Typical Nematodes - Roundworms
A soybean cyst nematode and egg
picture taken with a scanning
electron microscope of a plant parasitic nematode.
Galls (swollen regions) formed on vegetable roots
of infestation by parasitic nematodes
Meloidogyne incognita- a plant parasite
500X, in the process of penetrating a tomato root. The worm will
establish a feeding site once inside, which causes a nutrient-robbing
Other Nematodes - Roundworms
I can't bring
myself to call them "cool" like I have on other pages
in this section as many are just too gross
Typically 15 to 35cm in length, these
are parasitic of humans
Elephantiasis, a result of a parasitic infection of filarial
nematodes. The nematodes that cause this are microscopic
and cause a blockage of the lymphatic system and so fluid retention
with tissue and skin thickening. The legs and genitals are particularly
affected, the scrotum can swell to the size of a basketball in some
Ancylostoma caninum - a hookworm
attached to the
intestine of a dog, though species of hookworm infect other mammals
including humans. The name comes from them being bent over at the
head end so forming a hook.
Nematode infestation in the vomit of a cat
(yes I know
it's gruesome, but nematodes are like that)
used permission of Kalumet at de.wikipedia
GNU Free Documentation License, Version 1.2
Kingdom - Animalia
- Roundworms get their name from their round cross section
- Long thread-like bodies
- Usually very small to microscopic, some parasitic members
however may be a metre long
- Simple tube-like gut with a mouth and anus
- No circulatory system, gas exchange and excretion are by
diffusion across the body wall
- There is very little superficial difference between nematode
species, they all look pretty much like larger or smaller, somewhat
fatter or skinnier versions of each other
- Sexual reproduction, sexes separate, no asexual reproduction.
Males are usually smaller than the females, the females of some
species can deposit over 100,000 eggs per day.
Phylum - Nematodes
The group includes:
Nematodes are probably the commonest multicellular organism
that you've never heard of and have hardly ever seen. They occur
in all environments, in fresh and sea water, on land, in polar regions
and in deserts. They can be found in hot springs, high up mountains
and in the deepest oceans.
Most nematodes live in the "benthos" - sediments, whether
soil or aquatic sediments. A square metre of mud from an inshore
area near the coast of Holland was measured to contain over 4 million
nematodes. Good farm soil as well as containing a large number of
earthworms can also contain from several hundred million to several
billion nematodes per acre.
A single decomposing apple on an orchard floor has had 90,000
nematodes counted in it of several species and a single decomposing
fig around 50,000 of at least eight species.
Although there are so many free-living nematodes, they tend to
be microscopic and therefore unknown to all but the scientists who
study them. There are however a large number of nematodes that
are parasitic on a wide range of animals and plants, these are
rather better known than the free-living forms. These parasitic
forms have major economic and health influences in parts of the
world where they are found.
- Vinegar eels - tiny free living worms that can live
in vinegar or liquids that can become vinegary such as cider,
wine, beer etc.
- Caenorhabditis elegans - a 1mm long free living nematode
that has been used for many years (starting in 1974) as a "model
organism" for scientific research. At maturity all individuals
have exactly 959 cells. It was the first multicellular organism
to have its complete genome sequenced (in 1998), so leading
ultimately to the "Human Genome Project."
- Toxocara canis - the one that puppies are "wormed"
to remove and that can cause blindness in children if they come
into contact with animal faeces and are infected.
- Loa loa - the Africa eye worm
- Guinea worm
- Species that produce "filiariasis" in man -
What do nematodes eat?
Many free-living nematodes are carnivorous, they feed
on animals that are even smaller than they are including other nematodes.
Other free living nematodes feed on phytoplankton such as diatoms,
algae and fungi. Many terrestrial species feed on plant roots, penetrating
the cells and sucking out the contents. They are considered parasitic
in the way that fleas are parasitic on other animals, they can cause
great damage to the plants in this manner.
Species that live in sediments and other aquatic environments
ingesting particles of the substrate when they digest associated
bacteria and / or organic material.
Others feed more directly on dead organic material such as decomposing
plants and animals or dung where they may actually be eating the
bacteria or fungi that are feeding on the decomposing material
rather than being a decomposer directly.
The parasitic forms of nematodes show a great many variations
on the theme.
- Ectoparasite of plants - lives outside the plant
- Endoparasite of plants - lives and breeds inside
the plant having entered as a juvenile
- Saprophagous - juveniles and adults are free living,
juveniles enter invertebrate animal host and cause no harm,
but feed on dead tissues when the host dies.
- Zooparasitic juvenile stages only - juveniles
parasitize a host organism and then leave when it becomes
adult, the adult does not feed and may not have a
functioning gut. its job is reproduction only.
- Phytoparasitic juvenile, Zooparasitic adult - juvenile
lives as a plant parasite and is taken in by the animal host when it
feeds. The adult then parasitizes the adult and infects the
larvae of the host with juvenile worms which are then ready
to enter and parasitize the plant host again.
- Zooparasitic adult females only - young live in the
soil, males die after fertilizing females in the soil, pregnant
females then enter the animal host.
- ... and a whole range of other strategies with more than
one host to help the worms distribute themselves and complete
their life cycles
What eats nematodes?
Free living nematodes are themselves
prey for almost any predator that is larger than they are. This
will include other nematodes including those of the same species.
There is a surprising "predator"
of soil dwelling nematodes however and that is a fungus. This fungus
lays traps of rings attached to the fungal hyphae (thread-like growths).
Each ring is made of three highly specialized cells that swell up
rapidly when a nematode worm passes through them. The worm is captured
and the fungus passes hyphae into it to digest it.