Unknown freshwater ciliate — highly contractile, unusual posterior and anterior ciliature
I'm hoping someone with more ciliate expertise can help me narrow down this freshwater ciliate.
Sample: freshwater pond, Hawke's Bay, New Zealand
Microscope: Zeiss compound microscope, brightfield
Magnification: 40× objective
Camera: Phone camera through microscope adapter
I've been observing this organism for quite a while and I'm struggling to place it.
What I've observed:
• Definitely ciliated, with what appears to be ciliation around the whole body
• Free-swimming — I watched it for ~30–60 minutes and never saw it attach
• Swims relatively quickly and rotates around its longitudinal axis
• Very flexible/contractile; the entire cell can change shape quite dramatically within a few seconds
• When extended it is elongated/ovoid
• The posterior is particularly unusual: it can develop a pronounced U-shaped indentation, but this changes as the cell contracts/deforms
• The anterior has a conspicuous region of denser/longer, rapidly moving cilia
• I can see several internal structures/inclusions (roughly green, brown and clear), which seem to maintain approximately the same relative positions while the cell changes shape
• I haven't been able to see an obvious contractile-vacuole pulsation
I'm currently completely open on the identification. Nassulidae and Stentor have both crossed my mind, but I'm not confident in either.
I'd particularly appreciate comments on the morphological characters that support or rule out particular groups, rather than just a resemblance-based ID.
I have additional video footage if useful. Any suggestions about what features I should look for with my 100× oil objective would also be very welcome.
Thanks!