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I have a beautiful word for you; senescence.  It's the word for how, as multicellular organisms age, they die more and reproduce less.  It's the deterioration that comes from getting older.  But there are a few exceptions to this "get older and die" thing. Including, it turns out, tiny sea creatures called hydras.

We humans, like a lot of other animals, experience a peak period of fertility during the first third of our lifespans; the period of life in which we are least likely to die from most things.  Your body is working as well as it's going to, and you're as good as you're ever gonna get at firing out babies.  Then, senescence kicks in.  All of those things get less and less true until eventually you die.

For a long time senescence was thought to be universal, that all multicellular organisms capable of breeding more that once would get worse at it over time.  The older they get the more likely it is that they'll die.  Until scientists found a certain kind of jellyfish that can make itself young again. (link at 1:05) A power I would like to have.

And according to a study that came out this week from the Max Planck Institute in Germany, there's another exception to this rule, the hydra.  Not the mythical beast with all the heads. This hydra is a freshwater polyp that's a centimeter long and reproduces both asexually and sexually, can regenerate after it's been cut into pieces, including being able to regenerate it's entire head, and as it turns out, is pretty much immortal.  Not bad, being immortal and all that stuff, but I'd like to see it host  a YouTube show.

The hydra has fascinated scientists for centuries.  When they sequenced its genome back in 2010 they discovered that it had the same number of genes as humans.  We also have a surprising number of genes in common, including certain genes associated with Alzheimer's and Huntington's disease, so we've been studying these tentacular little guys ever since.  Because, like, maybe they can magically repair the neurodegeneration that  leads to Alzheimer's.  That would pretty much be exactly what we'd expect from hydras at this point. Like, why not?

Because we've been watching hydras for so long, scientists have expected that they don't senesce, but this was the first large scale study that focused on finding out for sure.  The team followed 2,000 individual hydras for a combination of 3.9 million days.  The hydras were between 0 and 41 years old, and they observed them for a period of 8 years.  The result; no decrease in fertility of increase in mortality was observed at any point in the hydra life cycle.  Although, if they don't age, it's not really a life cycle, it's more of a life-line that gets cut off when they get eaten by something.  And that is what I've been able to do that most hydras can't; get eaten, so far, at least.

These hydras reproduced, or budded, about once a month and they died at a rate of about one in 167 per year, no matter how old or young they were.  If the mortality rates in this study continued, that means in 3000 years, 5% of those hydras would still be alive if they were kept safe in the lab.  It would be a really long experiment.

So, how do hydras not decline with age?  Well, there are a lot of factors that might contribute.  They have a relatively simple body plan and relatively few types of cells, which probably helps.  But the most important factor is probably that hydras constantly produce new stem cells.  Every cell in a hydras body is replaced every 3-4 weeks by fresh cells produced by brand new stem cells.  It's essentially regenerating once a month, and in a completely different way from the jellyfish we talked about earlier, which reverts to an earlier stage in its life cycle when it's stressed.

There's a lot we might be able to learn from this.  What we now know about hydras suggests that they could hold the keys to big breakthroughs in research on stem cells, aging, and diseases like Alzheimer's.  While we're at it, we might as well leave little notes in hydra's genetic code for any  future aliens who might one day find our planet.  Maybe they'd sequence the genome of our immortal little buddies and it would say, "we were here, signed, humanity."  Then we could leave a link they could follow to the last working internet server, which would just be Rick Astley's "Never Gonna Give You Up."

Thank for watching this episode of SciShow News, and thanks especially to our patrons on Patreon like Emily Burrows and Mike Boyle who make this show possible.  We just picked a couple to say thanks to them specifically.  If you want to help us keep making videos like this, you can go to patreon.com/scishow , and don't forget to go to youtube.com/scishow and subscribe.

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