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NEMATODA

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((Rudolphi, 1808) Lankester, 1877 )

Sections
  • Background
  • General Description
  • Development
  • Ecology
  • Zoogeography
  • Background

    Phylum Nematoda contains the most species after Phylum Arthropoda, currently with around 20,000 described species. They occur in nearly every habitat, free-living, or as parasites of a variety of plants and animals. Identification of nematodes to species is a specialist task and in general zooplankton analysis they are usually only identified to phyla.

    Further information: Warwick et al. 1998.

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    General Description

    Nematodes have a varied morphology, so it is difficult to give a universal description, but their general worm-like shape is very characteristic. One unifying characteristic that makes the phylum unique is the lack of cilia or flagella. They are bilaterally symmetrical, usually transparent, the body encased in a strong, flexible noncellular cuticle (Figure 1) that may be smooth or with transverse striations. Sense organs are present on the cuticle and on the head, typically long setae that can be numerous, or shorter papillae. The cuticle is secreted by and covers a layer of epidermal cells, under which are muscle cells that run in a longitudinal direction only, contractions of which effect locomotion. Because of high internal hydraulic pressure, contractions cause the body to flex rather than flatten, and the animal moves by thrashing back and forth. The buccal cavity, if present, is at the anterior end, but the anus opens sub-terminally at the posterior, so there is a tail of variable length and shape beyond the anus. The buccal cavity has a very variable form, reflecting different feeding methods and may be armed with teeth or other projections. Some nematodes have paired pigment spots, or true ocelli with lens-like structures, on or in the anterior oesophagus. About half-way along the oesophagus is a nerve ring. Between the gut and body wall is a fluid filled pseudocoelom in which the reproductive organs are situated.

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    Development

    Development of fertilized eggs is usually direct and most marine species have a simple life cycle - an egg, then 4 juvenile instars before they become adult males and females. A few species are viviparous, the eggs hatching in the uterus. Parasitic species have developed a wide range of variations on this basic theme. The variations depend on whether there is a secondary host and the amount of time spent in one or either hosts. There is also considerable variability in the way that they move from one host species to another. Many species lay eggs that pass out of the primary host with the faeces and are eaten by the secondary host, which in turn gets eaten by the primary host after the nematodes have developed. Because it is not always predictable that the secondary host will be eaten just as the nematode larvae have developed into the infective stage, many species have the ability to encyst themselves in the muscle or cuticle of their secondary hosts. Size: Adults mainly ~1-2 mm.

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    Ecology

    Nematodes are occasionally found in plankton samples and may be free-living species that have been carried up from the sea bed by turbulent mixing. However, they may be present because they have been dislodged from zooplanktonic organisms, many of which are an intermediate hosts to parasitic nematodes. When rearing herring larvae on wild plankton Rosenthal (1967) found that about 10% of the actively feeding larvae died due to parasitic infection, including nematode (Contracaecum sp.) infection, the parasites taken in by the larvae in their food. Medusae, copepods, amphipods, cephalopods, chaetognaths, fish etc. are all known to harbor immature nematodes (Hutton et al. 1962) and Anisakis sp., a herring and seal parasite, is commonly found in  euphausiids (Sluiters 1974). 

    Most nematodes are dioecious and fertilisation is by copulation. Females may have one or two ovaries and, depending on the number and arrangement, the reproductive pore may be mid-way along the body, or closer to the anus. Males usually have two testes and fertilization takes place when males, using special copulatory spines that lie in a sac opening into the reproductive pore, open the females' reproductive tract and inject sperm into them. The sperm are unique in that they lack flagellae and move by pseudopodia, like amoebas.

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    Zoogeography

    Particularly shallow water for free living species, but almost anywhere if parasitic.

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