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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Isochaetides

Isochaetides Hrabe, 1966 emm. Brinkhurst, 1981 (ref. ID; 7201)

Family Tubificidae (ref. ID; 6651, 7854)

ref. ID; 7201

Type species

Limnodrilus baicalensis Michaelsen, 1901 (ref. ID; 7201)
  1. Isochaetides acapillatus (Fin., 1972) (ref. ID; 7201)
  2. Isochaetides arenarius (Michaelsen, 1926) (ref. ID; 6660, 7201)
    Syn; Limnodrilus arenarius Michaelsen, 1926 (ref. ID; 7201)
  3. Isochaetides baicalensis (Michaelsen, 1901) (ref. ID; 7201, 7254)
    Syn; Limnodrilus baicalensis Michaelsen, 1901 (ref. ID; 7201)
  4. Isochaetides excavatus Hrabe, 1982
    See; Tubifex excavatus (ref. ID; 7201)
  5. Isochaetides freyi Brinkhurst, 1965 (ref. ID; 6651)
  6. Isochaetides lacustris (Cernosvitov, 1939) (ref. ID; 6620, 7254)
  7. Isochaetides michaelseni (Last.) (ref. ID; 6634)
  8. Isochaetides neotropicus (Cern., 1939) (ref. ID; 7201)
  9. Isochaetides newaensis (Mich., 1902) (ref. ID; 7201)
  10. Isochaetides pseudogaster (Dahl) Hrabe, 1966
    See; Tubificoides pseudogaster (ref. ID; 5945, 7248)
  11. Isochaetides pusillus (Timm, 1977) (ref. ID; 7201)
  12. Isochaetides suspectus (Brinkhurst, 1964) (ref. ID; 6651)
  13. Isochaetides suspectus (Sokolsk., 1964) (ref. ID; 7854)

Isochaetides arenarius (Michaelsen, 1926) (ref. ID; 6660, 7201)

Synonym

Limnodrilus arenarius Michaelsen, 1926 (ref. ID; 7201)

Descriptions

Three specimens in the new material may belong here. Two resemble the specimens of I. baicalensis in size and shape, with broad anterior ends and small conical prostomia. The setae are fewer in number, mostly 4 anteriorly in one specimen but reaching 5 or 6 in the other (semimature) specimen. In the latter, the setae of II-III are mostly simple pointed with blunt worn tips, the rest in these bundles with short upper teeth; in III-IV the upper teeth of some setae are shorter than the lower, in others they are the usual equal length seen in the rest of the segments, spermathecal setae are developing. The third specimen was smaller (only 350 µm wide anteriorly) with 5-7 or 8 setae with teeth equally long. Segments II and III are clearly two ringed. The penis sheaths are thin and conical, of the same form as in I. baicalensis, about 100 µm wide at the base, 110 µm long (versus 150x180 µm or more for baicalensis). Spermathecal setae are present. Spermatozeugmata are of the same form as that illustrated for I. baicalensis. (ref. ID; 7201)

Comments

This species differs from I. baicalensis in having fewer setae (5 versus 5-8 anteriorly, 2 versus 4-6 behind the clitellum) in which the teeth are nearly equal in length. The atria may be longer than those of I. baicalensis (they are said to be not longer than the vasa deferentia in contrast to seven times the atrial width, descriptions that do not allow an accurate comparison). The penes are said to be tubular versus ovate. This species was originally said to have thin cuticular penis sheaths in contrast to I. baicalensis but my specimens show them to be present in the latter too. Both species are found in shallow water (10-14 m). I have only been albe to confirm the description of the setae using Hamburg material. There is an additional taxon, I. arenarius f. inaequalis, described by Michaelsen and Vereschagin (1930) in which the upper teeth of the setae approach the condition seen in I. baicalensis. Cekanovskaya (1962) lists this as a synonym of L. arenarius proper, but this does nothing to clarify the separation of the two species. Most of the original specimens (ZMUH, V10438) have dried up. (ref. ID; 7201)

Type materials

ZMUH (Zoological Museum of the University of Hamburg) V9969. Also ZMUH V10438 and V11945. (ref. ID; 7201)

Isochaetides baicalensis (Michaelsen, 1901) (ref. ID; 7201, 7254)

Synonym

Limnodrilus baicalensis Michaelsen, 1901 (ref. ID; 7201)

Descriptions

Robust worms, up to 2 cm long and 1 mm broad. Prostomium small and conical. Setae numerous from II, not increasing in number with successive segments, 5-9 or even 11 per bundle, upper tooth clearly longer than the lower. Modified spermathecal setae in X long, thin, and straight with basal curve, with associated glands; spermathecal setae up to 170 µm long, i.e., longer than in the illustrated specimen in which the tip may be broken. Vas deferentia long, about four times the combined lengths of penis and atrium, entering atria almost opposite the prostate stalks. Atria strongly rounded apically, then broadly cylindrical ending in wide penes set in deep penis sacs; penes with thin cuticular coverings barely distinguishable as modified sheaths. Spermathecae large, globular, with distinct voluminous ducts, large pores encompass spermathecal setae. Speramtozeugmata relatively short, broad, with some sperm tails protruding through external cortex. These specimens conform quite well to the original descriptions. The setal number may exceed that described in the original account (5-8) but the number varies. The relative widths of the atria and vasa diferentia and the form of the penes in the original illustrations strongly resemble those of maturing individuals, but are close enough to the fully mature forms examined here to permit confident identification. The problem raised by the redescription, however, is that I. baicalensis was named as the type species of Isochaetides by Hrabe (1966), and its now clear that the male ducts vary considerably in the species complex that is currently included in the genus. (ref. ID; 7201)

Type materials

Types amissus, but ZMUH (Zoological Museum of the University of Hamburg) V6587 from Lake Baikal, 1904, det. Michaelsen, 20 specimens. (ref. ID; 7201)