Plagiopyla frontata Kahl, 1930: 35

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 : 177

Plagiopyla frontata Kahl, 1930: 35 (®gures 34±37)

Ciliates of this genus are laterally compressed, with the shape of a kidney. The buccal region is large, and perpendicular to the main longitudinal axis of the cell (®gures 34, 35). Distinctive in Plagiopyla (and of other members of the Plagiopylidae ) is a structure of unknown function that extends anterio-posteriorl y along the right side of the cell. This is known as the striated band (®gure 36). The various species of Plagiopyla have been found in anaerobic freshwater, brackish, and marine habitats, and in the guts of echinoids.

Plagiopyla frontata is variable in size. It is basically kidney-shaped and laterally ¯attened, in the size range 75±110 m m long and 37±67 m m wide. Smaller specimens, 55± 60 m m long, are also frequent. The number of somatic kineties varies, usually between 60 and 68, and all are formed by monokinetids. Although the Australian population of P. frontata had fewer somatic kineties, around 40, the other morphological characteristics were indistinguishable from those of previously described P. frontata . All kineties on the right side of the cell, except three or four that ®nish near the opening of the cytoproct, reach the posterior end of the cell, where they meet the kineties from the left side, developing a suture. In the anterior part of the cell, the somatic kineties ®nish at the oral region, forming two`lips’ (®gure 34), and approximatel y 20 of these kineties enter the oral cavity forming a short infundibulum. There is typically one macronucleus with an associated micronucleus, although we also observed cells with three macronuclei.

The ®nal portion or anterior tips of the somatic kineties delimit the oral area (®gures 34, 35); these are separated from the somatic kineties by a narrow gap in the infraciliature (®gure 34). The oral kineties bend inwards in the oral aperture, and about 21 of them enter the buccal cavity, developing a short groove. The cytostome is at the end of the infundibulum.

There is a striated longitudinal band on the right side of the cell. This structure begins at the oral area (®gure 36), and extends beyond the cell equator to ®nish near the cytoproct. At the oral region the striated band bends about 90ss, then runs parallel to the somatic kineties towards the posterior part of the cell. It is formed by about 45 folds of the cell membrane. These folds are orientated at a right angle to the main longitudinal axis of the cell, and are supported by microtubules (pers. observ.). The cytoproct and contractile vacuole pores open near the end of the striated band (®gure 36).

Plagiopyla frontata lives in marine and brackish-water anaerobic sediments. It has also been found in the anoxic part of the water column of a Danish f jord (Fenchel et al., 1995), and we have also found it in anaerobic sediment in the Hopkins estuary near Warrnambool, where it grazes on anaerobic photosynthetic bacteria. Like the organisms retrieved from Europe, the Australian P. frontata bears endosymbiotic methanogenic bacteria in the cytoplasm (®gure 37) ( Fenchel and Finlay, 1991, 1995). The endosymbionts of P. frontata form clusters that ¯uoresce when excited with UV light (®gure 37). TEM sections of specimens from Europe show that each cluster is formed by alternating hydrogenosomes (essentially anaerobic mitochondria) and methanogens, with the components of each (and all) clusters dividing simultaneously immediately before the ciliate cell division ( Fenchel and Finlay, 1991). A photograph of clusters of silver-stained hydrogenosomes can be seen in ®gure 35. Details of the metabolism, growth rate, and symbiosis with methanogenic bacteria in P. frontata can be found in Finlay and Fenchel (1992), and Fenchel and Finlay (1991).