Eli5 ?If jelly fish can’t die of old age and reproduce why haven’t just taken everything over



A question my girlfriend asked and she’s not happy with any of my answers

In: Biology

They have no brains so they can’t do much. They’re also good food for predators and easy to pick off.

Just because you can’t die of old age doesn’t mean you can’t be eaten. There’s always a bigger fish, and jellyfish are no exception to that rule.

They may not die of old age, but they still die (being eaten by predators, for example). Their population growth is also limited by the resources in their environment—they still need to eat and they’ll starve to death if there isn’t enough food to go around. As long as the rate of jellyfish surviving to maturity is roughly equal to the rate of jellyfish deaths, their population will remain stable.

Because they are eaten by predators. I doubt a larger percentage of their babies survive to mating age. And as adults they get eaten which reduces their numbers.

It’s only particular species of jellyfish, and it seems like in the wild, most of those _can_ die of old age. It’s only under particular conditions that they revert to the juvenile form and reset the clock.

Edit: most are probably eaten or killed, as others said, but they can still die of old age.

It seems everyone here has failed to bring-up the fact that it’s only *one* species that’s “immortal” (so to speak): Turritopsis Dohrnii. *Not* all jellyfish.

Also, that particular species, apart from being able to be killed, reverts to a very primitive form. It’s not exactly the same thing as being immortal in the sense you and I would think of.

I do want to point out that they very well might take everything over in a few years; with climate change, warming oceans causes jellyfish eggs to hatch at a faster rate than before, which means higher jellyfish populations.

Already in Japan, I believe, a lot of fishing boats are having the problem where they catch a majority of inedible jellyfish and only a small amount of fish compared to the hauls they were getting in the past

Edit: here’s an ABC news article from 2019 about it: https://www.abc.net.au/news/2019-01-06/the-magic-and-mayhem-of-jellyfish/10377112