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Hydra are small aquatic invertebrates belonging to the order Hydroida, in the class Hydrozoa, phylum Cnidaria. This is the same phylum as the marine jellyfishes, corals, and sea anemones. Only a few species that belong to this phylum occur in freshwater due to the fact that they evolved in the sea. The body is a thin cylinder with several tentacles extending from the mouth (Five to seven tentacles are typical).
The name, Hydra, is derived from a monster in Greek mythology. One of the 12 tasks imposed on Hercules was to slay the Hydra of Lerna. It had nine serpent-like heads, one of which was immortal. So potent was its venom that even its smell was fatal to those who passed too close. Worst of all was that for every head Hercules cut off, two more grew in its place. He finally triumphed by cauterising the wounds with a firebrand as he chopped away. Just like the legendary monster, a single hydra may be cut into many pieces and if each piece contains a portion of the two body layers, ectoderm, and endoderm, it will develop into a complete animal.
In 1740, Abraham Trembley discovered the green species of hydra, Chlorohydra viridissima, and couldn't decide whether it was a plant or an animal. It was as green as a plant, yet it could move like an animal. To settle the question, he cut the polyp in half. It was known at the time that lizards could regenerate a tail and crayfish a claw, yet Trembley believed only a plant could regrow as much as half of itself. Tilting toward the animal theory, he was amazed to find the head half with a new bottom and the bottom half with a new and complete head. Continuing his regeneration experiments, he cut the polyp's head lengthwise in tow and the polyp grew two intact heads. He repeated the cuts until he had a seven-headed polyp and naturally enough compared his multi-headed creation to the mythological monster. In 1758, Linneaus, finding Trembley's comparison fitting, gave the name Hydra to this genus of freshwater polyp.
Hydra are commonly found hanging inconspicuously from vegetation or from the water's surface film in most streams, lakes, and ponds. Fully extended, its tentacles sweep the water for its prey, sometimes extending to a length of 5 cm. They consume small crustaceans, insect larvae, worms, or other tiny animals. In aquaria, they will thrive on any similar live food being fed to small rainbowfish fry - including the fry. Any aquarists who feed copious amounts of live brine shrimp nauplii will eventually find their tanks carpeted with hydra.
To capture these minute forms of life and to defend itself from larger animals, hydra are equipped with poison-containing structures in the ectodermal layer called nematocysts, the most complex cellular organs in the animal kingdom. Judging from its incredible potency and speed of paralysis and death, the poison is believed to be a nerve toxin. They have four different types of nematocysts, some stinging and paralysing the prey, whilst others discharge threads which coil round the prey and hold it. Small animals paralysed by the stinging cells are brought to the digestive centre by the tentacles and devoured. The mouth of the hydra is located at the centre of the tentacles. The body is hollow and the inner layer of cells digests the food by engulfing it. With prey almost as large as itself, the hydra stretches so thin in getting its digestive gut around it that the prey animal appears to be covered with a thin transparent film; only the tentacles tell you that you are looking at a hydra, although they generally keep their tentacles coiled up. A hydra that has recently fed will be shorter and more rounded than one that is hungry.
A number of different species of hydra are known, and they come in a variety of different colours such as green, grey, black, brown, orange, and transparent. The Green Hydra (Chlorohydra viridissima), lives in a symbiotic relationship with the small alga Chlorella vulgaris living within the body of the hydra. The algae make use of those substances that the hydra would normally excrete, such as carbon dioxide, phosphates and nitrogenous substances in this symbiotic relationship. For its part, hydra benefits by receiving the oxygen produced by photosynthesis. If there is a food shortage, the hydra will digest the plant cells and later ingest more living algae, which it does not digest. Due to the presence of these symbiotic algae, this hydra is attracted by light, contrary to other species and can survive long periods without food.
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Reproduction in hydras occurs most of the time by budding; in the photo above, you will see a brown hydra with two other budding hydras already completely formed but still attached. Photo: Jan Parmentier, Netherlands.
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Hydra reproduces both asexually and sexually. Asexual reproduction takes place in the winter in a process known as budding; the hydra simply forms a bud on the side of its body. Buds are produced every two to three days under favourable conditions. These simple organs are no more than lumps attached to the body of the female hydra. The embryo secretes around itself a hard, sticky shell and is carried by the female until the young hydra develop tentacles, and then breaks off, fully prepared to exist in its own. Hydras often have four buds at a time. Sexual reproduction occurs in the summer, though the brown hydra (Hydra fusca), prefers the autumn months. When reproducing sexually, one hydra forms testes to become a male, while the other develops ovaries. One thing that can cause sexual reproduction is the amount of carbon dioxide in the water.
In aquaria, hydra can be found attached to plants, gravel, stones, the sides of the aquarium, filter equipment, etc. They can also float at the surface of the water buoyed up by gas bubbles given out by the basal disc. They have a flexible cylindrical body and at one end is the "foot" by which the animal anchors itself to a surface. On the opposite end are from five to ten (sometimes more than ten) arms or tentacles, gently swaying in the water current. When disturbed they will rapidly contract into a blob of jelly, making them difficult to find.
Hydra can be introduced to the aquarium with live foods, snails, driftwood, plants, or water collected from natural freshwater areas. Most hydra in a normal aquarium go unnoticed, but within the confines of a small fry-raising tank, these little pests can be deadly, and can ingest a tankful of newly hatched rainbowfish larvae in less than a week. Hydra can kill small fry up to a size of around 10 to 15 mm. Newly hatched larvae of rainbowfishes and blue-eyes are bite-size for the average hydra. Larger fry can often pull away from the stinging tentacles, but will usually die in any case. Fry over 15 mm, however, really don't seem to have any problems. Hydra also competes with the fry for the live food thereby reducing growth rates.
Hydra only seems to appear in fry tanks being fed brine shrimp nauplii or similar live foods. They don't seem to appear in fry tanks that are fed primarily a dry or liquid diet. However, once you start feeding large amounts of brine shrimp nauplii, it isn't long before hydra appear, often in enormous numbers. They are usually tan or brown in colour and are not readily seen against a background of natural coloured gravel or on plants. Usually you don't notice them unless a heavy infestation has become established.
Most hydra are about 0.25 to 2.5 cm long and are fully capable of moving from place to place and are often seen free-swimming (floating) on the water surface in the aquarium. They probably remain in a free-swimming stage until they find a spot suited to their needs.
Finding a few hydras in your regular aquarium doesn't mean that the aquarium is unhealthy. Actually, it means you have a normal healthy aquarium, since hydra will not live in water of poor quality. A well-maintained aquarium will not provide enough food for a large number of hydras so keep the tank clean and you will probably never know that they are there. The best way to avoid introducing them into fry-growing tanks is to sterilise the tank and filter equipment with a chlorine solution before they are used.
In the past, the only way I found to eradicate hydra successfully was to sterilise the tank with a chlorine solution. Some copper medications claim that they will kill hydra but they can also be deadly to the fry. Other treatments usually recommended include ammonium nitrate, quinine sulphate, raising the temperature to 40°C for 15 minutes or more, and adding salt until the hydra are killed. An even more elaborate treatment I read once was the use of a 9V battery connected to leads that fed into the aquarium water. Most of these treatments simply do not work and in addition, they can be harmful to the young rainbowfish fry.
Dactycid (a product available in Europe) is an effective treatment against gill and body flukes, and other types of internal worms (hookworms and roundworms) including Camallanus cotti, hydra and planaria. The active ingredient is flubendazole. This product was available in powder form in capsules when I was using it, but I think it is only available in liquid form these days. Use according to manufacturer's recommendations.
Flubenol 15 (15%) is a product available in the United Kingdom. This product is used for the treatment of skin flukes, gill flukes, camallanus worms, tapeworms, anchor worms and other helminthics commonly found in tropical and coldwater fish. It has also been used successfully with 'skinny disease' in clown loaches.
Both of the products above have Flubendazole as the active ingredient. Flubendazole is a veterinary drug and can be found under a number of synonyms: Fluvermal; Flubenol; Flumoxal and Flumoxane. Flubendazole 5% is very effective in killing hydra and is very safe to use with small rainbowfish fry used at 1-2 mg/L. I have used this chemical with rainbowfish fry only 10 days old, without any problems. Pre-dissolve the flubendazole 5% in a container with some aquarium water, and then pour it in the aquarium. After 5-7 days, following treatment, do a 50% waterchange. Flubendazole is a "wettable powder" and as such does not completely dissolve. It may leave a white residue in the aquarium, but this can easily be removed with following waterchanges. Remove any deposit of white powder left on the aquarium walls with a sponge and siphoning the bottom with a gravel siphon. I did not have any problems with the residue and it also had no effects on brine shrimp nauplii when fed to the fry. However, this chemical is very toxic to snails.
Flubendazole is not available in Australia as far as I can ascertain. I use to get the chemical from Europe. However, a number of other products have been reported to have similar effects on hydra and other worms. Fenbendazole is a similar chemical that has also been reported as an effective control for hydra. This chemical is available in various formulations and trade names. Praziquantel is another chemical that has been reported to be effective against hydra and other aquarium worms at a recommended dose of 2 mg/L. Praziquantel is available as an aquarium formulation under a number of brand names. However, before using any of these products consult a veterinarian.
© Copyright Adrian R. Tappin
Updated February, 2007
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