Uronema nigricans, MuEller, 1786

Esteban, G. F., Finlay, B. J., Olmo, J. L. & Tyler, P. A., 2000, Ciliated protozoa from a volcanic crater-lake in Victoria, Australia, Journal of Natural History 34 (2), pp. 159-189 : 183-185

Cyclidium nigricans MuÈller, 1786

(®gures 46, 47)

Uronema nigricans is a small scuticociliate, 30±35 m m long, typically ellipsoidal and characterized by great variation in its oral and somatic infraciliature. The distinction between this species and U. marinum Dujardin, 1841 , is still unclear and is probably unjusti®ed (see Hoare, 1927; Te Âllez, 1981). The close examination of both species is not the objective of this paper as their comparison would involve the study of clonal cultures. Bearing this in mind, for the purpose of this article we leave the nominal species U. nigricans as such. The Australian population of U. nigricans was grown in culture, which facilitated the assessment of its morphological diversity. These cultures were also used as food for Pleuroplites australis (see above). The living organisms of U. nigricans are ovoid and ¯attened dorso-ventrally. U. nigricans bears one caudal cilium which the ciliate uses to attach to sediment particles and to whirl aroundÐto some extent like U. ®li®cum Kahl, 1935, but living specimens of the latter are easily discriminated (see Fenchel et al., 1995, for relevant details of U. ®li®cum). The number of somatic kineties of the Australian population of U. nigricans varies between 10 and 12, each kinety with (usually) 15±16 kinetids. Exceptionally, the last somatic kinety (kinety n) is the longest.

The variation within the oral infraciliature is striking, with the scuticovestige the most confusing character for species identi®cation. The structure and location of the latter is highly variable (®gures 46, 47). Apart from the normal variations in dividing and post-dividing organisms, the scuticovestige can be arranged as a line of mid-ventral post-oral kinetosomes (®gure 46), as an oblique row of three to ®ve kinetosomes at the end of the oral dikinetid, or as a disorganized group of kinetosomes beneath the oral dikinetid. A constant character in U. nigricans , however, is the location of the ®rst oral polykinetidÐa ®le of inconspicuous kinetosomesÐ always distant from the beginning of the paroral dikinetid and from the second polykinetid (®gures 46, 47). The paroral dikinetid is a zig-zag row of double kinetosomes, and it has the shape of an inverted question mark (). Its forward part is straight and ends at the level of the second oral polykinetid; its rear part curves to the left of the cell, as a hook.

The infraciliature of oral polykinetids 1 and 2 also varies, whereas the third polykinetid appears to be constant. The ®rst oral polykinetid can be a single row of kinetosomes or a split row developing two short oblique kinetosomal rows, and in some specimens these two rows eventually become parallel. The second polykinetid is generally a ®le of three vertical and parallel rows of basal bodies (®gures 46, 47), with variable length of the third (outermost) row. The third oral polykinetid is the least variable of all, and is well developed as a circular ®eld of kinetosomes. The cytoproct of the cell opens as a wavy line in impregnated cellsÐalong the middle line of the ventral surface (®gure 47). The contractile vacuole pore opens either at the end of the ®rst kinety or of the second, although sometimes somatic kinety 1 is so long that the contractile vacuole pore seems to open at the end of the 11th somatic kinety (in organisms with 12 kineties). There is usually one macronucleus and one micronucleus above it, both placed in the anterior half of the cell.