Nematomorpha are a phylum of parasitoid animals superficially similar to nematode worms in
morphology, hence the name. They range in size in most species from 50 to 100 centimetres
long and can reach in extreme cases up to 2 metres, and 1 to 3 millimetres in diameter.
Horsehair worms can be discovered in damp areas such as watering troughs, swimming pools,
streams, puddles, and cisterns. The adult worms are free living, but the larvae are
parasitic on beetles, cockroaches, mantids, orthopterans, and crustaceans. About 351 freshwater
species are known and a conservative estimate suggests that there may be about 2000 freshwater
species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates
to the fact that nematomorpha often tie themselves in knots.
Description and biology Nematomorphs possess an external cuticle without
cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory,
respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior
end of the animal, and a ventral nerve cord running along the body.
Reproductively, they are dioecious, with the internal fertilization of eggs that are then
laid in gelatinous strings. Adults have cylindrical gonads, opening into the cloaca. The larvae
have rings of cuticular hooks and terminal stylets that are believed to be used to enter
the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients
directly through their skin. Development into the adult form takes weeks or months, and
the larva moults several times as it grows in size.
The adults are mostly free living in freshwater or marine environments, and males and females
aggregate into tight balls during mating.
In Spinochordodes tellinii and Paragordius tricuspidatus which have orthopterans as their
vector, the infection acts on the insect's brain and causes it to seek water and drown
itself, thus returning the nematomorph to water. P.tricuspidatus is also remarkably
able to survive the predation of their host, being able to wiggle out of the predator that
has eaten the host. Taxonomy
Nematomorphs can be confused with nematodes, particularly mermithid worms. Unlike nematomorphs,
mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just
before the end apart from having a thinner, smoother cuticle, without areoles and a paler
brown colour. The phylum is placed along with the Ecdysozoa
clade of moulting organisms that include the Arthropoda. Fossilized worms have been reported
from Early Cretaceous Burmese amber dated to 100–110 million years.
Relationships within the phylum are still somewhat unclear, but two classes are recognised.
The five marine species of nematomorph are contained in Nectonematoida. Adults are planktonic
and the larvae parasitise decapod crustaceans, especially crabs. They are characterized by
a double row of natotory setae along each side of the body, dorsal and ventral longitudinal
epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.
The approximately 320 remaining species are distributed into two families and seven genera
within order Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial
habitats and larvae parasitise insects, primarily orthopterans. Unlike nectonematiodeans, gordioideans
lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels
are filled with mesenchyme in young animals but become spacious in older individuals.
References
Bibliography Pechenik, Jan A.. "Four Phyla of Likely Nematode
Relatives". Biology of the Invertebrates. Singapore: Mc-Graw Hill Education. pp. 452–457.
ISBN 978-0-07-127041-0. Further reading
Baker GL, Capinera JL. "Nematodes and nematomorphs as control agents of grasshoppers and locusts".
Memoirs of the Entomological Society of Canada 171: 157–211. doi:10.4039/entm129171157-1.
Hanelt B, Thomas F, Schmidt-Rhaesa A. "Biology of the phylum Nematomorpha". Advances in Parasitology
59: 244–305. doi:10.1016/S0065-308X(05)59004-3. Poinar GO Jr. "Nematoda and Nematomorpha".
In Thorp JH, Covich AP. Ecology and Classification of North American Freshwater Invertebrates.
San Diego, CA: Academic Press. pp. 249–283. Thorne G. "The hairworm, Gordius robustus
Leidy, as a parasite of the Mormon cricket, Anabrus simplex Haldeman". Journal of the
Washington Academy of Science 30: 219–231. External links
Hairworm Biodiversity Survey. Capinera, J. L. Horsehair Worms, Hairworms,
Gordian Worms, Nematomorphs, Gordius spp.. University of Florida IFAS. Published 1999,
revised 2005. Nematomorph worm - Behavior modfication of
cricket by nematomorph worm. YouTube.