Part Two

Discovering the Abundant Life in a Tank of Pond Water

Early spring has become a time for discovery in our household. We haul our aquarium and its wooden stand up from the basement and set it up in the living room. When my husband and I graduated from a 10-gallon tank to raise tadpoles and salamander larvae to a 30-gallon one, we allowed for much more airspace in the tank. This would prove to be a wise decision, as many of the creatures in our "pond microhabitat" metamorphosed into airborne insects.

Wiping out the dust from the inside of the tank, we arrange a pile of rocks as a focal point. The rocks will serve as an underwater shelter for many creatures. When moss and bark are added to the top surface of the pile, a place is created for froglets to emerge from the water. Branches, secured so that they will protrude from the water, provide the necessary metamorphosing perches for the flying insects. Scooping water from our backyard water garden, a neighbor's vernal pool, or a nearby pond, we include the leaf litter and muck, since an abundance of creatures exist there. We top it off to a depth of about five inches with tap water (well water containing no chlorine or water softeners). The small aquarium air pump is installed and the screened lid secured. Now it's time to sit back and wait for the soup to settle.

A friend had recommended we get our hands on the Golden Guide, Pond Life, to aid in our identification of aquatic life. This little gem of a book is loaded with illustrations and descriptions of plants, animals and fungi found in North American ponds and lakes, including microscopic life forms. It would become our most valuable tool in discovering what was residing in our aquarium.

First discovery: we had snails; boy, did we have snails. From the typical conical-shelled snails to the tiny 1/8" flattened shells of freshwater limpets, the tank was snail heaven. When the limpets traveled along the glass sides of the aquarium we were privy to what was going on under their shells: a rasping mouth worked along the surface of the glass, consuming algae or decaying matter. Peering through a hand lens at such a mouth, one can see where science fiction writers get their ideas. Over time we found small clear egg masses attached to the leaf litter or the tank glass. Minute shelled snails developed within.

Hydras, fascinating animals related to marine corals, sea anemones and jellyfish, were attached to surfaces like leaf litter or the tank glass. The ones in our aquarium were no larger than one-half inch. We had two species, one that was white, and one that was green from the algae living within its cells. Tentacles at the top of the body reach out to capture prey. We watched as mosquito larvae and other tiny creatures became entangled in the hydra's tentacles and were consumed. One way hydras reproduce is by forming buds that branch into a new, smaller version of the adult animal. We had many branching hydra in our tank.

One year, in the silt and coarse sand at the bottom of the aquarium, we discovered a small patch, probably an area no larger than two square inches, of tiny dancing worms. They protruded from the silt, these fine red strands of undulating life - Tubifex worms! The Pond Life guide says that they are tube builders and their heads are buried in their silt tubes while their tails wave above. Are these the same tubifex that are dehydrated and used as aquarium fish food? They were great fun to watch, as the slightest disturbance would cause them to zip down into their tubes in unison. When all calmed down, their tails would slowly emerge again and begin the dance anew.

Another year, as a shaft of sunlight shone through the tank, a small milky cloud formed in the water. Tracing it to its point of origin, I saw that one of the 1/4" fingernail clams was shooting out minute particles from between its shells. Was it spawning?! Usually by the end of the season, when we empty the aquarium back into the pond or vernal pool where we originally got the water from, the fingernail clams in the tank have died, so I didn't hold out much hope for reproductive success within the aquarium. Either fingernail clams have a very short lifespan, or the tank was not the perfect re-creation of the habitat they needed. Still, we found them very interesting to observe.

Several years we watched a whitish growth, usually on leaf litter, once on the tank glass, form in the aquarium. Freshwater sponge! Now how did that get in there? The Pond Life guide says sponges are colonial animals and "feed on floating or swimming microscopic animals and plants that are trapped in their pores as water circulates through." The colony dies in the winter, but not before the sponge drops gemmules to the pond bottom. In spring each gemmule becomes a new sponge colony. We must have scooped up a gemmule from our water garden.

Isopods, crustaceans related to land-dwelling pill bugs, helped to break down the leaf litter and other decaying matter in the tank. They thrived, providing necessary food for larger predators in the tank.

Moving through the water were many minute creatures that required a hand lens to observe. The aquarium was teeming with tiny crustaceans, such as copepods, ostracods and water fleas. One group of copepods with a terrific name, Cyclops, inhabited our tank. Yes, they have one eye in the middle of the head. The females carry eggs in a pair of sacs, like saddlebags. Ostracods, also known as seed shrimp, actually look like microscopic clams that zip through the water. Within the bivalve is a shrimplike creature with legs and antennae that protrude from between the shells, aiding in its propulsion through the water. The ostracods are never visible in the aquarium when we first add the pond water in April. But within a couple days the tank is swimming with them. Do they hibernate in the winter as mature adults, emerging when the water warms? We'll need to look into that. We've also observed that they disappear from the tank, and natural bodies of water, by early summer. A professional naturalist I spoke with said he has observed this as well.

Water fleas, like the Daphnia in our tank, move in a jerking motion through the water. With a hand lens you can observe their internal workings, including the eggs within the female, through their transparent bodies. All these crustaceans feed on algae, bacteria and organic debris. They, in turn, are food for many animals, including hydra. I once saw a red water mite grab an ostracod, eat the creature, then drop the empty shell. Life in the food chain.

I experienced an epiphany of sorts while viewing ostracods through a hand lens. As I was watching their behavior, lost in their world, an enormous creature swam by. A shark, a whale?! No, a tadpole. It took my breath away. There is an entire world that lives and survives where tadpoles are the megafauna! And it hadn't really dawned on me until that moment of clarity. Biodiversity is so much more than lions and tigers and bears. There are worlds of life on this planet that survive in a puddle of water, in a handful of soil, in a decaying log. How wonderful is that?

Over the years we have had many larval insects develop in our aquarium. Crane flies and mayflies emerged as adults from our tank, often before we knew the nymphs were in the water. Caddisfly larvae would crawl along the bottom, living within a protective tube fashioned from bits of leaves, twigs, sand or pine needles. (Each species uses specific materials and construction design and can be identified by them.) We had an adult caddisfly emerge once in June, a fairly drab brown mothlike insect that did not inspire in us the same fascination as the larva.

On the other hand, both larval and adult stages of the damselflies and dragonflies were equally captivating. The damselfly nymph, with its three paddle-shaped gills extending from the tip of its abdomen, looks so delicate, yet it is a voracious predator. Like the dragonfly nymph, it will capture and eat almost anything that moves. Luckily, the damselfly nymphs in our tank were too small to be a threat to the wood frog tadpoles. Not so the tremendous dragonfly nymph that was hiding under the rocks! We did not know it was in the tank until I saw it grab a tadpole and immediately begin to eat it. I realize that this is, once again, life in the food chain, but we wanted to watch the tadpoles' different stages of development. We'd have no tadpoles left within a week if we didn't do something. That was the year that we bought a tank divider to keep the dragonfly nymph in one half of the tank and the tadpoles in the other. We supplemented the nymph's diet with small earthworm pieces and it continued to grow and molt. After thirteen weeks in our tank, it emerged. I came into the living room one morning to find a gorgeous dragonfly perched on one of the protruding branches. It was a darner species, though I didn't keep it long enough to find out which one. When it was ready, I released it. (In later years, we knew there was a dragonfly nymph in the tank by the small dark splatters on the inside of the tank glass in the airspace. You see, the splatters were dragonfly nymph excrement. The nymph will raise the tip of its abdomen and shoot the excrement into the air. More than you wanted to know?)

Gliding along the surface of the water we had small water striders, which are true bugs. True bugs have piercing and sucking mouthparts. The young are smaller versions of the adults. We watched these small striders molt their outer skin several times as they grew larger. One of my favorite things was to feed them by taking the mosquito, or horse fly, that had just bitten me, and drop it into the aquarium. The water striders would converge on it and suck it dry.

Finally, a creature that grows on you, no pun intended. The leech. The first time that I scooped one from our backyard water garden, I was mildly surprised. Of course, it was only a half-inch long. But when a writhing four-inch monster surfaced with a scoop of leaf litter, I was astonished. How can something so big live in a 3 X 5 foot body of water? I put it in the aquarium. Some leeches are carnivores, some scavengers, and some parasitic bloodsuckers. Wishing to find out what this one was, I asked my husband and a friend to stick their hands into the aquarium water for awhile. They declined. From the illustrations in the Pond Life guide, this leech looked like an erpobdella, which feeds on invertebrates, fish, frogs, and occasionally, humans. I won't go into the sorry details of adding a goldfish to the tank as an experiment, but I will say that the leech showed no interest in it whatsoever. It did attach its mouth parts to an earthworm, presumably sucking sustenance from it. Twice we've observed a leech actually consume earthworm when it was offered in small pieces. Minutes after eating, one of the leeches undulated against the tank glass, a small section of its body constricted by something holding it against the glass. It slipped its body from what was holding it and we saw that it was a clear oval egg sac attached to the glass! The leech then used its mouth to secure the edges of the sac in place. Inside were about eight miniscule white eggs. By the next day the papery egg sac had turned brown. How long does it take for a tiny egg to grow to be a four-inch leech? If anyone has the answer, we'd love to know.

So there you have it, a fascinating alternative to television on spring and summer evenings. I recommend a hand lens and a flashlight for better viewing, and a mind open to the fact that there are countless amazing creatures that we share this wondrous planet with.

Submitted by Cindi Kobak

Go to Microhabitat, Part 1

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Last updated 18 September, 2004 .