Roundworms, eelworms

Code II999999

Key terminates here

Nematodes are unsegmented, elongate worms. The body is round in cross-section, generally blunt or tapered anteriorly and tapered posteriorly, or else uniformly threadlike. Mouth terminal, usually surrounded by 6 or 3 lips. The gut is straight; anus situated before the 'tail'. Sexes are separate but males are rare or unknown in many free-living species. Most nematodes are microscopic but aquatic free-living species range from well under 1mm to about 12mm long. Many but not all nematodes move with a characteristic whip-like motion.

The phylum is ubiquitous. The higher taxonomy is unstable with a tendency for names to be applied on the basis of ecological guilds rather than phylogenetic relationships, which for the most part remain obscure. There is no reliable catalogue of species and the aquatic fauna is mostly unknown.

At this stage we cannot offer a key to the aquatic, macroscopic Nematoda. We comment that such a key should include taxa known from damp soil and other 'semi-aquatic' habitats as well as some plant parasitic species and species which alternate between parasitic and free-living phases or generations. Identification to ordinal level generally will require mounted specimens and a compound microscope with good optics, but electron microscopy may be necessary for small specimens.

Identification

Most nematodes can be identified to phylum on the basis of body form and structure. A few aberrant, highly setose or annulated species might be mistaken for small annelids (generally Oligochaeta) but the unusual bulbous forms which occur amongst plant parasite groups would not normally be found in aquatic samples.

Members of the phylum Nematomorpha (horsehair or gordian worms) might be mistaken for nematodes of order Merminthida. Both taxa are exclusively parasites of arthropods, are long and thread-like, and have an aquatic, free-living, non-feeding adult stage. Adult merminthids are relatively shorter than nematomorphs, usually under 200mm, and never more than about 500mm long, as against 200-1000 mm for Nematomorpha. Merminthida always possess remnants of mouthparts, and have the intestine developed into a fat-body. Nematomorpha take their food by absorption through the integument: the intestine is non-functional and the oesophagus solid even if vestigial mouthparts are present. Nervous systems also differ across the two groups: Merminthida have a typically nematodan nervous system with ventral, dorsal and lateral nerve cords; Nematomorpha have the somatic nervous system reduced to a ventral cord only.

Seventeen orders of Nematoda might reasonably be expected in aquatic samples. These orders are:

1

Narrow, anterior tapered bluntly, the posterior finely pointed. Mouth terminal surrounded by 6 lips. No obvious annulations but sometimes a light cuticular patterning. Body setation sparse. Head region with a doubled cuticle forming a distinct helmet or cap, or set off by a groove. Buccal cavity well-developed, with 0, 1 or 3 moveable teeth. Lips distinct and striated or low and inconspicuous. Pharynx cylindrical, without bulb-shaped thickenings. Five oesophagal glands (1 dorsal, 4 subventral), with the dorsal and anterior subventral glands opening into the buccal stoma and the posterior subventral glands into the pharynx anterior to the nerve ring. Amphids pouch-like, the openings transverse elliptical. Cephalic slits present in addition to amphids. Cephalic sensory organs in 2 whorls (6, 10) or 3 whorls (6, 6, 4). Male typically with paired testes and two spicules. Females with 1 or 2 ovaries.

2

As Enoplida but cephalic cuticle simple not duplicated. Buccal cavity large. One dorsal and two subventral teeth, sometimes also small denticles in transverse rows. Teeth massive in Oncholaimidae, oesophagal glands opening through the teeth. Left sublateral tooth largest in Eurystominidae.

3

As Enoplida but cuticle often strongly annulated; no helmet or cap; buccal cavity cylindrical or funnel-shaped and lightly sclerotised, with or without teeth.

4

Predatory nematodes. Cuticle smooth, body setation sparse. Lips somewhat expanded, angular, distinct. Amphids small and cup-like, located just posterior to lips. Cephalic sensory organs in 2 whorls (6, 10). Buccal cavity very large, strongly cuticularised, with a single massive dorsal tooth and often with large ventrolateral teeth. The teeth sometimes opposed by denticles in transverse or longitudinal rows. Pharynx cylindrical or conoid with a greatly thickened lumen. Five oesophagal glands all opening posterior to the nerve ring. Excretory pore degenerate. Males with supplements and two equal spicules. Females with 1-2 ovaries.

5

As Monochida except buccal cavity somewhat smaller and with 1 or more massive teeth.

6

Plant parasitic nematodes with a two-part, hollow axial spear or stylet in the buccal cavity. Lips separated from each other and bearing the cephalic sensory organs. Lip region marked off from the remainder of the body. Amphids placed well back at this junction.

7

As Dorymaimida, except the stoma vestigial and no axial spear.

8

Long, slender, smooth nematodes parasitic in insects, with a free-living adult stage. Length to 20 (rarely 30) cm. Head usually with two lateral and four submedian papillae, often with additional circumoral papillae. Lateral organs always present. Alimentary canal incomplete, the oesophagus reduced to a simple, narrow, cuticular tube and the intestine modified to a sausage-shaped fat body. Very similar to gordian worms (Phylum: Nematomorpha) but shorter and always with a smooth cuticle.

9

Ornamented nematodes with a simple body annulation without punctations. Amphids circular, spiral or shaped. Stoma funnel-shaped, followed by a tubular oesophagus generally ending in a terminal bulb.

10

As Araeolaimida except the lips amalgamated so that only three lips are evident. Stoma well developed and divided into a series of chambers, often with denticles in the final chamber.

11

Ornamented nematodes, sometimes annulated but always with punctations. Amphids transversely elongate, circular or a simple spiral. Cephalic sense organs at the extreme anterior in 1 or 2 whorls. Stoma cylindrical or funnel-shaped with a dorsal tooth, jaws, or extrusible rugae. Oesophagus with a cylindrical corpus and expanded, heavily sclerotised postcorpus.

12

As Chromodorida except the amphids tightly looped and multispiral, body almost always annulated, soma with three teeth, expanded postcorpus sometimes weak or absent.

13

Conspicuously ornamented nematodes with either thick concretion rings or else scales, warts and bristles. Amphids occupying much of the head region. Cephalic sense organs reduced in number and in 1-2 whorls. Oesophagus only slightly expanded posteriorly.

14

Ornamented nematodes with a cephalic helmet formed by duplication of the anterior cuticle. Prominent annulation but no punctations except in the tail region of some males. Some taxa have anterior and posterior adhesive tubes used in locomotion.

15

A poorly defined group. Ornamented or smooth-bodied nematodes with simple amphids; cephalic sensory organs in two whorls (6, 10) or else only one whorl of 4; often with sub-cephalic setae; the soma funnel shaped and lightly sclerotised or else spacious with heavy walls and large teeth; oesophagus generally cylindrical or with a slight posterior swelling. In females, the ovaries always are outstretched, not folded back on themselves.

16

Nematodes with a distinctive, three-segmented oesophagus which comprises an anterior muscular corpus, a lighter and narrower isthmus, and a posterior postcorporeal bulb containing a three-component muscular valve. Other characters of the order include: amphids pore-like, dorsolateral on the lips; cephalic sensory organs in two whorls (6, 10) on or very near the lips; usually a pair of sensilla at the level of the nerve ring and a second pair in the caudal region; cuticle often transversely striated and with longitudinal lateral ridges. Rhabditida are principally a terrestrial group.

17

Somewhat like Rhabditida except ornamented with annulations, longitudinal striations, and occasionally punctations. Stoma usually with a spear, oesophagus proportioned as in Rhabditida but the glandular postcorpus without a muscular valve.