Not all the plankton are utterly invisible. The largest free-swimming rotifers are just discernible, if you set a glass of tank water into a sunny window. They're too small to count as visitors (in the "Invertebrates" folder), but they're a major component of the food web.

Rotifers are the most abundant and cosmopolitan of freshwater zooplankton. Eventually they will turn up, even in an unplanted aquarium, partly because they can resist desiccation by secreting a protective gel envelope and blowing in from outdoors in a spore-like fashion. Rotifers can survive freezing as well as drying, in a kind of suspended animation, which has enabled them to be distributed, windblown, from deserts to the poles, wherever there is occasionally enough free water that lasts long enough for rotifers to hatch, feed and reproduce. Which isn't very long.

Rotifer means "wheel-bearer," because a rotifer's mouth is surrounded by many cilia, moving too fast to be individually distinguishable but working in coordination to whirl suspended particles of food into the mouth. So rotifers count as "filter feeders." In fresh water, we don't generally have many larger, more obvious filter-feeders, which are so characteristic of coral reef systems, unless we happen to keep freshwater mussels or Atyid shrimp.

Though rotifers are multicellular creatures, they are on the same scale as ciliates: they average 0.1 to 0.5 mm long, the largest of them just visible with a 10x magnifier. Though rotifers are universally distributed in the freshwater plankton, each ecotope supports a limited range of species that are particularly well-suited to that particular environment. In your aquarium, you'd expect to find many rotifers, but of only a few species.

Rotifers are among the primary grazers in fresh water. (There are few marine rotifers, perhaps fifty species.) They subsist on bacteria and on the various photosynthesizing organisms. So planktonic rotifers join with the ciliates in keeping aquarium water from turning green. Sessile rotifers attach with a sticky foot to plant surfaces (making them literally "littoral"), or in the interstices of the biofilm, and on flocs of humus and grainy sediment. Rotifers are colorless, but a recent meal can tint them rusty brown or green.

The 2000 species described so far can't be more than a fraction of the rotifers that exist. Rotifer species run a broad gamut. Rotifers provide a major food source for fish larvae, but they also participate in the decomposition cycle. Proales species colonize the outer shells of daphnids and can settle quickly onto snail eggmasses or hitch a ride on the globular colonies of Volvox. Free-living rotifers feed on the bacteria that coat suspended organic floc. Some prey on single-celled protists and even on other rotifers. But Albertia is a parasite in the digestive tract of annelid worms (though harmless to fishes).

 Link. A good brief not-too-technical introduction to the biology of rotifers is Roy Winsby's article "Rotifers and how to find them" archived at the Microscopy website.

Zooplankton: Crustaceans.

Crustacea are a huge division of the arthropods. Crustaceans fill most of the roles in water that insects fill on dry land.

Copepods are Crustacea. You'll be doing well if you can divide them into two major orders: Cyclopoid and Calanoid. The Cyclopoid copepods are the ones "like Cyclops," a species of copepod favored by old-time fishkeepers as a food for fry. Thus Cyclops stands in for the whole group. In tropical freshwater, there are comparatively few copepod species, but those species are widespread. One small wet campo on a fazenda belonging to the University of Brasilia supports more than 30 species, a current world record for copepod species richness at a single location. Copepods play varied roles in the trophic web. Some are filter-feeding grazers in detritus, or feed on bacteria and other smallest organisms. When they are present at the top of the planktonic food web, their predations help clear your hazy water. Other rotifers are predators that can even attack the smallest fry of some fish. And in a more sinister vein, still others are vectors of disease, as the intermediate hosts of some parasites that mature in fish that eat the copepods.

One way or another, copepods concentrate the nutritional value of bacteria and plant matter into a package available to young fish, and this is their most important ecological function. Copepods form a major food of young fish, once the fry have reached a size where they can turn the tables on their former predators. We often intrude at this stage and substitute newly-hatched brine shrimp.

Daphnia and its kin ("daphnids") are also crustacea. Primarily freshwater creatures, they are present in pelagic, littoral and benthic communities. They are so diverse that they are grouped by taxonomists into four Orders. In the aquarium, our fish pounce so quickly on them that they rarely get a chance to take up their functional niche in the ecosystem, unless we loose them in a fish-free tank that's plagued by green water, which they will clear in no time at all. In fact, part of the stability of a mature aquarium is based on this interaction between consumers and the algal resource; Daphnia or copepods will keep the algal resource far below the level set by nutrients and light. Prof. William Murdoch at UCal Santa Barbara is at work on this phenomenon: "In lakes where Daphnia predators are not important, the zooplankter suppresses the algae far below the limits set by nutrients... Such systems are stable over a wide range of stability." He's out to explore this stability. See his brief notes at the U.Cal Santa Barbara website.

The Environmental News Network reported in Oct 1999 that Daphnia in Lake Constance (between Switzerland and Germany) have adapted in the last thirty years to add toxic cyanobacteria to their diet, that they have developed metabolic ways to detoxify it, and that they're even keeping it under control in this somewhat polluted lake. (Yes, I know! wouldn't it be great!)

Links. The Cladoceran homepage is slanted towards biologists, but it includes rarified anatomical details of Cladocerans and lots of handy links.

If you have a microscope at hand and you're curious about Daphnia, Richard Fox provides a description of Daphnia anatomy, though Dapnia are environmentally so plastic that amateurs are unlikely to be able to identify them by species.

Here at the Skeptical Aquarist, you'll find more information on daphnia as a live food.