Hydra vulgaris

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Equally poisonous is the bite of the stinging cells of the hydra, capable to paralyze in a few instants preys much greater than the hydra itself. The genus Hydra is diffused in the fresh waters of the temperate and tropical zones of all the continents. To the group Hydra vulgaris belong numerous species and subspecies of uncertain distinction present in all the continents.

The hydra lives in clean and well oxygenated waters of ponds, small lakes and also resurgences. In Italy, the species is relatively common, even if the pollution has strongly reduced its presence. Despite its diffusion, the hydra is very difficult to observe in nature, since it lives anchored usually on the aquatic vegetation.

Its small size and the fact that, if upset, it contracts further reducing the size render very difficult its finding. In Italy, besides Hydra vulgaris is present Hydra oligactis Pallas and the Green hydra Hydra viridissima , Pallas, , sinonimi Chlorohydra vividissima Pallas, , Hydra viridis Linnaeus, This species is very interesting as it presents an obliged symbiosis with a unicellular green alga, of the genus Chlorella.

The algae are hosted inside the cells of the gastroderm. The nourishing substances the hydra gets from the symbiont help remarkably its metabolism and this species is a less greedy predator, and its nematocysts are smaller than those non symbiont. The predators of the hydras include plathelminths, crustaceans and aquatic insects. At the centre of the hypostome opens the mouth, strongly extensible, which leads to the coelenterons, the cavity occupying the whole length of the body, with digestive function.

Through the hypostome, which is the only opening, are ingested the preys and are egested the digestion residuals. The body as well as the tentacles is formed by only two layers of cells it is a diploblastic organism , the ectoderm, external, and the gastroderm, inner, separated by a non cellular gelatinous layer called mesoglea. The gastroderm is rich of glandular cells producing digestive enzymes which go into the gastric cavity. The ectoderm contains myoepithelial cells, with covering and contraction function, nervous cells and stinging cells typical of the phylum Cnidaria , the nematocytes or cnidocytes.

The nematocytes contain the stinging organs, called nematocysts or cnidocysts. The tentacles are extremely mobile and extensible: when contracted they seem small protuberances of the head, whilst, when extended they may reach a length five times more than the length of the body. Around the opening of the hypostome is present a ring of nervous cells. Despite the extreme anatomical simplicity, the nervous system of the hydra is formed by neurons functionally overlapping to those of the vertebrates.

Also the system of transfer of the nervous impulses between cell and cell utilizes the same mechanisms operating in more complex organisms and the molecular mechanism of the synapses uses most part of the neurotransmitters and neuromodulators known in the mammals. For the locomotion, the hydra utilizes three different methods. The first, described in detail by Trembley, consists in a serious of somersault movements: keeping the foot anchored to the substratum, the hydra bends the body to touch a support with the tentacles.

Now it releases the foot, does with the body a complete turnaround and anchors again the foot further on, and then releases the tentacles. The manoeuvre repeats with a series of somersaults. A second mechanism consists in anchoring the tentacles to the substratum, releasing the foot. Now the hydra walks on the tentacles, using them as legs. Now, the hydra is transported by the water currents. The hydra is an efficient predators mainly nourishing of small crustacean such as Daphnia or Cyclops.

In captivity, it can be easily fed with nauplii of Artemia salina. The seizing of the preys, often bigger than the hunter, is borne by the nematocysts of the tentacles. When the preys touches a sensory cilium cnidocil protruding from the stinging cell, the nematocyte or cnidocyte , the explosion of the nematocysts takes place.

There are different types of nematocysts, but the one performing the key role is the penetrating nematocysts, or stenoteles. This is equipped with an acuminate dart, formed by three merged stylets, which is everted at vey high speed, perforating the teguments of the prey. The three stylets then bend outside and from the nematocyst is everted a long tubule which releases into the body of the victim a cocktail of toxins which quickly paralyze the prey.

Other types of nematocysts present in the battery cell help in holding the prey. Now the tentacles begin a series of movements carrying the still alive prey up to the hypostome which will proceed to the ingestion.

Often when the prey is too big and resistant, like a Daphnia , equipped with acuminate chitinous appendages, the ingestion is very laborious and at times the walls of the body of the hydra may be perforated, without, however, definitive damages, seen the regenerative capacity. It is quite interesting to understand how the hydra, having no complex sensorial organs, can recognize a fresh prey, useful as food, from a debris or anyway from a non-living material.

The system is based on a substance present only in the living beings, the reduced Glutathione GSH. This is an unstable molecule, produced by the metabolism, which rapidly deteriorates after the death. The tentacles of the hydra, thanks to specific receptor molecules, recognize the presence of the GSH in the fluids released by the wounds produced by the nematocysts. The link of the GSH with the receptors causes the movements of the tentacles which fold towards the mouth.

If we put some GSH in the water, in absence of any prey, the tentacles fold and move towards the mouth, which, in turn, opens and receives the extremities of the tentacles! The egg forms a chitinous exoskeleton and waits for favorable conditions before the organism will emerge. Budding occurs when conditions are favorable; proper water temperature and enough food are the major factors.

A miniature adult hydra will grow out of the body of the parent hydra. Once fully developed, it will separate from the parent and continue the life cycle. Hydras use this kind of reproduction more often because it is easier and will create more hydras than other methods. The third method of reproduction is more of a survival mechanism than a reproduction mechanism. When a hydra is cut into segments, each segment, if large enough, will grow into an individual hydra sized relative to the size of the segment.

This happens in other animals, such as the starfish. From Wikipedia, the free encyclopedia. Species of cnidarian. Pallas , [1]. Schuchert P ed. World Hydrozoa database. World Register of Marine Species. Retrieved Springer Netherlands. DOI: May , "Mortality patterns suggest lack of senescence in hydra. September Experimental Gerontology. PMID S2CID Muscle activity map in Hydra gives insight".