Hydras: Main characteristics of these inmortal creatures

Meet these wonderful creatures called Hydras, which are a genus of hydrozoas hydroids from the Hydridae family of freshwater. They measure a few millimeters and are predators that capture small prey with their tentacles loaded with stinging cells.

The Hydras have an amazing power of regeneration and reproduce both sexually and asexually and are hermaphrodites. Their name comes from the Greek mythology´s creature of the same name, to which, if one of its heads was cut off, two would grow in its place.

 The Hydras’ Main Characteristics

Generally, Hydras have a few millimeters in length, which range from 1 mm to 20 mm when they are fully extended. Moreover, they have a tubular body that ends in a simple adhesive foot called basal disc whose cells segregate a sticky liquid that confers them their adhesive properties. Their mouth is located at the free portion of their bodies, surrounded by one to twelve slender mobile tentacles.hydras. Schematic drawing of a discharging nematocyst

Each tentacle is covered by very specialized stinging cells called cnidocytes, which contain specialized structures called nematocysts that look like bulbs with a thread rolled inside. There is a sort of short hair-shaped releaser at the narrow outer edge of the cnidocyte. Once in contact with the preys, the contents of the explosive nematocyst are poured, by shooting a dart along with the internal wire, and by injecting neurotoxins. For humans, these neurotoxins only means a nuisance in the worst case, however, for some other species, they can be paralyzing.

The Hydras’ nervous system is a neural network, which is structurally simple compared to the mammalian nervous system. Moreover, they have no recognizable head or brain.


When hydras are alarmed or attacked, they can retract their tentacles and their entire body folds back into a small gelatinous sphere. In general they react in the same way, regardless of the direction of the stimulus, which may be due to the simplicity of their nervous system.

Hydras are generally sedentary or sessile, but occasionally they manage to move easily, especially when they hunt. To do this, they bend forward and adhere to the surface with their mouth and tentacles, and then release their foot, bend their body and fix their foot in another place. In this way they can move up to 100 mm in a day. In addition, they often break off from the substrate and let themselves be carried by the current.

How Do Hydras Reproduce?

Hydras can reproduce by making other smaller hydras. They do so by gradually spawning a smaller hydra, using mitosis.

But the Hydras, like some unicellular eukaryotes, can also be reproduced using meiosis. Usually the hydras are neither males nor females, but when they are going to reproduce using meiosis, a hydra develops testicles to make the sperm cells and the other hydra develops ovaries to make the egg cells.

These sperms and ovules can melt to gestate a new Hydra. This has the advantage of giving the new Hydra a combination of DNA different from that of their parents, allowing them to evolve quickly to respond to the changes around them.

How did Hydras Evolute

Some types of hydras have developed a relationship with unicellular prokaryotes that can photo synthesize food for them. The Hydras let these algae live inside them and the algae eat what the Hydra doesn´t  want (carbon dioxide) and spit out what the Hydra wants (oxygen to help digest food)


It should be noted that the  hydras’ body  is not stable, but on the contrary dynamic, since their cells carry out mitosis constantly and are displaced to the extremities of the column where they are stripped. In this way, each cell plays different roles over time. If the body of a hydra is cut into different parts, each part will regenerate a head on its original apical side and one foot is its original terminal basal side.

One of the keys to understand the hydras´regenerative capacity, is by also understanding their  ability to acquire stem cell characters from their epithelium, which continuously carries out self-renewing mitotic divisions and also has the option of following differentiation metabolisms.  Apart from the continuous production of cells with a high phenotypic plasticity, there is also a production of signaling factors in the adult tissue.

At the end of the 20th century, Professor Daniel Martínez reported that Hydra vulgaris did not age. After studying this species for four years, he saw that they looked as good as the first day. Its cells didn´t deteriorate. The Hydra is eternally young and potentially immortal and its power relies in its stem cells. Thanks to them, it regenerates and reproduces without sex.

It can recover damaged parts, and create small clones of itself. Recently, Professor Thomas C. G. Bosch and his team have discovered a protein in stem cells that seems to be key in anti-aging. «If the FoxO gene is removed, Hydra ages,» says Bosch, although it is not yet clear how it works.

Actually, beyond Hydra, FoxO could be the universal source of eternal youth. Humans have versions of this gene, and some are more common in people who live more than 100 years.

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