Hydra can reproduce sexually

Large numbers of freshwater polyps (hydrons) can often be found on aquatic plants in early summer. They multiply very strongly during this time, as they benefit from the large food supply in summer. With their adhesive disc, the hydras attach themselves to plants, wood or stones and stretch the catching tentacles into the water. You use it to catch water fleas and other small animals. Hydras can reproduce asexually by forming side buds that separate from the mother and grow into independent polyps. During sexual reproduction, fertilized eggs develop into larvae that swim around in the open water for a short time before they attach themselves and grow into new polyps.


All cnidarians (hydra, anemones, corals, jellyfish, etc.) are very simple in structure. They consist of a two-layer tube with an adhesive disc at the bottom and a mouth opening at the top. A tentacle crown sits around the mouth opening. Cnidarians have nerve cells that are not yet organized in their own "nervous system". A hydra has no brain, not even major nerve nodes. Hydra have neither a heart nor gills nor any other internal organs. The oxygen supply occurs only through diffusion. Hydras have muscle cells that allow the animal to contract, stretch, and bend over. The muscle cells are not organized in bundles (actual "muscles")

above: Scheme of a freshwater polyp
right: Freshwater polyp from Lake Zurich with an egg on the right.
Nat. Size about 0.5 cm

A cnidarians consists of 2 layers of tissue, an outer (ectoderm) and an inner (endoderm) and a middle lamella between the two layers. The nettle capsules are located in the ectoderm, unicellular green algae can sometimes be stored in the endoderm, which live in symbiosis with the polyp, similar to the zooxanthellae in stony corals. With the freshwater polyps, however, this symbiosis is not essential as it is with the hard corals.

Left: Symbiotic green algae in the endoderm
above: Polyp at 40 times magnification

Nettle capsules

The tentacles of all cnidarians contain tiny, unicellular poison cartridges, the nettle cells (nematocytes). When charged, the stinging cells can have an internal pressure similar to that of a full diving cylinder (over 150 bar)! The nematocytes sit in groups (batteries) packed together on the tentacles. If a prey touches the sensory bristle of a nettle capsule, the lid of the nettle cell pops open and a harpoon-like stylet with barbs hits a microscopic wound in the victim's skin. This process takes less than 1 / 40,000 of a second! A tube is then put into the wound, through which a strong neurotoxin flows into the prey. Since the stinging cells of a battery are connected to one another by nerves, the entire battery is fired every time a sensory bristle is touched. Each time a tentacle is touched, many batteries with thousands of stinging cells are discharged, which hold, paralyze and kill the victim. The Australian box jelly (Chironex fleckeri), a box jellyfish, can kill people with such small weapons! Nettle cells can only be shot once. New cartridges of this type are therefore constantly being formed in the catching tentacles. The cnidarians are a very primitive group. Its blueprint has been tried and tested for millions of years and has hardly changed. The nettle capsules, however, represent one of the most complex cell types in the animal kingdom. It is astonishing that we find these highly developed cells in such a “primitive” group of animals.