Table 2.

Generalized morphological, biological and physiological aspects of free-living nematodes in sediments and soils (Van de Velde and Coomans 1987, Bird and Bird 1991, Turpenniemi and Hyvarinen 1996, Traunspurger 2000, Eyualem-Abebe et al. 2008).

	Marine nematodes	Freshwater nematodes	Soil nematodes
Morphology	Caudal adhesive glands present in most species with primary secretory function, to aid attachments of nematodes and eggs to substratum and agglutination of sediment particles in burrows	Unicellular caudal glands (up to 3) producing secretions that facilitate attachment to substratum	Epidermal and caudal adhesive glands generally absent
Reproduction	Mostly amphimictic (i.e., reproduction in which sperm and eggs come from separate individuals), fertilization by copulation	Mostly amphimictic, fertilization by copulation; occasionally parthenogenetic (i.e., asexual reproduction without fertilization); hermaphroditism is rare	Mostly amphimictic, fertilization by copulation, meiotic and mitotic parthenogenesis as well as self-fertilizing hermaphroditism are fairly common
Feeding	Feed on diverse food sources, including bacteria, microalgae, protozoans, small metazoans (including other nematodes); soil nematodes may also feed on fungi and higher and lower plants (including algae)
Chemosensory organs	Amphid (complex sense organ in the head region that is exposed to the external environment by a pore in the nematode cuticle) functions as primary chemoreceptor; amphids vary in shape and size		“Adenophorea”: Amphid similar to marine and freshwater nematodes but often smaller; phasmid absent “Secernentea”: Amphid; phasmid (complex sense organ in the tail region), similar to amphid but smaller
Secretory–excretory (S–E) system	Relatively simple S–E system consisting of a single ventral gland cell, a renette, usually with a noncuticularized terminal duct; S–E system mainly involved in secretion of glycoproteins that coat the cuticle surface and act as a lubricant to assist movement		“Adenophorea”: Relatively simple S–E system similar to marine and freshwater nematodes “Secernentea”: More complex, tubular H-shaped S–E system with a cuticle-lined duct; S–E system mainly involved in osmotic regulation as well as secretion of glycoproteins
Osmoregulation	Usually isosmotic to seawater; high cuticular permeability for water; species-specific differences in the efficiency and rate of osmoregulation	Slightly hyperosmotic to surroundings; high cuticular permeability for water	Hyperosmotic to surroundings; low cuticular permeability for water, slowing down the rate of water flux allows time for osmoregulatory mechanisms to operate

Note: Given that phenomena such as convergent evolution, and secondary loss and gain are widespread among nematodes, attributes should not be taken as fixed characteristics for either of the three major habitat types.