University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications from the Harold W. Manter Laboratory of Parasitology Parasitology, Harold W. Manter Laboratory of 2001 Worms, Nematoda Scott Lyell Gardner University of Nebraska - Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/parasitologyfacpubs Part of the Parasitology Commons Gardner, Scott Lyell, "Worms, Nematoda" (2001). Faculty Publications from the Harold W. Manter Laboratory of Parasitology. 78. https://digitalcommons.unl.edu/parasitologyfacpubs/78 This Article is brought to you for free and open access by the Parasitology, Harold W. Manter Laboratory of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from the Harold W. Manter Laboratory of Parasitology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Published in Encyclopedia of Biodiversity, Volume 5 (2001): 843-862. Copyright 2001, Academic Press. Used by permission. Worms, Nematoda Scott L. Gardner University of Nebraska, Lincoln I. What Is a Nematode? Diversity in Morphology pods (see epidermis), and various other inverte- II. The Ubiquitous Nature of Nematodes brates. III. Diversity of Habitats and Distribution stichosome A longitudinal series of cells (sticho- IV. How Do Nematodes Affect the Biosphere? cytes) that form the anterior esophageal glands Tri- V. How Many Species of Nemata? churis. VI. Molecular Diversity in the Nemata VII. Relationships to Other Animal Groups stoma The buccal cavity, just posterior to the oval VIII. Future Knowledge of Nematodes opening or mouth; usually includes the anterior end of the esophagus (pharynx). GLOSSARY pseudocoelom A body cavity not lined with a me- anhydrobiosis A state of dormancy in various in- sodermal epithelium. vertebrates due to low humidity or desiccation. spicule Blade-like, sclerotized male cop- cuticle The noncellular external layer of the body ulatory organ, usually paired, lo- wall of various invertebrates. cated immediately dorsal to the cloaca. gubernaculum A sclerotized trough-shaped struc- synlophe In numerous Trichostrongylidae, ture of the dorsal wall of the spicular pouch, near an enlarged longitudinal or oblique cuticu- the distal portion of the spicules; functions in guid- lar ridge on the body surface that serves to ing the spicules. hold the nematodes in place on the gut wall. hypodermis The cellular, subcuticular layer that vermiform Worm-shaped with tapering form both secretes the cuticle of annelids, nematodes, arthro- posteriorly and anteriorly. NEMATODES are the most speciose phylum of complex setae; and parasitic species manifest amaz- metazoa on earth. Not only do they occur in huge ingly great reproductive potential and large body numbers as parasites of all known animal groups, size. Nematodes are one of the major synanthropic but also they are found in the soils, as parasites of metazoans, with some species such as pinworms plants, and in large numbers in the most extreme having coevolved with humans and their relatives environments, from the Antarctic dry valleys to the since the beginning of the lineage of the primates. benthos of the ocean. They are extremely variable in While estimates of the numbers of known species their morphological characteristics, with each group hover around 20,000 actual numbers of taxono- showing morphological adapta tions to the environ- mists/systematists with expertise in this group are ment that they inhabit. Soil-dwelling forms are ex- decreasing yearly. This is despite the fact that the tremely small; many marine species have long and Nemata are probably the last great group of Meta- 843 844 G ARDNER IN E NCYLCOP E DIA OF BIODIV E RSITY , VOL . 5 (2001) zoa to be well-documented and described. Estimates lined with mesodermally derived cells and they are of the actual number of species of nematodes that re- triploblastic. Possessing no circular body muscles, main to be described include several thousand from movement is accomplished by contraction and relax- insects and millipedes, several thousand from verte- ation of longitudinal muscles in apposition to a hy- brates, and perhaps millions from marine habitats. drostatic skeleton. The body wall is flexible and very strong. All nematodes maintain their form because I. WHAT IS A NEMATODE? DIVERSITY their body fluids (in the hydrocoel) are under a pos- IN MORPHOLOGY itive pressure relative to their environment, analo- A. General Characteristics and Synapomorphies gous to the way a water balloon maintains its shape. The cross-section in Figure 8 shows Vexillata and the Despite numerous assertions that appear in the lit- round shape of the body under the cuticle that sup- erature stating that nematodes are morphologically ports spines or aretes (the whole struc ture is called conservative, the contrary is actually true: species of the synlophe). Nematodes have a complete digestive the phylum Nemata are extremely variable in their system with an anterior stoma just behind the mouth morphological characteristics. Because of this di- and usually a triradiate or triple muscle pumping versity, almost any broad statement regarding their type of esophagus (Figures 1a, and 1c) that can be anatomy probably should be tempered or qualified. muscular or glandular in structure (or both). The in- Nevertheless, nematodes are nonsegmented worms testine, tubular in form (Figure 1c), is usually a sin- that generally lack external appendages. Most are gle cell in thickness lined on the peritoneal side with vermiform, with tapering anterior (Figure 1a) and a thin collagen-like material, internally lined with posterior (Figure 1b) ends, cylindrical in cross-sec- microvilli (Grassé, 1965; Maggenti, 1981, 1991a). The tion, and covered with a usually translucent, flexi- tube extends from the esophagus straight to the anus ble, acellular cuticle (Figure 1c) secreted by an un- or cloaca, sometimes showing out pouched cecae or derlying cellular hypodermis. The cuticle may be diverticulae near the esophageal end. smooth (Figure 2) or ornamented with rings (Fig- Most nematodes are sexually dimorphic with sep- ures 3a-3c), longitudinal striations, spines (Figure arate sexes (diecious or amphigonus) and are ovipa- 3b), or spikes, or it may have well-developed wing- rous; however, some are ovoviparous and some are like structures called lateral alae that are very com- viviparous. Males usually have a cuticularized spic- mon on the externo-lateral surfaces (Figure 4). Some ule or pair of spicules that are used to assist in the marine species have long setae (genus Draconema) transfer of sperm to the females. Many species also (Figures 5a, 5b, and 6), modified sensory papillae have a gubernaculum (seen extended from the clo- that are probably used in movement and in detect- aca of a species of an aspidoderid in Figure 23) that ing their environment. In contrast to all other nem- guides the spicule during copulation and spicular atodes, some marine species have eye spots that en- eversion. Some are hermaphrodytic; in these cases able them to detect light in their environment (e.g., the nematode produces both sperm and ova from genus Draconema) (Figure 7). the ovotestis of the same individual at different times The fine structure of the external body-cuticle is during ontogeny (Maggenti, 1991a; Malakhov, 1994). complex, acting to protect the animal from the ex- A synapomorphy of the Nemata is the presence ternal environment and allowing it to remain ho- of noncontractile axon-like myoneural processes meostatic inside. The cuticle can be extremely resis- or exten sions that run from the contractile or body tant, and de pending on the nematode species and portion of muscle cells to the neural junctions of the its life-history attributes. The cuticle of the nema- nerve cords. The width of nematodes is usually less tode may be able to resist digestion in the most in- than 2 mm, even when extremely long. However, hospitable stomachs in the vertebrate world. On the even here there are exceptions, with the giant kid- other hand, nematodes may be extremely delicate, ney worm, Dioctophyme renale, of canids attaining a able to exist intact only within the osmotically bal- size of 15 mm x 1,000 mm. The eggs of nematodes anced tissues of other animals (e.g., members of the are also very similar in size, with most ranging from order Filaroidea), and if moved from the isosmotic 50 to 100 μm long by 20 to 50 μm wide, with an ex- solution to one with fewer salts, they may explode ception being the antarctic marine nematode Deon- and die in a most amazing display. tostoma timmerchioi (40 mm in length of body) that Nematodes are called “pseudocoelomates” be- has eggs that range from 870 to 1,100 μm long by cause in most forms, their coelom is not completely 240 to 350 μm wide (huge by nematode standards). W ORMS , NEMATODA 845 Figure 1 (a) Anterior end of Didelphoxyuris, the pinworm of South American marsupials, showing the small anterior stoma at the narrow end, followed by the well-developed muscular esophagus with a large posterior bulb. The esophagus is tripartite and serves as a muscular pumping organ, pumping food into the intestine. (b) Whole drawing of a predatory nematode of the genus Mononchus showing the general structures of a nematode including the tapering anterior and posterior ends and the ventral vulval opening just posterior to midbody. The stoma is heavily cuticularized and bears a spike or spine that is used to puncture the cuti- cle of the prey (modified from Cobb, 1914). (c) Confocal microscopic image of the esophageal bulb and posterior part of the esopha- gus of a species of Pratylenchus. The cuticle can be seen as the outer light outline of the nematode with an underlying brighter layer showing the cellular hypodermis. The bulb-shaped posterior part of the esophagus attaches via a valve to the single cell thick-tu- bular-intestine, shown here as an optical section just posterior to the esophageal bulb.