Free-living ciliates

Any sample of pond water will likely turn out at least a few ciliate species, many of which will be small and hard to identify at sight, but some of which are quite “charismatic” once their specific quirks are recognized.

Contents

Cinetochilum
Coleps
cf. Disematostoma
Halteria
Lacrymaria
Litonotus
Mesodinium
Scuticociliate (probably Cyclidium)
Hypotrich ciliates
Urocentrum
Green ciliates
Ciliates separating
Unidentified ciliates


Cinetochilum


Active ciliates, common, said to be bacterivores. The cell body is oval in outline, relatively flattened, grooved lightly on the dorsal side. Three or four long caudal cilia (“spines”) project from the tail end. The edge of the cell is also fringed with cilia. Prominent contractile vacuole is visible in some of these pictures. The cell “mouth” is on the ventral surface, and positioned off-center. The “mouth” is at least partly surrounded by an undulating membranelle.

Cinetochilum is a scuticociliate, but does not have the same degree of twitchiness as Cyclidium (see below). It uses its ventral cilia to “walk” across substrates, much like Euplotes and other hypotrichs.


Coleps

Barrel-shaped, rigid cells with distinctive rectangular plates evenly covering the surface. The “mouth” is at the anterior end of the cell. They are colored brown. Found among detritus, scavenging on dead animal carcasses. They were abundant in a plankton tow sample from Kranji Reservoir, after many of the copepods and other crustaceans had begun to die and decay.


cf. Disematostoma

Appears to match description of Disematostoma or its relative Frontonia. Body is rugby-ball shaped, with tapered ends. Just under the cell surface is a layer of trichocysts, visible as small tapered lines. Ciliation is more or less even over the body surface. In the cytoplasm the nucleus and contractile vacuole are visible, as well as some other inclusions and organelles. There is a shallow depression about 1/4 the way down from the tapered end, which looks like a cytostome.

The organelles in the cytoplasm are beautifully displayed when the ciliate rotates:


Halteria

Rounded cell bodies, moving in short bursts with pauses in between. Anterior of cell is surrounded by an “anterior zone of membranelles” (AZM). Membranelles are flap-like structures formed by the fusion of multiple cilia. Projecting from the equator of the cell are stiff spine-like structures. In some of these pictures, the cell is seen from end-on, so the radial arrangement of the equatorial spines is clearly visible.

Heterotrich / Spirotrichea

Video of the darting behavior of Halteria

Video of a Halteria which has become anchored to the slide and begun spinning around rapidly.


Lacrymaria

Spindle-shaped cell body with a long extensible neck that is able to stretch and move around actively. The “mouth” is at the end of the cell. Commonly known as “swan’s tears”. The surface of the cell is ornamented with spirally patterned kineties. May be confused with Litonotus (below), but the position of the mouth differentiates the two.


Litonotus

Rugby-ball shaped cell body with an elongated swan-neck. Rapidly moving, neck bends to the left and right. Two macronuclei (with the micronucleus said to be between them) in middle of cell body, and a contractile vacuole in the posterior end. “Mouth” extends as a long region anteriorly along the neck; below the surface are trichosomes, organelles used in prey capture (visible as faint parallel hatching on the neck).

Click for animation


Mesodinium

A small ciliate darting about actively, shaped like a bottle gourd, with cilia emerging from its pinched waist. The tentacles emerging from the anterior end of the cell are forward-pointing and forked at the ends. When seen head-on or end-on, the cilia radiate outwards in all directions, much like Halteria. In addition to the rapid darting motion, it also exhibits a slower crawling motion, as seen in the video below:


Scuticociliate (probably Cyclidium)

Small, twitchy ciliates with long cilia covering cell body. Twitching motion can be seen in the video embedded below. In the middle of the cell is a feeding groove from which a membranelle, composed of numerous cilia compressed into a sail-shaped structure, unfurls and is used in feeding.

Scuticociliate binary fission

A cell in the process of dividing by binary fission. It was still motile and twitchy like the rest of them!

Unidentified scuticociliate

Unidentified scuticociliate which could be Cyclidium or Pleuronema.


Hypotrich ciliates

Hypotrichs are rigid-bodied ciliates, often dorso-ventrally flattened. Cirri present in groups on the ventral surface, they use these to crawl on substrates. Dorsal surface of the cells has “sensory bristles”.

Euplotes cf. woodruffi

Large hypotrich ciliate found in small forest stream. The ciliate is comparable in size to, and even larger than, many aquatic invertebrate animals. There are large thick cirri used for movement, and a margin of smaller cirri lining the peristome. Compare the size and behavior with the smaller species of Euplotes below.

The contractile vacuole is off-center and can be seen changing in size in the above sequence of images.


The macronucleus is shaped like a long ribbon arranged in a “U” within the cytoplasm, visible above as a grainy curved strip.

The cell is capable of moving quite quickly in the water


Aspidisca

This ciliate had cirri on its ventral surface and an apparently unciliated dorsal surface that bore prominent ridges. The cell body was rigid and relatively flattened. It was highly active and used its ventral cilia to crawl over surfaces like algal filaments and debris, giving it the appearance of a tiny insect. Projecting from the rear margin of the cell were three or so “spines” or cirri.

(Previous version of this page misidentified this as an Euplotes sp. Thanks to WordPress user anbaeser for the correction. – 9 May 2013)

cf. Oxytricha

Hypotrich ciliate with cirri around margin. This cell was actively swimming, crawling, and squeezing about and it was hard to get a definitive look at its pattern of cirri, so the identification is only tentative.

Stylonychia


Hypotrich with three long and stiff cirri from the caudal (tail) end. Margin is lined with a row of flexible cirri used for motion and crawling; these also set up a water current for feeding:

Ridged green ciliate

This ridged ciliate was also highly motile, rigid-bodied, and hard to photograph. When swimming freely it tumbles in a spiral, and on approaching a substrate does so with one of its flatter sides. It was moving too fast to clearly see whether its cilia were differentiated or not. The cytoplasm was also filled with green material: these may be its food, or may be chloroplasts harvested from prey organisms.


Urocentrum

Top-shaped ciliate with a constriction at the “waist” and cilia borne in two bands girdling the two halves. The caudal (tail) end has a pointed tail.


The contractile vacuole is visible close to the tail end (above).



Green ciliates

These ciliates clearly contain chloroplasts, which are likely harvested from their food. Many protists, including amoebae, flagellates, and heliozoans, retain chloroplasts from their food organisms and appear to use them for photosynthesis. Other individuals of the same species may thrive just as well on a heterotrophic diet, without using photosynthesis. The “green” ciliates observed here are holotrichs and quite changeable in form.


Ciliates separating

These ciliates, probably Tetrahymena, were in the process of pulling apart from each other. A thin thread of cytoplasm is visible connecting the two cells. They eventually separate from each other after much squirming. Look carefully for the contractile vacuoles to notice that the anterior end of one cell is connected to the posterior end of the other. Have we just watched them finish binary fission?


Unidentified ciliates

Unidentified small ciliate

Small holotrich ciliates with interesting behaviors but hard to identify.

Unidentified peritrich

Large (about 75 µm across) ciliate with a prominent ring of cilia surrounding one end of the cell (red arrow). The cell surface also has longitudinal ridges. This ciliate became trapped under the cover slip as the slide dried out, and was soon surrounded by a swarm of hungry bacteria.

Unidentified holotrich

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One Response to Free-living ciliates

  1. Kiara Rosa says:

    Wow! I’m currently doing a survey of pond water for a microbiology project and your work has helped so much in identifying most of the freshwater organisms I’ve found! Thank you!

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