Theristus

  

Contents

  

Rev: 03/31/2025

  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

    Synonyms:

Cylindrotheristus
     
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Morphology and Anatomy:

Tail without terminal setae. One of two genera in the Xyalidae having species inhabiting freshwater, however, most species are marine or in brackish water, especially in interstitial habitats.
  • 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
  • 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

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:

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)

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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.

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

 

 
Copyright 1999 by Howard Ferris.
Revised: March 31, 2025.