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Nematode Hyperdiversity

Nematode Hyperdiversity

Nematodes (phylum Nematoda), are one relatively unknown component of a group of animals called the meiofauna.

Nematodes are both widespread and abundant, free-living and parasitic.

They are also a very diverse group, with around 25,000 described species representing a small fraction of the total estimated numbers of species.

• Why are nematodes sodiverse?

Although nematodes have a simple body plan, their nutritional and morphological diversity, coupled with their generally small size and diverse life histories, are all factors leading to species radiations and diversity in nematodes.

Ascaris lumbricoides

Ascaris lumbricoides is anintestinal human parasite which can grow up to 50 cm in length.

A typicalinfection is 30 - 50 worms per host, and a female can produce 200,000eggs per day.

• Whatimpact does this parasite have?
  

Filarial Worms

The tropical nematodes Wuchereria bancrofti and Brugia malayi live as adults in the human lymph system and producelarvae which circulate in thebloodstream.

The presence of these worms in the lymph vessels causes severeinflammation.

Pinworm

The pinworm, Enterobius vermicularis, is a very common intestinal parasite which can reach very high infection rates, especially in young children.

The condition can be almost symptom-less until the time comes for reinfection, when adult female pinworms emerge from the anus and lay their eggs all around the peritoneal area, causingintense itching.

The eggs also become scattered on bedclothes and clothing and can even survive in dust where they can infect by being breathed.

Guinea Worm

The guinea worm, Dracunculus medinensis, lives under the skin and ramifies through the joints of its host.

This images shows male and female of Dracunculus medinensis; the male is 2 cm long and the female 30 cm long (the scale is in centimetres).

Since ancient times, a guinea worm infection has been treated by making a small incision in the skin near one end of the worm and slowly extracting it by winding it onto a stick.

The symbol for the medical profession is probably derived from the image of a guinea worm being wound onto a stick (see: www.st-mike.org/medicine/caduces.html).

It is also possible that theOld Testament "fiery serpents", which descended on the Israelites, were actually guineaworms.

Root-knot Nematodes

Many plants suffer from root-knot nematode parasites, which cause tremendous damage to commercial crops.

For this reason they are extensively studied and a great deal is known of their life histories, distribution and abundance.

Nematode Control of Slugs

Nemaslug is a naturally occurring parasitic nematode of slugs, supplied commercially as a means of biological slug control.

The nematode, Phasmarhabditis hermaphrodita, was first described in 1859, having been found in decaying slugs, but it was not at first thought to be parasitic.

It is now known that specialised larvae can infect slugs by entering near the slug's mantle, where they develop into hermaphroditic adults and reproduce. They cause swelling as they multiply, eventually killing the slug [Wilson et al. 1993] .

A Model Nematode

Caenorhabitis elegans is an opportunistic nematode species which naturally colonises animal dung.

It was the first metazoan to have its entire genome mapped, and is a goodmodel for studying genome function, cell development and ageing (see: http://www.ageing.org/news/story5.html), as well as many aspects of nematode biology.

Vinegar eelworm

The vinegar eelworm, Panagrellus redivivus, is a nematode that can tolerate low pH, and lives in vinegar and has also been found in damp and soggy beer mats.

Vinegar eelworms are also easy to culture and reproduce rapidly, and so are widely used as a live fish food ("Tubifex") by aquarium keepers.

Taxonomists and Taxon Diversity

Given the status of nematodes as one of the main hyperdiverse taxa and the economic importance ofparasitic nematodes, lack of knowledge on nematode systematics might be considered surprising.

One explanation lies in the relativelysmall number of taxonomists that work on nematodes, compared to insects. Another reason lies in theirmicroscopic size.

[Gaston & May 1992] show that their is a striking mismatch between the known distribution of biological diversity, and both the geographical location of taxonomists and the nature of the taxa they study.

Meiofauna

The term "meiofauna" is derived from the Greek word "meion", meaning smaller, and was first coined by [Mare 1942] to describe those animals passing through a 1 mm sieve, but being retained on a 63 um sieve.

Thus the meiofauna lie between the microfauna and macrofauna in size range.

The ecological importance of meiofauna in benthic ecosystems was reported by [Gerlach 1971] .

• What animal phyla are usually considered to be part of the meiofauna?

Nematode habitats

Nematode Distribution

Nematodes can be found in practically allecosystems, living ininterstitial spaces, and it can be difficult not to find them.

As well as free-living forms, they can beparasites of plants and animals.

Soil core showing soil grains

Soil

In soils, nematodes often live in the spaces between the individual grains of soil often associated with plant roots.

Sand

Sand

In sandy sediments nematodes often live in the spaces between the individual grains of sand - called interstices.

Aquatic habitats

Nematodes live in the sediments in aquatic habitats, such as rivers and lakes.

Antarctica

Nematodes have been found living in snow and ice in association with algae which can also grow in this habitat.

Mud

Mud flats, as in these from the Somme estuary, France, are a surprisingly diverse and important habitat for free-living nematodes.

• What other habitat for nematodes is represented by this image?

Nematode Abundance

• Howabundant are nematodes?

The record figure is 22,860,000 m^-2 on amudflat [Warwick & Price 1979] .

• How many nematodes may there be on earth?

Assume that, on average, there are 1 * 10 ⁶ nematodes m^-2, and that the total surface area of the planet is 5.1 * 10¹⁴ m².

This gives a figure of 5.1 * 10²⁰ nematodes over the entire surface of the earth, which is a verybig number and biodiversity, as a science, is full of very big numbers. The importance of this, or a similar estimate, is what the number means.

On earth, if all the metazoan animals could be counted, roughly 80 % might be nematode worms!

Nematode Species Numbers

There are 25,000 nematode species currently described [Hawksworth & Kalin-Arroyo 1995] , and some estimates put this number at well below 1 % of the actual total.

• What are the implications of parasitic nematodes on such estimates?

Many parasitic nematodes are species specific. If only a relatively small percentage of all other animal and plant species on the planet have a nematode parasite, then the number of nematode species would be very large. This does not consider the fact that the free-living nematodes are more abundant and speciose than the parasitic ones.

Nematode Body Plan

The basic body plan of a nematode is a relatively simple, thread-like structure. Nematodes are usually eutelic animals.

They have two "tubes" - the epidermis and the gut - with reproductive organs in between and a hydrostatic skeleton.

Nutritional Diversity in Nematodes

Nematodes live in a habitat matrix, which they move through to find the widerange of food types theyeat.

• How might diversity of mouth parts and food items lead to speciation in nematodes?
  

Type 1a Bacteriovore

Type 1a nematodes have minute buccal cavities, which can be very selective allowing only food particles to be ingested.

For some species, this type of buccal cavity is used as a precision instrument, whereas for others, it is used as a single-aperture sieve.

Type 1b Detritivore

Type 1bnematodes generally have an open, cup or flask-shaped buccal cavity which lacks any teeth. They are generally non-selectivedeposit feeders.

However, some species have been seen to swallow diatoms whole, and one species, according to [Warwick 1981] , was capable of a form of"gardening".

Type 2a Epigrowth Feeders

Type 2anematodes have small, fixed teeth, which can be used to scrape bacteria and algae from the surfaces of sand grains.

Some type 2a nematode use the teeth to crack or prise apart the valves ofdiatom shells, allowing the cell contents to be ingested.

Some nematodes have a spear in the buccal cavity which can be used to puncture cells and suck out their contents.

Type 2b Predators

Type 2b nematodes are either true predators or omnivores, with large fixed teeth or mandibles, and various feeding mechanisms.

Nematodes from the genus Mononchus feed on other nematodes by performing a "kiss of death".

In nematodes of the genus Enoploides each of three mandibles has a double hook, giving it six grasping teeth and making them formidable predators.

Morphological Diversity in Nematodes

Nematodes show a great diversity of shape, and have a diverse range of cuticular ornamentation, hairs and bristles.

• How might this diversity drive nematode speciation?

Nematode Size

Most free-living forms are essentially microscopic, being one or two millimetres long.

• Why might the small size of nematodes lead to great species diversity?

Some parasitic nematodes can be large, and the largest known nematode is the parasitic Placentonema gigantissima, which can reach eight metres in length and be as thick as a finger.

• What animal might it be a parasite of, and what conservation issues does it pose?
  

Nematode Reproduction and Life Histories

Nematodes have two sexes. They show direct development, may be viviparous, have no active dispersal phase and a generally short lifespan.

Complex copulation and intromission structures provide a very specific "lock and key" mechanism that keeps mating to the same species, and can drive speciation.