Phylum Nematoda

• "Thread Worms", commonly called roundworms
• 20,000 described species
• Now recognized as 2nd largest phylum:
  • Conservative estimates 500,000 to 1 million spp.
  • Perhaps as diverse as arthropods (10-20 million)
• Free-living species are abundant as decomposers in soil
• Parasitic forms widespread, economically important
• Occupy wide range of habitats (as with arthropods)
  • Beer-soaked mats in pubs
  • Permafrost soils in Arctic and Antarctic tundra
  • Soil abundance is so great that fungi "feed" on them

Nematode Characteristics

1. Body is thin, elongated, cylindrical, both ends tapered

2. Mostly small (few mm), parasites can be large (pseudocoel more pronounced)

3. 3-6 labia surround mouth and bear sensillae

4. Collagen cuticle, bears 4 sensory ridges internally

5. Brain is circular around pharynx (Cycloneuralia)

6. Buccal cavity may have styles or teeth

7. Long muscular pharynx, triangular intestinal lumen

8. Usually dioecious, parasites may be hermaphroditic

Nematode Development and Cryptobiosis

• Development
  • Protostomes - blastopore forms mouth, but cleavage irregular
  • Strongly determinate development
  • Fixed number of cells - eutely
  • Caenorhabditis has 1031 after development
  • Programmed cell death - Caenorhabditis 671 in embryo, 113 die before hatching, 419 are added, 18 more post-hatch
• Cryptobiosis
  • Ability to withstand extremely dry, cold conditions
  • Well-developed in nematodes, rotifers, tardigrades

Nematode Lifestyles

• Ecto- and endoparasites of plants
• Mixed free-living and animal parasites, or alternate between plant and animal hosts
• One-host animal (Ascaris, Trichinella)
• Two-host animals (Wuchereria)
• Free-living
  • Soil productivity depends on roundworms
  • Food web in Antarctic soil dominated by nematodes

Loads of 'todes

Benthic marine substrate: 4.4 million / m2

1 gram soil: 200 species nematodes

"If all the matter in the universe except the nematodes were swept away, our world would be still be dimly recognizable, and if, as disembodied spirits, we would investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin layer of nematodes..."

- N. A. Cobb, nematologist

Phylum Tardigrada

• "slow walkers" or water bears
• < 1 mm in size
• 600 species
• mostly freshwater, temporary ponds, and moist terrestrial surfaces (mosses)
• new molecular evidence places this phylum next to the arthropods

Characteristics of Tardigrades

1. Cylindrical, segmented body

2. Four pairs of ventral stubby legs

3. Cuticle contains chitin

4. Body is a pseudocoel

5. Brain and ventral nerve ganglia

6. Digestive - mouth, pharynx, intestine

7. Malphigian glands (excretory)

8. Dioecious, reproductive tract common opening with anus

Evolutionary Implications

• Aschelminths have several features in common (pseudocoel, eutely, cleavage)
• Now all (rotifers, nematodes, tardigrades) are clearly aligned with protostomes
• However, aschelminth phyla that molt (Nematoda, Tardigrada) appear to be more closely related to arthropods (Ecdysozoa - includes 8 phya) than are Rotifera and Acanthocephala
• Moreover, a clade occurs within Ecdysozoa - the Cycloneuralia - which contains five phyla (contains nematodes but excludes arthropods, onychophorans, and tardigrades)