Metopus setosus var. minor Kahl, 1927

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 : 174-177

Metopus setosus var. minor Kahl, 1927

(®gures 26±30)

Ciliates of the genus Metopus are anaerobic organisms with some tolerance to dissolved oxygen, so migrants are occasionally found in oxygenated waters ( Esteban et al., 1995). All Metopus species have similar body shape, characterized by the torsion of the anterior part of the cell, and by a frontal lobe protruding over an ascending adoral zone of membranelles (AZM, ®gures 26±30). The size of the cells, the general shape and degree of torsion, and the presence/absence of caudal cilia, are the main species identi®cation features.

Metopus minor is a small ciliate; cells in the Australian population were 30±40 m m long. Our population ®ts within the description of Kahl (1930 ±1935). As we manage d to keep a culture of this organism for some days, we were able to observe further features of this species. Metopus minor has three groups of long cilia and a conspicuous, protruding anterior lobe (®gures 26±30). The three groups of long cilia are located as follows: one group is at the front of the cell (when the organism is seen from the side), just above the tip of the frontal lobe (®gures 27, 28); one group is opposite the latter, at the anterior part of the dorsal surface (®gures 28, 29); and ®nally, there is one group of (usually) ®ve, long, caudal cilia (®gures 26±30). The ciliate uses these three groups of cilia to attach to sediment particles (®gures 28±30), where it is normally observed ®lter feeding. Metopus minor has ®ve or six somatic kineties with few cilia per kinety (as seen with Nomarski interference contrastÐ silver impregnation was not feasible due to the small number of specimens found). The single macronucleus is ellipsoidal, and is placed in the anterior half of the cell (®gure 28); the single micronucleus lies next to it. The contractile vacuole is situated at the posterior end of the cell.

Metopus minor was found in brackish water in the crater-lake, together with a few specimens of other Metopus species. It has previously been recorded in Europe ( Kahl, 1930 ±35; Foissner and Adam, 1979).

Oxytricha salmastra Dragesco and Dragesco-Kern Âis, 1986 (®gures 32±33)

Most hypotrich ciliates are di cult to identify, especially from living material. Almost all are adapted for crawling on surfaces (e.g. sediment, detritus, mosses), and they have a large ®lter-feeding oral apparatus (the adoral zone of membranelles, AZM). Their main morphological feature is that the cilia in the ventral surface of the cell (which is in contact with the substratum) are gathered to form packed groups called cirri (®gure 32). Cirri are responsible for the principal type of locomotion of these ciliates, which appear as if they are`walking’ on surfaces. Hypotrichs can also swim, by means of the adoral zone of membranelles in the oral apparatus. These ciliates have wide morphological plasticity, and they are fragile. Quite frequently, bits of cells with odd shape and capable of independent movement are found in natural samples .

Like many hypotrichs, O. salmastra displays broad morphological variability, mainly dependent on the amount and the type of food ingested. Oxytricha salmastra can be elongate and slender (®gure 32), or fat and wide, about twice the breadth of a slender one. Some are pear-shaped, wider at the anterior end, and tapering towards the posterior pole. We kept O. salmastra in culture. Typically, the cells are elongate and quite slender, 110±155 m m long, and 48±56 m m wide. The distinctive feature in living, immobile organisms, is a marked row of cirri running along each edge of the ventral surface (these are the marginal rows of cirri; ®gure 32). The infraciliature in this species varies widely.

The somatic infraciliature (®gures 32, 33) is formed by: (a) two frontal cirri; (b) typically eight frontal cirri that include: one frontoterminal cirrus very close to the beginning of the adoral zone of membranelles (and appearing to be a part of it), one buccal cirrus, and 2 +2 cirri in the anterior half of the cell, spreading out down to the cell equator. This arrangement varies in larger individuals of the same population. Apart from the frontal cirri, there are two to four ventral cirri, located post-orally, and three to ®ve transverse cirri. Within the latter, if ®ve cirri are present four comprise an oblique row and one is on its own (®gure 32). When three, two are in a row, and one is on its own. There is one right marginal row of cirri (RMR, ®gure 32), and one left marginal row (LMR) of cirri running along each side of the ventral surface of the ciliate. These two rows never make contact at the posterior end of the cell. The LMR bears from 27 to 32 cirri, although other numbers are possible. The RMR bears 30±32 cirri. There are two or three caudal cirri, and four dorsal kineties with dikinetids. Three of the dorsal kineties run bipolar along the dorsal surface, with each kinety ®nishing in one caudal cirrus. The fourth dorsal kinety is short and does not reach the cell equator (®gure 33). The dorsal kineties bear 14, 15, 14, and six pairs of kinetosomes, respectively.

The oral region occupies the anterior one-third portion of the cell, with 27±35 membranelles in the adoral zone, one paroral membrane, and one endoral membrane (variable in length). In some organisms the paroral and endoral membranes cross each other; in others, the membranes do not intersect, the endoral membrane is very short (®gure 32) and is located at the beginning of the paroral membrane. The nuclear apparatus is formed by one long macronucleus or by two macronuclei, and two micronuclei.

Remarks. Oxytricha salmastra has previously been recorded only from tropical Africa, where it was found in brackish water. We also found this species in brackish water, and our observations provide the ®rst record of this species outside of Africa.