregation and other social behaviours, depending on each acaroside, their combination and the concentration being exhibited. It’s also entirely possible that more information may be being passed on and read, 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.

 

Nematode, South Devon

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. And yet, they showed every sign of being adapted for that specific environment.

Nematode, South Devon

Top