Nematodes, commonly known as roundworms, represent one of the most abundant animal phyla on Earth. These non-segmented organisms are found in nearly every habitat, from marine trenches and freshwater to mountain soils and polar regions. A single acre of fertile topsoil can contain billions of individuals, often outnumbering all other animals combined. This success is due to their extraordinary dietary diversity, which allows them to occupy nearly every trophic level in an ecosystem. Their varied feeding strategies make them significant players in decomposition, nutrient cycling, and agriculture.
The Role of Nematodes as Microbial Consumers
The majority of free-living nematodes in soil and aquatic environments feed on microorganisms, playing a foundational role in decomposition and nutrient release. This group includes bacterivores, which graze extensively on soil bacteria, and fungivores, which pierce and consume the contents of fungal hyphae. These microbial-feeding activities are fundamental to the health of the soil food web.
Bacterivorous nematodes possess a simple, non-spearing mouth opening, which they use to suck in bacterial cells. They consume bacteria at a rapid pace, effectively regulating microbial populations in the soil. Fungivorous nematodes use a specialized mouth structure to puncture the tough cell walls of fungi and extract the internal cytoplasm.
The consumption of microbes is an important mechanism for nutrient mineralization, particularly nitrogen. Bacteria and fungi incorporate nitrogen into their body mass, but nematodes require less nitrogen relative to their diet’s carbon content. As they consume microbes, they excrete the excess nitrogen as ammonium (\(\text{NH}_4^+\)), which is readily available for plant uptake. This grazing activity ensures that nutrients tied up in microbial biomass are cycled back into the soil solution to support plant growth.
Nematodes That Feed on Living Plants
A distinct group of nematodes feeds exclusively on living plant tissues, making them significant agricultural pests worldwide. These herbivorous nematodes are characterized by a specialized, syringe-like mouthpart called a stylet. This structure is used to pierce plant cell walls, allowing the nematode to inject enzymes that partially digest the cellular contents before ingestion.
Plant feeders are classified based on where they feed relative to the root. Ectoparasites, such as stubby-root and needle nematodes, remain outside the root, extending their stylets deep into the tissues to feed on multiple cells. Endoparasites, conversely, enter the root tissue to feed, causing more extensive damage.
Endoparasitic nematodes are further divided into migratory and sedentary types. Migratory species, like lesion nematodes (Pratylenchus spp.), move destructively through the root cortex, feeding on individual cells. Sedentary species, like root-knot nematodes (Meloidogyne spp.) and cyst nematodes, are the most specialized.
Once a sedentary nematode settles, the female induces surrounding plant cells to transform into specialized feeding structures, such as giant cells or syncytia. These structures continuously supply the parasite with nutrients. This specific feeding habit causes distinct symptoms, such as the characteristic root swellings or galls induced by root-knot nematodes. The resulting damage, which includes reduced nutrient uptake and susceptibility to other pathogens, results in billions of dollars in crop losses annually.
Predators and Specialized Animal Parasites
Beyond free-living and plant-feeding forms, other nematodes have developed specialized diets, including predation and obligate parasitism of animals. Predatory nematodes are a diverse group that feeds on other nematodes, rotifers, and small invertebrates. Many predators have large, armed mouth cavities, sometimes featuring teeth or cutting plates, used to seize and consume prey whole.
Other predatory species, such as some in the Mononchida order, use a large tooth to slice open their prey before consuming the contents. Entomopathogenic nematodes (Steinernema and Heterorhabditis) are parasites used in biological control. They invade insect larvae and release symbiotic bacteria that multiply quickly, killing the host, and allowing the nematode to feed on the resulting microbial biomass.
The final major feeding group includes those that parasitize vertebrate hosts, including humans and livestock. Species like hookworms and pinworms are obligate parasites that feed on the host’s blood or tissues. They rely on the host for survival and often have complex life cycles where larval stages must develop externally before re-infecting a new host.