Theristus

  

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Rev: 02/18/2026

  Classification Biology and Ecology
Morphology and Anatomy Life Cycle
Return to Theristus Menu Ecosystem Functions and Services
Distribution Management
Return to Xyalidae Menu Feeding  References
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Classification:

Chromadorea
  Chromadoria
   Monhysterida
Monhysterina
Sphaerolaimoidea
                 Xyalidae
       Theristus Bastian, 1865

Type species of the genus: Theristus acer Bastian, 1865

    Synonyms:

Cylindrotheristus
     
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Morphology and Anatomy:

With general characteristics of family Xyalidae, and:
  • Body length 0.6 to 1.7 mm for freshwater species
  • Cuticle finely striated; thin setae
  • Head continuouis with body contour or slightly offset, bearing 10 to 12 setae in 6 groups
  • Ocelli sometimes present
  • Stoma small and narrow without obvious denticles
  • Tail conoid, short spinneret


    Females:
  • Monodelphic prodelphic
  • Vulva at about 55- 65% of body length

  
   

Theristus longisetifer  Male A. Anterior region; B. Head,; C. Posterior region; D. Spicule and gubernaculum; E. Spicule; F. Lateral piece of the gubernaculum. Female G. Head; H. Posterior region, right side view
Drawings from Kito and Aryuthaka, 1998

Males:  

  • Monorchic or diorchic
  • Spicules usually arched, sometimes very long.
  • Gubernaculum slipper-shaped, with or without a caudal extension.
  • No terminal setae on tail

Ref: Andrassy, 1981; Kito and Aryuthaka, 1998; Huang and Guo, 2022

Body size range for the species of this genus in the database - Click:
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Distribution:

Bottom sediments, including bottom sedimensts in sea-water shrimp culture pondes.

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Feeding:

Food Sources and Feeding strategies for the genus Theristus

Microbial epistrate feeders.

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Biology and Ecology:

Some species of Theristus inhabit freshwater. For example, T. tessae is reported from the Fish River of Namibia (Heyns and Coomans, 1989).

Theristus polychaetophilus is an ectosymbiont of the polychaete worm Scolepis squamata (Hopper, 1966). Other examples of nematode ectosymbionts of polychaetes are in the subfamily Harpogonchinae of the Chromadoridae: Harpagonchus averincevi and H. similis on the gills of Aglaophamus macroura (Nephtyidae), and Harpagonchoides crassus between parapodia of Hemipodus digitifera (Glyceridae).  (Hopper, 1966; Tchesunov, 2013).

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Life Cycle:

 
For Ecophysiological Parameters for this genus, click 
 
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Ecosystem Functions and Services:

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Management:

 
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References:

Andrassy, I. 1981. Revision of the order Monhysterida (Nematoda) inhabiting soil and inland waters. Opusc. Zool. Budapest XVII-XVIII:13-47.

Andrassy, I. 1984. Klasse Nematoda (Ordnungen Monhysterida, Desmoscolecida, Araeolaimida, Chromadorida, Rhabditida). Gustav Fischer Verlag, Stuttgart. 509 pp.

Heyns, J and Coomans, A. 1989. A new freshwater species of Theristus from South West Africa/Namibia (Nematoda: Xyalidae)  South African Zoology 24:95-98.

Hopper, B. E. 1966. Theristus polychaetophilus n. sp. (Nematoda), an external parasite of the spionid polychaete Scolelepis ( Scolelepis) squamata Muller, 1806. Can. J. Zool. 44: 787-791.

Huang, Y., Guo, Y. 2022. Order Monhysterida Filipjev, 1929. In: Free-living Marine Nematodes from the East China Sea. Springer, Singapore. https://doi.org/10.1007/978-981-16-3836-7_9

Kito, K., Aryuthaka, C. 1998.  Free-living marine nematodes of shrimp culture ponds in Thailand. I. New species of the genera Diplolaimella and Thalassomonhystera (Monhysteridae) and Theristus (Xyalidae). Hydrobioloia 379:123-133

Tchesunov, A.V. 2013. 7.12 Order Chromadorida Chitwood, 1933. In Schmidt-Rheas and de Gruyter, W. Handbook of Zoology.Gastrocticha, Cycloneuralia, Gnatifera. Volume 2: Nematoda

 

 
Copyright 1999 by Howard Ferris.
Revised: February 18, 2026.