Hydra (biology)
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Hydra americana |
Hydra is the genus name of a simple, fresh-water animal possesing radial symmetry. It is a member of the phylum Cnidaria and the class Hydrozoa.
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Form
Hydras are small animals with a body length ranging from 1 mm 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 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. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release.
Hydra mainly feed on small aquatic invertebrates such as Daphnia. Some species of hydra exist in a symbiotic relationship with a type a green algae. The hydra offers the algae protection from predators and in return, the algae uses photosynthesis to give the hydra a food source.
Morphology
Hydras have two main body layers separated by mesoglea, a gel-like substance. The outer layer is the epidermis and the inner layer is called the gastrodermis. The cells making up these two body layers are relatively simple cells.
The nervous system of Hydra is a nerve net, which is simple compared to mammalian nervous systems. They do not have a recognisable brain. Nerve nets connect sensory photoreceptors and touch sensitive nerve cells that are found in the body wall and tentacles of hydras.
Respiration occurs by diffusion through the epidermis. Some excretion and transportation also occurs in this manner.
Many members of the Hydrozoa go through a body change from a 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 regroup to form 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. Due to the simplicity of the nerve net, hydras generally react in the same way, regardless of the direction of the stimulus.
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 and simply break away when they are mature. When conditions are harsh, often before a cold winter, sexual reproduction occurs in some hydra, producing unfertilized eggs. These eggs are then fertilized by sperm from testes which form on the external surface of the stalk. The fertilized eggs secrete 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 manuevered 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 have already joined in the attack to kill struggling prey. Within 2 minutes, the tentacles will surround the prey and move it into the opened mouth aperture. Within 10 minutes, the prey will be enclosed within the gastrovascular cavity and digestion will have started. The hydra is able to turn itself inside-out in order to digest prey more than twice its size. 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, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas.
References
- Gilberson, Lance, Zoology Lab Manual, 4th edition. Primis Custom Publishing. 1999
- Solomon, E., Berg, l., Martin, D., Biology 6th edition. Brooks/Cole Publishing. 2002he:הידרה (ביולוגיה)
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