Nematodes are round worms. They move like snakes. Nematodes need a layer of water in which to live and so can be restricted to soil with adequate water films. In soil, they live on the films surrounding soil particles and organic matter. Beneficial nematodes are strict aerobes and cannot tolerate reduced-oxygen conditions, whereas root-feeding nematodes seems to have no such restriction. Nematodes should be counted and classified according to how many are seen per field, but usually there simply will not be that many of these organisms in compost, extract, or tea. There can sometimes be huge numbers of bacterial-feeding nematodes and root-feeding nematodes, but rarely are fungal feeders or predatory nematodes present at numbers greater than two to four per slide. Size is not important as characteristics because nematodes get larger with each juvenile stage they pass through. Nematodes shed their exoskelton and grow new ones. At times when nematodes seem to have a loose "outer skin," they may be in various stages of shedding that skin.
Bacterial Feeding Nematode
1. Bacterial Feeding Nematode
Most important is that bacterial-feeding nematodes have simple mouths, with no stylets or other special structures easily observed. They can have elaborate lip structures, but most have long, narrow mouths (Cylindrical); some are V shaped, and some just have round openings. Google "nematodes" and look at more pictures. These nematodes consume bacteria by filter-feeding them from soil suspension. As a result of their bacterial food containing a much greater concentration of N, P, S, Ca, Fe, and so on, than the nematode requires, bacterial-feeding nematodes release nutrients in plant-available forms. Because the plant typically grows huge numbers and varieties of bacteria around root systems, through release of exudates ("cakes and cookies"), the plant controls nutrient cycling in its root system if bacteria and nematodes are allowed to grow without toxic chemical interference.
2. Fungal Feeding Nematode
This is a fungal-feeding nematode. On the left side are the lips, a bit offset from the rest of the body, but inside the mouth, the big, broad stylet can be seen. It comes to a point at the working end, pointing out of the body. The other end has muscle attachments, but big knobs are not needed to give the big push that root-feeding nematodes must generate to get through the really thick cell walls of the plant. Thus, in fungal feeders, the stylet is simple, broader, but typically shorter than in the root feeders.
Fungal Feeding Nematode
Root Feeding Nematode
3. Root Feeding Nematode
These can be distinguished relatively easily by the fact they have a spear used to puncture plant root cells and suck out the internal contents of that cell. Plant root juice keeps root feeders alive. Because the root cell has thick cell walls, root feeeders have big spears, with big knobs at the ends, opposite the pointy end that stabs the root cell. those knobs are distinctive and relatively easy to see. Thus is you see a shiny, light-refractive, long, narrow spear, pointy at one end and knobbed at the other, you know it is a root-feeding nematode. You also know the plants are in trouble.
4. Predatory Feeding Nematode
Predatory nematodes eat other nematodes. ALl predatory nematodes have large, open mouths, like this one, with thick outer layers around the mouth, and thus are quite easy to pick out from other nematodes in soil. They may have teeth, from one single one, as in this nematode, to many smaller ones, in their large mouths. Some predatory nematodes are known to prefer to consume root-feeding nematodes, and thus these species would be the ones that would be best for agricultural systems.
Predatory Feeding Nematode
5. Nematode, Fungi, Actinobacteria
Total magnification of 100X (10X objective) showing a root (upper left across to mid-right); nematode just below the root on the left side, several small strands of most-likely fungi, actinobacteria, and different size organic matter bits and aggregates. No positive ID can be made of the nematode, the fungal hyphae or actinobaceria without moving to a higher magnification.
Nematode, Fungi, Actinobacteria
Nematode, Fungi, Actinobacteria
6. Nematode with unusual constricted "Waist"
Nematode on the right hand side of picture with an unusual constricted "waist" about half way down the body where the nematode most likely escaped from a nematode-trapping fungus trap.