Pleuroplites australis Foissner, 1988

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 : 179-182

Pleuroplites australis Foissner, 1988

(®gures 38±45)

The living ciliates of this and related genera are very variable in shape, number of nuclei, mode and degree of silver impregnation, cell size, and in the number of somatic kineties. These ciliates are well known as rapacious carnivores (see Corliss, 1979). Starved cells are normally elongate with the oral aperture at the top of the anterior end (®gures 38±45), and with no obvious caudal cilia. A large number of very similar nominal species (an example is shown in ®gure 45) have been described, with di erences between them as slight as the number of macronuclei. Taxonomic revision of the genera involved falls outside the scope of this paper.

The amount of food ingested, and the stage of the life cycle, dictate the shape and size of P. australis (®gure 38). It varies between 45 and 60 m m long, and 17 and 25 m m wide although very small cells were occasionally observed. The full morphological diversity of P. australis was assessed by preparing di erent cultures and using Uronema nigricans as food (®gure 39). The shape can be ellipsoidal (e.g. 45 m m long by 23 m m wide); elongate, tapering towards the posterior end (e.g. 60 m m long by 17.5 m m wide); pear-shaped with broader anterior part of the cell, pointed at the anterior end and rounded at the posterior; or with a protruding cytostome (®gure 38). Pleuroplites australis has the characteristics of the family Enchelyidae (as established by Corliss, 1979): there is a ®eld of clavate sensory cilia, and the toxicysts are localized, typically in or near the oral area. Pleuroplites australis has sparse somatic cilia and no obvious caudal cilia, although the cilia in that area do appear straighter than the rest (®gure 38). There is a single group of extrusomes (toxicysts?) gathered in a circle close to the oral aperture at the anterior part of the cell (®gures 41±43). This structure is detectable in silver-impregnated specimens, but hard to observe in living organisms (even in immobilized cells). In our experience, and assuming a good silver impregnation (either protargol or silver carbonate), this feature is observable only in one out of ®ve cells. There are other extrusomes scattered in the cytoplasm (®gure 41), which, again, were evident in only a few of the silver-impregnated cells examined.

The somatic kineties are bipolar and dimorphic (®gures 38, 43). Half of them bear sensory bristles ( Corliss, 1979), and therefore the infraciliature of these kineties di ers from that of the other half (see below). In our population, the number of somatic kineties varied between 17 and 19, each kinety with few kinetids. Approximately half of the somatic kineties contain dikinetids with clavate cilia (about 1.9± 2 m m long, ®gure 44) along their entire bipolar length, and these are easier to di erentiate in the area around the oral region. These clavate cilia constitute the sensory bristles (usually inaccurately termed`brosse’ in the literature), and the ciliates can bend them. The kinetids in these kineties are closer to each other at the anterior end of the cell (®gures 38, 43). The arrangement of the clavate cilia within a kinety varies between individuals and among kineties of the same specimen (®gure 38). It can be two clavate cilia together per kinetid, such as those around the oral aperture; one clavate cilium per kinetid; two clavate cilia plus a longer cilium (~ 7±8 m m long) per kinetid; or one clavate cilium with one normal cilium per kinetid. The remaining somatic kineties bear single, scattered kinetosomes. The circular ®eld of extrusomes lies on this side of the cell (®gures 38, 42, 43).

The oral region is encircled by the circumoral kinety which has 17±19 paired kinetosomes lying side by side within each pair (®gures 38, 42, 43). The nematodesmata in the mouth are inconspicuous. In some living cells the mitochondria were observed arranged in bipolar rows, between the kineties (®gure 38). The contractile vacuole can appear round, elongate, or triangular, and situated at the cell posterior end. The number and shape of the macronuclei is variable too (®gure 38), the number varying from one to four macronuclei, although there is only one micronucleus. Pleuroplites australis has previously been reported from Kenya, Australia, and Antarctica ( Foissner, 1998).

Living cells of P. australis are indistinguishable from ciliates of the following genera: Coriplites, Enchelys , Enchelyodon , Foissnerides , Fuscheria , Pleuroplitoides , from some of Kahl’s Spathidium species , and from Placus ovum (Kahl, 1926) . As stated above, a thorough revision of all these genera goes beyond the scope of this article. However, the peculiarity of the infraciliature of P. australis , with a ®eld of clavate cilia that involves more than half of the somatic kineties, and the existence of a localized group of toxicysts, resembles Pseudoprorodon spp. (see ®gure 45). The infraciliature of P. australis is also nearly identical to that of Foissnerides heliophagus Song and Wilbert, 1989 (see ®gure 45, and Song and Wilbert, 1989) with a single group of localized toxicysts near the oral aperture standing as the only di erence between the two. Song and Wilbert (1989) made no mention of this structure (but note that in our preparations it was observable in one out of ®ve silver-impregnated cells). Foissnerides heliophagus was found in Germany.