Hydra americana
Hydra attenuata
Hydra canadensis
Hydra carnea
Hydra cauliculata
Hydra circumcincta
Hydra hymanae
Hydra littoralis
Hydra magnipapillata
Hydra minima
Hydra oligactis
Hydra oregona
Hydra pseudoligactis
Hydra rutgerensis
Hydra utahensis
Hydra viridis

Hydra is the genus name of a simple fresh-water animal of the phylum Cnidaria and the class Hydrozoa which is radial in symmetry.

Form

Hydras are small animals with a body length ranging from 1mm to 20 mm when fully extended. They have a tubular body secured by a simple adhesive foot. At the free end of the body is a mouth opening surrounded by a ring of 5 to 7 ( to 12) thin mobile tentacles. Each tentacle is clothed with highly specialised stinging cells called nematocysts. Nematocytes look like miniature light bulbs with a coiled thread inside. At the narrow, outer edge is a short trigger hair. When this is touched, the contents of the nematocyst are explosively discharged firing a dart like thread containing neurotoxins into whatever triggered the release.

Their main prey are small aquatic invertebrates such as Daphnia

Morphology

Hydras have two main body layers separted by a gel like substance, the mesoglea . The outer layer is the epidermis whilst the inner layer is called the gastrodermis . The cells making up these two body layers are relatively simple cells but frequently include coloured chloroplasts or possibly unicellular algae which gives hydra species their distinctive colours.

The colelentron , also known as the gastrovascular cavity, is the place where the digestion process occurs.

The nervous system of Hydras is a nerve net, which is simple compared to mammalian nervous systems. They do not have a recognisable brain. Nerve nets connect sensory photoreceptor and touch sensitive nerve cells that are found in the body wall and tentacles of hydras.

Many members of the Hydrozoa go through a body change from polyp to an adult form called medusa. However all Hydras remain as a polyp throughout their lives.

19th century biologists reported that Hydra was so simple an animal that it was possible to force an animal through gauze so as to separate it into individual cells and then, if the cells were left to themselves, they would re-group and reform a hydra again. This experiment has never been repeated succesfully in the 20th or 21st centuries - all that is produced is Hydra soup. A similar experiment with some sponges may be more successful.

Motion and locomotion

If Hydras are alarmed or attacked, the tentacles can be retracted to small buds and the body column itself can be retracted to a small gelatinous sphere.

Hydras are generally sedentary but they do move location quite readily. They do this by bending over and attaching themselves to the substrate with their mouth and tentacles and then release their foot which provides the normal attachment. The body then bends over and makes a new place of attachment with the foot. By this means a hydra can move several inches (c. 100 mm) in a day.

Reproduction

When food is plentiful, many Hydras reproduce asexually by producing buds in the body wall which grow to be miniature adults which simply break away when they are mature. When conditions are harder or when winter approaches, sexual reproduction can be triggered with some hydras producing an egg and some producing sperm from testes which form on the external surface of the stalk. Free swimming sperm is released into the water and fertilizes eggs in other Hydras. The fertilised egg secretes a tough outer coating and, as the adult dies, this resting eggs falls to the bottom of the lake or pond to await better conditions when it will hatch once again into a miniature adult.

Feeding

When feeding, Hydras extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, the tentacles of hydra are extraordinarily extensible and can be 4 - 5 times the length of the body. Once fully extended the tentacles are slowly manouevered around waiting for a suitable prey animal to touch a tentacle. Once contact has been made, nematocysts on the tentacle fire into the prey and the tentacle itself coils around the prey. Within 30 seconds most of the remaining tentacles are also engaging with the prey which may still be struggling. Within 2 minutes all the tentacles will have been deployed, surrounding the prey and moving it into the opened mouth aperture. Within 10 minutes the prey will be enclosed within the gastrovascular cavity and digestion will have started. After two or three days, the undigestible remains of the prey will be discharged by muscular contraction through the mouth aperture again.

The feeding behaviour of the Hydra demonstrates the sophistication of what appears to be a simple nervous system.

Hydras are beautiful low power microscopical objects and well worth study by biologists. They can be found in most freshwater ponds and lakes in the temperate and tropical regions by gently sweeping a collecting net through weeds in clean, healthy ponds or very slow moving rivers.

References

• Gilberson, Lance, Zoology Lab Manual, 4th edition. Primis Custom Publishing. 1999
• Solomon, E., Berg, l., Martin, D., Biology 6th edition. Brooks/Cole Publishing. 2002