The "roundworms" or "nematodes" (
phylum Nematoda) are the
most diverse phylum of
pseudocoelomates,
and one of the most diverse of all
animals.
Nematode
species are very difficult to
distinguish; over 80,000 have been described, of which over 15,000
are
parasitic. It has been estimated that
the total number of described and undescribed roundworms might be
more than 500,000. Unlike
cnidarians or
flatworms, roundworms have a
digestive system that is like a tube at
both ends.
Habitats
Nematodes have successfully adapted to nearly every
ecological niche from marine to fresh
water, from the polar regions to the tropics, as well as the
highest to the lowest of elevations.
They are ubiquitous in
freshwater, marine, and terrestrial environments, where they often
outnumber other animals in both individual and species counts, and
are found in the locations as diverse as Antarctica
and oceanic
trenches. They represent, for example, 90% of all life
on the seafloor of the Earth. Their many parasitic forms include
pathogens in most plants and animals
(including
humans.) Some nematodes can
undergo
cryptobiosis.
Taxonomy and systematics
Eophasma jurasicum, an extinct
nematode
The group was originally defined by
Karl
Rudolphi in 1808 under the name
Nematoidea, from
Ancient Greek νῆμα (
nêma, nêmatos,
'thread') and -eiδἠς (
-eidēs, 'like'). The vernacular word
"nematode" is a corruption of this taxon, reclassified as
family Nematodes
by Burmeister in 1837 and ordo
Nematoda
by
K. M.
Diesing in 1861.
At the origin, the taxon
Nematoidea applied to both
roundworms and horsehair worms. Along with
Acanthocephala,
Trematoda and
Cestoidea, it formed the group
Entozoa. The first differentiation of roundworms from
horsehair worms, though erroneous,
is due to von Siebold (1843) with orders
Nematoidea and
Gordiacei (
Gordiacea). They were classed along
with
Acanthocephala in the new phylum
Nemathelminthes (today obsolete) by
Gegenbaur (1859). Then the taxon
Nematoidea has been
promoted to the rank of phylum by
Ray
Lankester (1877) including the family
Gordiidae
(horsehair worms). In 1919,
Nathan Cobb
proposed that roundworms should be recognized alone as a phylum. He
argued that they should be called
nema(s) in
English rather than "nematodes" and defined the taxon
Nemates (Latin plural of
nema). For ITIS,
the taxon
Nematoda is invalid.Since Cobb was the first to
exclude all but nematodes from the group, the valid taxon should be
Nemates Cobb 1919 or
Nemata Cobb 1919.
Phylogeny
The relationships of the nematodes and their close relatives among
the
protostomian Metazoa are unresolved. Traditionally, they were
held to be a lineage of their own, but in the 1990s it was proposed
that they form a
clade together with
moulting animals such as
arthropods. This group has been named
Ecdysozoa. However, the
monophyly of the Ecdysozoa was never unequivocally
accepted: while most researchers consider at least the placement of
arthropods as more distant relatives of
annelids — with which they were formerly united — to
be warranted, the presumed close relationships of the nematodes and
relatives with the arthropods has been a major point of
contention.
Even though the amount of data since accumulated in regard to this
problem is staggering, the situation seems if anything less clear
these days.
DNA sequence data,
initially strongly supporting the Ecdysozoa hypothesis, has become
rather equivocal on ecdysozoan monophyly, and is simply unable to
refute either a close or a more
distant relationship between the arthropod and nematode lineages.
That the roundworms have a large number of peculiar
apomorphies and in many cases a parasitic lifestyle
confounds morphological analyses. Genetic analyses of roundworms
suggest that — as is also indicated by their unique morphological
features — the group has been under intense
selective pressure during its early
radiation, resulting apparently in accelerated rates of both
morphological and molecular
evolution.
Furthermore, no distinctive apomorphies of Ecdysozoa are known;
even moulting has recently been confirmed to occur outside the
presumed clade.
Conversely, the identity of the closest living relatives of the
Nematoda has always been considered to be well resolved.
Morphological characters and molecular phylogenies agree with
placement of the roundworms as
sister
taxon to the parasitic
horsehair
worms (Nematomorpha); together they make up the
Nematoida. Together with the
Scalidophora (formerly Cephalorhyncha), the
Nematoida form the
Introverta. It is
entirely unclear whether the Introverta are, in turn, the closest
living relatives of the enigmatic
Gastrotricha; if so, they are considered a
clade
Cycloneuralia, but there is much
disagreement both between and among the available morphological and
molecular data. The Cycloneuralia or the Introverta — depending on
the validity of the former — are often ranked as a
superphylum.
Nematode systematics
Due to the lack of knowledge regarding many nematodes, their
systematics is contentious. Traditionally, they are divided into
two
class, the
Adenophorea and the
Secernentea, and initial
DNA sequence studies suggested the existence of
five
clades:
As it seems, the
Secernentea are indeed
a natural group of closest relatives. But the "Adenophorea" appear
to be a
paraphyletic assemblage of
roundworms simply retaining a good number of
ancestral traits. The
Enoplia do not seem to be monophyletic either but to
contain two distinct lineages. The old group "
Chromadoria" seem to be another paraphyletic
assemblage, with the
Monhysterida
representing a very ancient minor group of nematodes. Among the
Secernentea, the
Diplogasteria may
need to be united with the
Rhabditia.
while the Tylenchia might be paraphyletic with the Rhabditia.
The understanding of roundworm systematics and phylogeny as of 2002
is summarised below:
Phylum Nematoda
Reproduction
Reproduction is usually sexual. Males are usually smaller than
females (often much smaller) and often have a characteristically
bent tail for holding the female for
copulation. During copulation, one or more
chitinized spicules
move out of the cloaca and are inserted into genital pore of the
female.
Amoeboid sperm
crawl along the spicule into the female worm. Nematode sperm is
thought to be the only
eukaryotic
cell without the globular protein
G-actin.
Eggs may be embryonated or unembryonated when passed by the female,
meaning that their fertilized eggs may not yet be developed. In
free-living roundworms, the eggs hatch into larva, which eventually
grow into adults; in parasitic roundworms, the life cycle is often
much more complicated.
Nematodes as a whole possess a wide range of modes of reproduction.
Some nematodes, specifically
Heterorhabditis spp., undergo a process
called
endotokia
matricida:
intrauterine birth
causing maternal death. Some nematodes, like other animals (for
example segmented worms), are hermaphroditic. The hermaphroditic
nematode keeps its self-fertilized eggs inside its uterus until
they hatch. The juvenile nematodes will then ingest the parent
nematode. This process is significantly promoted in environments
with a low or reducing food supply. The nematode model species
Caenorhabditis
elegans and
C.
briggsae exhibit
androdioecy,
which is very rare among animals. The single genus
Meloidogyne (root-knot nematodes) exhibit a
range of reproductive modes including sexuality (
amphimixis),
facultative sexuality, meiotic
parthenogenesis (
automixis) and mitotic
parthenogenesis (
apomixis).
Free-living species
In free-living species, development usually consists of four molts
of the cuticle during growth. Different species feed on materials
as varied as algae, fungi, small animals, fecal matter, dead
organisms and living tissues. Free-living marine nematodes are
important and abundant members of the
meiobenthos. They play an important role in the
decomposition process, aid in recycling of nutrients in marine
environments and are sensitive to changes in the environment caused
by pollution. One roundworm of note is
Caenorhabditis elegans, which
lives in the soil and has found much use as a
model organism.
C. elegans has had
its entire genome sequenced, as well as the developmental fate of
every cell determined, and every neuron mapped.
Parasitic species
Nematodes commonly parasitic on humans include
ascarids (
Ascaris),
filarids,
hookworms,
pinworms (
Enterobius) and
whipworms (
Trichuris trichiura). The
species
Trichinella spiralis, commonly known as the
trichina worm, occurs in rats, pigs,
and humans, and is responsible for the disease
trichinosis.
Baylisascaris usually infests wild
animals but can be deadly to humans as well.
Dirofilaria immitus are
Heartworms known for causing
Heartworm disease by inhabiting the hearts,
arteries, and lungs of dogs and some cats.
Haemonchus contortus is one of the
most abundant infectious agents in sheep around the world, causing
great economic damage to sheep farms. In contrast,
entomopathogenic nematodes
parasitize insects and are considered by humans to be
beneficial.
One form of nematode is entirely dependent upon
fig wasps, which are the sole source of
fig fertilization. They prey upon the wasps, riding them
from the ripe fig of the wasp's birth to the fig flower of its
death, where they kill the wasp, and their offspring await the
birth of the next generation of wasps as the fig ripens.
Plant parasitic nematodes include several groups causing severe
crop losses. The most common genera are
Aphelenchoides (
foliar nematodes),
Ditylenchus,
Globodera (potato cyst nematodes),
Heterodera (soybean cyst
nematodes),
Longidorus,
Meloidogyne (root-knot
nematodes),
Nacobbus,
Pratylenchus (lesion nematodes),
Trichodorus and
Xiphinema (dagger nematodes). Several
phytoparasitic nematode species cause histological damages to
roots, including the formation of visible galls (e.g. by root-knot
nematodes), which are useful characters for their diagnostic in the
field. Some nematode species transmit plant viruses through their
feeding activity on roots. One of them is
Xiphinema index, vector of GFLV
(
Grapevine Fanleaf Virus),
an important disease of grapes.
Other nematodes attack bark and forest trees. The most important
representative of this group is
Bursaphelenchus xylophilus,
the pine wood nematode, present in Asia and America and recently
discovered in Europe.
Nematodes in agriculture
Depending on the species, a nematode may be beneficial or
detrimental to plant health.
From an agricultural perspective, there are two categories of
nematode: predatory ones, which will kill garden pests like
cutworms, and pest nematodes, like the
root-knot nematode, which attack
plants and those that act as
vectors spreading
plant viruses between crop plants.
Predatory nematodes can be bred by soaking a specific recipe of
leaves and other
detritus in water, in a
dark, cool place, and can even be purchased as an
organic form of
pest control.
Rotations of plants with nematode resistant species or varieties is
one means of managing parasitic nematode infestations. For example,
marigolds, grown over one or more
seasons (the effective is cumulative), can be used to control
nematodes. Another is treatment with natural antagonists such as
the fungus
gliocladium
roseum.
Chitosan is a natural
biocontrol that elicits plant defense responses to destroy
parasitic cyst nematodes on roots of sobyean, corn, sugar beets,
potatoes and tomatoes without harming beneficial nematodes in the
soil.. Furthermore
soil
steaming is an efficient method to kill nematodes before
planting crop.
CSIRO has found
[720894] that there was 13- to 14-fold reduction of
nematode population densities in plots having Indian mustard
(
Brassica juncea) green manure or
seed meal in the soil.
Hundreds of
Caenorhabditis
elegans were featured in a research project on NASA's
STS-107 space mission (which ended in the
Space Shuttle Columbia
Disaster).
Epidemiology
[[Image:Intestinal nematode infections world map - DALY -
WHO2002.svg|thumb|
Disability-adjusted life year
for intestinal nematode infections per 100,000 inhabitants in
2002.
]]A number of intestinal nematodes affect human beings. These
include
ascariasis,
trichuriasis and
hookworm disease.
See also
Footnotes
References
- (1973): The Respiratory Physiology of the Marine Nematodes
Enoplus brevis (Bastian) and E. communis
(Bastian): I. The Influence of Oxygen Tension and Body Size.
J.
Exp.
Biol.
59(1): 255–266. PDF fulltext
- (2003): Worms survived Columbia disaster. Version of
2003-MAY-01. Retrieved 2008-NOV-04.
- (1982): The Masterpiece of Nature: The Evolution and
Genetics of Sexuality. University of California Press.
- (1998): A molecular evolutionary framework for the phylum
Nematoda. Nature 392: 71–75.
- (2007): Deep-sea species' loss could lead to oceans'
collapse, study suggests. Version of 2007-DEC-27. Retrieved
2008-NOV-04.
- (1951): "Giant nematoda from the placenta of Cetacea;
Placentonema gigantissima nov. gen., nov. sp.".
Proc.
USSR
Acad. Sci.
77(6): 1123–1125 [in Russian].
- (1999): Endotokia matricida in hermaphrodites of
Heterorhabditis spp. and the effect of the food supply.
Nematology 1(7–8): 717–726. (HTML
abstract)
- (2006): Giant Kidney Worm Infection in Mink and Dogs.
Retrieved 2007-FEB-10.
- (2002a): Bilateria. Version of 2002-JAN-01. Retrieved
2008-NOV-02.
- (2002b): Nematoda. Version of 2002-JAN-01. Retrieved
2008-NOV-02.
- (1976)): The Structure of the Ventral Nerve Cord of
Caenorhabditis elegans.
Phil.
Trans.
Roy.
Soc.
B 275(938): 327–348. PDF fulltext
External links
on the
UF
/ IFAS Featured
Creatures Web site
Wikis
Quiz
Facts
Map
Search
