Tropical Fish Hobbyist

The Planted Tank
Aquarium Invaders

aquarists most When think of freshwater invertebrates, those that come to mind most often are probably shrimp and snails, or perhaps crabs or crawfish. There are, however, many other invertebrates that live in fresh water and can be introduced to the home aquarium, planned or not. Many animals can come in on plants, and it is particularly important for those of us keeping planted tanks to be aware of what animals might pop up in them unexpectedly, and what to do about them.

hydra food as well. I do occasionally see hydra in my tanks, but they have never become populous enough to be a problem. Gouramis are one possible solution, as there have been claims that they eat hydra. There are also several commercially available medications reported to be effective on them. In most decorative planted or community aquariums, just keeping the tank clean and not overfeeding should keep hydra from multiplying to the point that medications are needed.

Rhonda WILSON
photographs by the author

Rhonda Wilson keeps about 80 planted aquariums collected over more than 30 years of fish keeping. She is co-author, with Terry Barber, of the T.F.H. book, The Simple Guide to the Planted Aquarium. She’s also written several articles published both in the US and abroad. Rhonda has been active in local and national fish and aquarium clubs including serving as past chairman of the American Livebearer Association. She has a lifelong love of animals and nature. She currently lives with a zoo of children and pets in the Phoenix area of Arizona. You can see her website and post questions on her bulletin board at http://naturalaquariums.com.

Hydra

Hydra look like very tiny sea anemones, and the two are actually related. Hydra, along with jellyfish, corals, and sea anemones, are among the animals in the phylum Cnidaria (formerly Coelenterata). Hydra are biologically unusual in that they don’t have eyes, yet they can sense light. They can be white, brown, or green, and can attach to almost anything in the aquarium, such as plants, decorations, gravel, and the tank itself—they can also reproduce easily in the aquarium.

Hydra are carnivorous and eat pretty much any animal they can manage to catch. They can kill small fry and will be happy to eat any live foods you are trying to feed your fish. To kill their prey, hydra shoot individual stingers called nematocysts from their bodies and tentacles. The nematocysts paralyze and kill the prey, which are then digested by the hydra.

Hydra can do particularly well in the aquarium if there is an abundant source of small live foods. This can be a devastating combination for some fish breeders: the food for the fry ends up being eaten by hydra, while the fry themselves become

Moss Animals

More correctly called bryozoans, moss animals are widespread colonizing animals that are also abundant in the fossil record. Most bryozoans are marine, but some have moved into fresh and brackish waters, and so some can find their way into your planted aquarium.

Bryozoans are made of colonies of individual zooids, and the bryozoans most often seen in the aquarium form small branching colonies. Some species form very different types of colonies—for example, very large ones that look like giant egg masses, which are found in some lakes.

The presence of moss animals in the aquarium usually lasts no more than a few months, so they shouldn’t be a problem— although in some aquariums they may occur somewhat regularly. Colonies will not only grow, but sometimes zooids will produce statoblasts, functioning as an asexual form of reproduction. Statoblasts, which are somewhat similar to seeds in plants, are resistant to adverse conditions and can germinate to form a new colony when conditions improve.