ENEMIES
Hydra
This little creature, anywhere from a third to one inch in length, is illustrative of how the guppy hobby carries us into related bypaths. Hydra are often introduced into aquaria with pond water and have become the "number one" enemies of guppies. To call them animals may seem like stretching a point if we judge from their appearance. A whole chapter could be written about them-here only the high spots. The animal attaches itself with a sticky substance to the glass or an object in the water. It cannot swim but it can move by a looping movement. The free end of the body has a conical projection called a hypostome on which its mouth is located. At its base is a circlet of long tentacles, five to ten in number. The hydra pulls the fry to its mouth by means of the tentacles, after interesting devices called menatocystshave caught it. Some of the menatocysts contain long spines in long rows. The prey is pierced and paralyzed by the spines which inject poison. Other cells have thread tubes in them which can lash out and wind around any protuberances on the fry to hold it securely, aided by a glue-like substance which it secretes. In a daphnia culture hydra create havoc. Hydra, when at rest, contract into soft balls. After eating, too, they retract into irregular globular forms. Because they reproduce both by budding and by fertilized eggs, hydra breed very rapidly. They are difficult to eliminate from a tank by washing it or even drying because their eggs have a hard outer shell which resists both drying and washing. The contents of the shell come to life quickly once favorable conditions are established.
A heavy infestation of hydra can destroy or retard the growth of many fry both by killing them and by eating their food which starves them. Hydra are easily seen protruding from their attachments and swaying about.
Treatment: (1) Hydra cannot stand chlorine or heat. If you have city water, remove the fish, drain tank and fill with fresh water. Return guppies two or three days later to the tank. (2) Remove fish, raise water temperature to 105' for 24 hours. When the water has returned to normal, return the fish. (3) Use of ammonium nitrite or ammonium sulphate at the rate of 5 grains per gallon of water will destroy hydra. Dissolve crystals in small amount of water first, then pour this solution into the larger tank. In 3-5 days all hydra will be dead, the guppies remaining unharmed by the solution.
Almost all of the other guppy enemies are introduced unintentionally along with tubificids, daphnia or live plants. and belong to the insect kingdom. Nost of them are outdoor enemies, invading pools or large tanks in which fish are kept. There is no treatment for them that will not additionally effect your fish, so all you can do is put mosquito screening over the tanks to prevent entrance by the insects. Here are the principal insect enemies: Dragon fly larvae, Damsel fly larvae, Dobson fly larvae, Water scorpions, Water boatmen, Back swimmers, Water tigers which are larvae of whirligig beetles, Water scavenger's larvae, Predacious diving beetle larvae and the Giant water bug.
If you are not acquainted with these insects, read about them in a good book which you may obtain at your local tropical fish store or the public library.