Animals

Taxonomic Classification

Animals are members of the kingdom Animalia. Unlike the chaotic collection of unrelated life-forms that is “algae,” the animal kingdom is a monophyletic group. All animals are characterized by a set of fundamental biological traits, including eukaryotic cell structure and multicellularity.

Freshwater assemblages of microscopic animals include representatives of distinctly different animal lineages. Thus, as with algae, it is still difficult to make general statements about morphology (at least about any features we can see with a light microscope) that would apply to all species. But don’t worry, we’re going to try to make some of those general statements anyway! 

Types of microscopic aquatic animals observed by NJCWST include rotifers, gastrotrichs, crustaceans (such as copepods and water fleas), and tardigrades.

Characteristics

Color

The types of microscopic animals listed above have mostly minimal coloration or appear essentially colorless. Muted shades of brown, greyish-brown, or greyish-pink may be present. Internal anatomical features may be visible within otherwise colorless bodies.

Size

In general, an individual animal is larger than an individual algal cell. Algal colonies, however, may be comparable in size to microscopic animals.   

Form

Animals, by definition, are multicellular organisms. In almost all animals, cells are organized into different tissue types. These specialized tissue types are further organized into reproductive, digestive, nervous, and other body systems. Some of the anatomical features associated with these bodily systems may be visible, such as eyes, eggs or egg sacs, intestines, and feeding structures. Crustaceans like water fleas and copepods also possess miniaturized versions of some more familiar animal attributes, like legs and antennae.   

Movement

All of these organisms are capable of movement. Many move their entire bodies, either along surfaces or through the water; others live attached to surfaces (sessile), but move certain body parts, like feeding structures. 

Non-Algal Protists

Taxonomic Classification

“Protist” is something of a taxonomic orphanage: unclaimed eukaryotes can be found here. To put it in more scientific terms, eukaryotes that are neither plants nor animals nor fungi are referred to as protists. There is a significant amount of biological diversity contained within this unnaturally-defined group, both phylogenetically and morphologically speaking.
Notice that this section is titled “Non-Algal Protists”—that’s because many kinds of algae are also considered protists (not our prokaryotic friends, the Cyanobacteria!). The descriptions given here will apply to protists that are not also considered algae. 

Groups of non-algal protists observed by NJCWST include ciliates, flagellates, and amoeboid protists. 

Characteristics

Color

These organisms are often colorless, though this is not always the case. 

Colorless protists sometimes contain variously colored inclusions (these may even be algae that they have eaten!), and it is also possible for the organisms themselves to be pigmented, as is the case with the turquoise-hued Stentor coeruleus (Slabodnick & Marshall, 2014). Paramecium bursaria is one example of a non-photosynthetic protist that actually houses live algae inside of itself. As a result, this ciliate takes on an overall bright-green color, though a closer look reveals that this is due to the presence of many algal endosymbionts. Some amoeboid protists are enclosed within a shell-like structure called a test, which may be colored. For instance, Arcella are enclosed within yellowish-brown tests. 

Still, many non-algal protists do take on a colorless appearance—which is not the case with live algal cells.

Size & Form

Very wide variety of forms. Protists are unicellular (though some form colonies), and the majority are microscopic in size. At about 1 mm in length, Stentor coeruleus is unsually large (Slabodnick & Marshall, 2014).   

Movement

Many protists are motile, and different types of protists exhibit different mechanisms of movement. Some use the beating of hair-like structures—flagella or cilia—to generate movement; others seem to flow over surfaces in a fluid manner, reaching out with projections of the cell to pull themselves along (Encyclopaedia Brittanica, 2019).