Hydra - The immortal monster of the micro world

A hydra might look a bit like a miniature octopus, but it is actually a lot closer related to jellyfish. It is relatively common in both tropical and temperate regions where it can be found in slow flowing streams, ponds and other freshwater habitats. At one end the animal has one to 12 tentacles which all have their origin around the mouth. In the opposite end is a ‘foot’ allowing the hydra to attach to a substrate. The body can measure up to 10 mm when fully grown and stretched.

Note the small buddig of a new hydra on the lower side.

The hydras tentacles are covered in special stinging cells called cnidocytes which it uses to paralyze its prey. Each cnidocyte is equipped with a structure called a nematocyst and a trigger hair. When the cnidocyte is triggered it fires a thread from the nematocyst containing neurotoxins capable of paralyzing and even killing its victims. Now the hydra slowly retracts its tentacles to bring the paralyzed victim towards its mouth to be eaten, often while still alive. Hydras mostly feed on small crustaceans.

 

Note the trigger hair on top and the nematocyst inside the cell. 

Hydras multiply by an asexual process, called budding, the vast majority of the time. This means that the animal will grow a clone of itself somewhere along the trunk. When the clone is mature the two will separate leaving two genetically identical hydras. If conditions are right and food is plenty, a hydra is able to grow a new bud every two days, and sometimes even several at once. However, to get some genetic variation, these animals also multiply sexually. This typically happens when conditions get less optimal.

Several hydras. One in the upper right corner showing the beginning of a bud, the one to the left shows a more developed bud. Note also the presence of newly ingested victims in the two animals on the right

Another interesting fact about the hydra is that it is “immortal”. The animals cells does not age like our cells does, and in turn, the hydra won't die of old age. Furthermore, the hydra has incredible regenerative abilities. If cut in half, both pieces will regenerate and live on. Even if sliced into more pieces, each piece is able to regenerate and develop into a complete individual. So if the hydra can avoid starvation, desiccation and being eaten, it can in theory live and produce clones of itself forever.

Hydras with both brightfield and darkfield microscopy. 4x and 10x objectives