Ciliates

Ciliates, as their name suggests, are characterized by the presence of cilia: short, hairlike structures, which typically occur in large numbers (unlike flagella). Cilia are involved in both feeding and movement. 

Characteristics of Ciliates

The number and arrangement of cilia varies, and is related to the functional role of the cilia (Sanders, 2009). For instance, cells of Paramecium are completely covered with a fringe of cilia, which beat in a coordinated fashion to propel cells through the surrounding water. Vorticella, on the other hand, are sessile, and their cilia are used for feeding: concentrated in a ring at one of the end of the cell, these cilia create a current that draws food into the organism’s “mouth.”

Some ciliates are characterized by gliding, crawling, or free-swimming movement, while others are sessile. Sessile forms may be attached to the substrate by a stalk or by a stalk-like, elongated posterior end; some of these ciliates are colonial, with individuals attached by branching stalks (Foissner & Berger, 1996).

There is significant variation in general appearance among ciliate species. In terms of an individual’s shape, for example, some are oval-shaped and flattened; some have a trumpet-like shape; some are barrel-shaped and covered in armored plates; and that is far from an exhaustive list of the possibilities! Coloration also varies among ciliates, though many species are colorless. Some ciliates appear colorful because they produce pigments; this is true for Stentor coeruleus, a species characterized by an overall blue-green appearance (Slabodnick & Marshall, 2014). Other ciliates appear colorful because they harbor dense populations of endosymbiotic algae; Paramecium bursaria, for instance, looks green because it contains many endosymbiotic green algae. Colorless ciliates often contain visible remnants of ingested algae (which are colorful).

Ciliates in Freshwater Environments

Ciliates are especially abundant in sediments, but are common in the plankton as well (Finlay & Esteban, 1998). Vorticella species are observed fairly regularly in water samples collected by NJCWST. Other ciliate taxa observed by NJCWST include Paramecium bursaria, Codonella, Nassula, and Coleps. 

Above: Paramecium bursaria. The numerous green spheres packed densely together inside this P. bursaria cell are symbiotic green algae.

Above: A large ciliate (about 200 µm long in these photos, but it is not fully elongated in these pictures), possibly Stentor. Scale bar is 20 µm.

Above: Vorticella, a genus of stalked/sessile ciliates.

Above: Vorticella attached to clusters of Dolichospermum (cyanobacteria) filaments.

Above: free-swimming ciliates

Amoeboid Protists

Amoeboid protists are characterized by the plastic (meaning variable in shape) nature of their cells (Sanders, 2009), and by their utilization of mobile cytoplasmic extensions, called pseudopods (or pseudopodia), for movement and feeding (Lara et al., 2020). Note that some species of algae in the class Chrysophyceae also have amoeboid cells (characterized by a variable cell shape and the formation of pseudopods) (Nicholls & Wujek, 2015).

Characteristics of Amoeboid Protists

Pseudopods are blunt and finger-like in some taxa, while in others they are fine and threadlike (Sanders, 2009). Testate amoebae are surrounded by an outer shell, called a test. Tests vary in shape, and may or may not be colorless (some are brownish-orange).

Heliozoans are a polyphyletic group (Nikolaev et al.,2004) characterized by the presence of axopodia, which are stiff, slender, radially arranged pseudopods (Finlay & Esteban, 1998); due to the appearance of their axopodia, heliozoans are sometimes called “sun animalcules”.

Amoeboid Protists in Freshwater Environments

Heliozoans are commonly found in the plankton, while other types of amoeboid protists are usually associated with sediments or surfaces (Sanders, 2009). Heliozoans have been observed by NJCWST on several occasions.

Above: testate amoeba. Note the colorless, lobe-like pseudopods that can be seen emerging from within the orange-brown test.

Above: Axopodia-bearing amoeboid protists (also known as "Sun-animalcules") 

Above: An engulfed cyanobacterial filament (pale greyish-green) visible within the cell body of an amoeboid protist (colorless).

Heterotrophic Flagellates

Flagellates are exactly what they sound like: protists with one or more flagella! Unlike flagellated algal protists, heterotrophic flagellates must consume other organisms (like bacteria, for instance) to obtain energy and nutrients. Heterotrophic flagellates use their flagella for movement, feeding, or attachment to surfaces (Sanders, 2009).

Characteristics of Heterotrophic Flagellates

The number, arrangement, and relative length of flagella varies among species. The size of heterotrophic flagellates varies widely (Jeuck & Arndt, 2013), but most flagellates are relatively small in comparison to ciliates and amoebae (Finlay & Esteban, 1998). The cells of some flagellate species are housed in a shell-like lorica, which may be cone- or bell-shaped; other species may be surrounded by a membranous layer called a theca, which may be tube-, cup-, or flask-shaped (Jeuck & Arndt, 2013). Some flagellates are attached to substrates, whereas others are unattached and move by gliding or swimming (Jeuck & Arndt, 2013). Both colonial and unicellular species exist (Finlay & Esteban, 1998).

Heterotrophic Flagellates in Freshwater Environments

Groups of flagellates commonly found in the plankton of freshwaters include choanoflagellates and colorless heterotrophic members of the class Chrysophyceae (Finlay & Esteban, 1998). Choanoflagellates possess a collar (composed of projections called microvilli) and a single flagellum, and are often attached to substrates (Jeuck & Arndt, 2013). Chrysophytes have two flagella of unequal length, though the shorter flagellum may be difficult to see (Jeuck & Arndt, 2013). The class Chrysophyceae also includes many photosynthetic (i.e., algal) flagellates, which are described in detail on our Golden Algae page.

A Note on Flagellated Protists...

While many algae—including cryptomonads, euglenophytes, and others—are flagellated protists, they are usually analyzed as part of the algal population, rather than as part of the broader “flagellated protists” population. This is because algae function as primary producers, whereas heterotrophic flagellates and other heterotrophic protists function as consumers. Additionally, as photosynthetic organisms, algae respond similarly to the availability of key resources required for photosynthesis to occur (like light and carbon dioxide). Thus, from an ecological perspective, it is logical to consider all algae together as part of the same functional group. Note that some organisms we consider “algae” are in fact mixotrophic, meaning they can obtain energy and nutrients both by photosynthesis (autotrophy) and by feeding on other organisms (heterotrophy). 

Choanoflagellates growing on filamentous green algae.