Zooplankton: protists.

The zooplankton, or "animal"-like organisms are a diverse group of minute, weakly-swimming organisms defined by their small size and their function in the energy system, rather than by any shared taxonomic affiliations. Many of the zooplankton are suspension feeders, straining out the phytoplankton and keeping your water clear. They may be single-celled organisms like flagellates or ciliates, or multicellular animals, such as microscopic nematodes or rotifers, or the small crustaceans, such as copepods and daphnids. They are adapted to drift with the current (these are pelagic, a word that originally referred to the open sea), or to make a living attached to vegetation or other surfaces (these are littoral, a word that originally referred to the shoreline). Similar animals that carry on a sedentary existence attached to the silt and detritus of the substrate are referred to as benthic.

The zooplankton in your aquarium, which take advantage of these three kinds of watery environments--— pelagic, littoral or benthic--— are very largely composed of three major groups of organisms: single-celled protists, rotifers, and crustacea. As we look in turn at each of these main groups, we are working upwards through the food web.

Protists.

Protists include the whole range of single-celled eukaryotes. Protists don't include prokaryotic bacteria and cyanobacteria on the one hand, and on the other they exclude all multicellular animals, even the simplest hydra or smallest rotifer.

Ciliates are perhaps the major group among the protists. Ciliates are not primitive. That's an old misconception of the Victorian age, when the great ladder of creation, a natural counterpart of the contemporary social hierarchy, placed ciliates near its bottom rung. Quite the converse, ciliates have the most complex structures of any single-celled organisms. Typically they are larger than most protists, too. Metazoans, the multi-cellular creatures like us, have evolved specialised cells for various purposes, and we assemble them into organs. Ciliates have developed in an alternative direction: they have evolved specialised structures within the single cell instead. That makes them fascinating and often beautiful under the microscope. The hairlike "cilia" that give them their name move so rapidly they seem like a blur, and they are so minute they can only be individually distinguished under the electron microscope.

The ciliates don't engage in sexual reproduction, but they can recombine chromosomes through a unique kind of conjugation.

Most ciliates have a mouth, but some kinds make a living digesting materials dissolved in the surrounding water, which they absorb through their cell walls. It's not much of a jump from there to the kind of parasitic absorption of liquid cellular contents displayed by the notorious ciliate Ichthyophthirius multifiliis. Most ciliates, however, are free-living in the plankton, or form part of the biofilm, though some associate with larger organisms, generally hitching a ride as harmless ecto-commensals, say on the carapace of a daphnia. Many ciliates have a cystic life-stage that serves to tide them over in hard times, when food becomes scarce, or even to survive desiccation. Cysts help distribute them from one watercourse to another, perhaps in mud clinging to the feet of water fowl.

Ciliates fill many trophic niches. Individual ciliate species vary so greatly in their tolerance of pollution that biologists can use their presence or absence, as indicator species, to assess the health of the aquatic environment. They are essential "keystone" species in microbial food webs. Ciliates are prime consumers of bacteria or algal cells, diatoms and other members of the smallest plankton. Some are omnivorous. Some of the largest of them prey on other protists or even the smallest multi-cellular organisms. In turn, ciliates such as paramecia become important prey of rotifers or copepods and daphnia--—or of fish fry in their first feeding stages. In their useful role as food for larval fish, ciliates were called "infusoria" by old-fashioned aquarists. Large populations of ciliates are not generally welcome because they are a characteristic sign of organic enrichment of the water ("eutrophication"). The grazing of ciliates and rotifers stabilizes the population of bacteria and of the algae and euglenoids that cause "green water," but when you are plagued by cloudy water in the aquarium, it's mostly ciliates that are to blame.

Links.There's a general zooplankton site maintained by Southwest Missouri State University.

There's an introduction to ciliates at the giant site maintained by U.C.Berkeley Museum of Paleontology-- which is the best portal for the natural history of invertebrates.

Richer, more detailed material is at D.H. Lynn's Ciliate Resource Archive.

Flagellates are the other major group of single-celled planktonic organisms. Some few are photosynthesizers, like Euglæna and its kin. The non-photosynthesizing ("heterotrophic") flagellates are extremely various, making livings by eating bacteria and the smallest protists, or by absorbing their nutrients from detritus, an "osmotrophic" technique that has given an opportunity for some flagellates to become parasites of multicellular organisms. The only flagellate that freshwater aquarists are likely to be aware of is Oodinium, though in marine aquaria the symbiotic photosynthesizing zooxanthellae of some corals and of Tridachna clams are also flagellates.