Amazing Planet: Are hydra really immortal?

It is likely that there is one near you right now if you live close to freshwater. These small invertebrates can be found in all kinds of ponds, lakes, streams and rivers, usually on the underside of rocks or leaves. And there, right under your nose, they get older without aging biologically. 

The animal in question is called hydra, like the Greek mythological water monster that has multiple heads, one of them immortal. Legend has it that the monster grows several new heads if one is cut off.  

Real-life hydra are a far cry from this serpentine creature. They are only about an inch long (25mm) and have tentacles at the end of their tubelike body to catch prey like brine shrimp or insect larvae.  

But like the Greek monster, they do have regenerative capabilities. You can cut off an actual hydra's head and it will grow a new one. Or you can cut it into several pieces — then you have several hydra. These freshwater polyps can also reproduce asexually, meaning they can essentially produce duplicates of themselves. Researchers recently looked into how hydra regrow their heads. They found hydra use different genes when they regenerate a new head because of an injury than when they grow one during budding, a part of asexual reproduction. The gene activity also differs in the two processes: It is dynamic during regeneration, but increases slowly when budding. 

Of course it is a massive win that they can regrow any part of their body that has been amputated. That is because they constantly produce new stem cells, which produce fresh cells every three to four weeks. This also means that, under the right circumstances, hydra don't grow weaker, get any worse at catching shrimp or reproducing. They simply don't age the way we do.  

Scientists studied this in 2015 by creating ideal environments for the little creatures, with regular feeding and water changing. They observed the hydra over eight years and found that the death rate remained constant and extremely low throughout. The fertility rate also didn't change. 

That doesn't mean hydra can't die. If they run out of food, a fish kills them or their water is polluted, they too will perish. In fact they are so sensitive to environmental pollutants like metals, pharmaceuticals and endocrine disrupting compounds that they have been used tomeasure the impact of these substances, because they can alter hydra's growth, reproduction and behavior, among other things. 

 But, in the ideal environment, they can be forever young.  

Edited by: Sarah Steffen