Those teenage years all over again

Now I’m going to cover two oddly related topics—one concerns a very strange transformer and the other is about plants and animals that might be essentially immortal, but I’ll also briefly touch on the question of what makes an individual an individual, and a clone a clone. As different as these things seem, they are connected. And they’re also connected to sex.

Just as caterpillars turn into butterflies, jellies go through several major transformations as they age. One of its juvenile stages is perhaps the most bizarre transformation in nature. When they’re in the adult stage of their lives that we’re familiar with, they’re called medusas. In their polyp stage they’re usually anchored to something and have a stalk with their mouth and tentacles on top like a hydra. Some remain in this form without ever turning into medusas.

Juvenile moon jelly medusas. W.carter.

Many sea creatures—perhaps most of them—go through several unusual stages as they grow up. As an example, I’ve chosen the moon jellyfish. They look pretty much as you’d expect a typical jellyfish to look like when it’s an adult. It’s mostly translucent with a main body that resembles roughly resembles a bell or umbrella and is shaped similar to the top of a mushroom. From its body hang stinging tentacles. Its body pulses to push itself through the water and to create a current around its tentacles, bringing in food. They are found throughout the oceans, but are usually encountered at beaches and in bays, estuaries, and harbors.

Metamorphosis is common in sea creatures, amphibians, and insects. When they reach a certain stage in their life they transform into what looks like another life form. Most of their cells do remake themselves to take on different functions, but a few remain. This is most important in the nervous system, since the animal’s behavior and neural duties have to drastically change with their new bodies. Most of the neurons remain, but scientists are just learning whether the animal’s memories do. They’ve tested fruit flies, which have a simple nervous system, and they seem to lose their memories during metamorphosis, but butterflies and beetles with more complex systems retain some memories.

The leading theory of why some creatures drastically alter their bodies is that it eliminates the competition for food and resources between each stage. Whatever the reason, those who do transform themselves are extremely successful—think of flies, bees, and ants.

A typical jelly’s stages of life with metamorphosis at stages H to I, J to K, and K to L.

Moon jellies start out as eggs carried by their mother until they hatch into tiny oval- or paramecium-shaped larva that are covered with cilia, the little hairs that they beat rhythmically in order to move around or to move water around it so it can catch food. We also have cilia in many parts of our bodies, including our lungs, kidneys, intestines, and we use them to hear. These larvae are already armed with stinging cells.

They float around as plankton until they attach themselves to something, such as a rock or a shell. Then they turn into a polyp and enter the stationary stage of their life. It now looks a bit like a hydra, with a tube-like body that tapers down to where it is attached and with tentacles at the other, open end. If it has trouble finding food during this period, it can go up to three years without eating. Some stop here, never becoming medusas, but others go on to the next stage.

Now here’s the interesting part. When there is food around, the tentacles go away and this top portion of its body grows into a stack of saucers with eight or more arms sticking out of its central disk, each of which end with two fingers. As new saucers form at the bottom of the stack, the ones at the top mature and eventually break off. These go on to become a juvenile jellyfish and then an adult. The hydra-like stage produces up to twenty clones of itself. That’s up to twenty moon jellies from a single egg.

Imagine if you were about thirteen years old and you suddenly attached yourself to your couch until food arrived, then you began sprouting more of you until you had twenty twin brothers or sisters stacked on top of you. But that’s not all. As you continue to grow you may bud off a few more clones out of your side or split in half to make another copy of you.

And not only that, in the case of at least a couple kinds of jellies, once you’ve floated around for a while as a juvenile or adult medusa, if you get injured, stressed, sick, or old, or if your environment gets too hot or too cold, over a period of about a week, you—or part of you, if some of you was eaten by a predator—could reabsorb some of your tissues, reverting to a young mass of cells and reattach yourself to your sofa as a polyp so you can begin budding off up to twenty clones once again, one on top of the other. Your friends and relatives might think you were strange, but all this is just a normal part of growing up for a jellyfish.

Who wants to live forever?

Replicating oneself through cloning—dividing in half or budding off new individuals—is fairly common among small creatures and plants. Whether you think of these as being immortal depends on how you define an individual. With humans, when embryonic cells split in two, you end up with identical twins. Of course they are two individuals. Because of mutations and environmental conditions, their genetic code will drift apart in small ways. The older they get, the less alike they are.

The same thing happens with plant and animal clones, although they might not be as different if they have shorter lives. Even bacterial clones are different from one another in many ways, even though their DNA is the same. In addition, genetic mutations accumulate over time, which means the longer they are separated, the more different they become. So we might not consider the existence of clones as making an individual immortal.

But what if clones bud off and don’t separate from the original individual. If they remain connected, then they could be considered to still be part of that individual. Some plants do this. Bermuda grass sends out runners that sprout into more grass, but they remain connected until that runner is severed. Aspens and elm trees do this, so an entire forest of them could be thought of as one individual. One aspen in Utah is thought to weigh 6,600 tons (6,000 tonnes) and consists of over 40 thousand trees that are part of one root system.

A ribbon-weed meadow being patrolled by a bream. David Harasti, © State of New South Wales. For current information go to www.nsw.gov.au. CC BY 4.0 (adjusted.).

Perhaps the largest instance of this is a seagrass at Shark’s Bay, Western Australia that is one individual with a bunch of separated parts around it, all with identical DNA. Researchers estimate it’s about 4,500 years old and it covers an area seventy-seven square miles (200 km²), which is larger than Washington, DC. That means this seagrass could be the largest individual organism in the world, although it hasn’t been determined how much of this ribbon-weed meadow is connected and how much are clones since many of its connections are buried, but the researchers consider it to be all one genetic individual. As I said, it depends on how you define an individual.

A challenger might be fungus growing under forests and fields that are all interconnected, but the actual size is still unknown. Things get a little trickier with colonial animals that are made up of zooids. Corals can be considered one creature since the polyps are interconnected, but is it an individual? The polyps could be viewed as being similar to our cells, just less specialized. They die and are replaced, but some deep-sea corals have lived for tens of thousands of years and will continue to do so until conditions change.

A sea walnut comb jelly. NIH.

And there’s one other thing to consider that makes things even more interesting. When two comb jellies are injured, they might attach to one another, whereupon they merge into one individual. Researchers discovered this when they noticed that one of these jellies—known as sea walnuts—disappeared and another was larger than the others. On a closer look, it had two sensory organs and two rearends, although the researchers later found that sometimes sea walnuts fuse seamlessly. The two individuals join together to become one individual.

Comb jellies are very different from jellyfish—which are also known as jellies—but there’s one jellyfish that we know of so far that can also do this, as well as one tapeworm. That jellyfish is one of the “immortal” jellies that I’ll tell you about shortly.

The key to an extremely long life is regeneration. Cut off a crab’s leg and it will grow a new one. If a sea star loses all its legs, it grows new ones. A more extreme case is where a couple of types of sea slugs decapitate themselves, casting off their bodies so they can grow new ones. Perhaps it’s a method to get rid of parasites, but it still seems pretty drastic. And there are worms that decapitate themselves, leaving their heads behind to be eaten by predators as a distraction.

I’ll discuss all of these another time.

A hermit crab’s acquired whelk shell colonized with snail fur, along with a closeup.

You can chop up certain types of worms and so long as the pieces aren’t too small, each will grow into a new worm. You can do the same thing to a creature that lives on snail shells called snail fur, a colonial hydroid that often grows on rock and whelk shells, particularly those that are occupied by hermit crabs. Basically they’re a tube with a mouth and tentacles on top. Cut off its top and it grows a new body, while the body grows a new top. Now we’re getting into a situation that’s like cloning, where one individual has become two that go off and live their separate lives.

But if the regeneration happens over and over again in a single individual, can it live forever? This happens with snail fur. Snail fur can also keep regenerating all their parts, while the old ones are reabsorbed.

So far there are at least two jellies that are considered to be immortal, with the possible addition of moon jellies, but they regenerate in a different way. Of course, they can be killed by getting eaten by a fish, turtle, shark, a larger species of jellyfish, or some other predator. They might even get eaten by barnacles and mussels when in their planktonic stage, and many of them do grow old and die.

As far as we know, the lifespan for most jellyfish is about a year. Even the huge jellies that have bells that are eight feet across (2.4 m) attain that size in six months and die each autumn. Moon jellies are no different. With the arrival of fall, flood tides begin carrying moon jellies ashore to die on the sand. But the strange thing is that we have no way to know how long some individual jellies can live. A few individuals might actually be immortal.

When damaged, stressed, or they become old, these jellies can reverse their metamorphosis. Earlier I explained the different stages they go through as they age. Well, they can revert to an earlier developmental stage, effectively becoming young again. The medusa sinks to the seabed, gathers up its bell, and shrinks down to a slimy ball that either enters a cyst-like stage before becoming a polyp months later, or it goes straight to the polyp stage, which turns into a hydra, then a medusa.

Continuing the earlier analogy, it’s as if you melted down on a couch and became a sedentary again, growing tentacles from the top instead of the bottom. It would be as if our specialized cells turned back into stem cells that could once again become specialized as another part of our body.

Moon jellies can do something just as amazing, but in a different way. A Chinese marine biologist found that they, too, can re-form themselves from a juvenile medusa back to their polyp stage, but its not yet known whether this is the same as what the two previous immortal jellies are doing. But what is especially interesting here is that he kept one of his moon jellies that died from old age and three months later it sprouted a new polyp.

As ocean scientist and author Juli Berwald put it, the life cycle of jellies is similar to caterpillars transforming into butterflies, but with a few additional transformations. What’s going on here is as if three months after a butterfly dies, suddenly a piece of it sprouts a new caterpillar.

This is quite stunning, but needs more research. Was it all dead or was part of it dormant? Was the development a controlled stage of the life cycle or was it regeneration, in which case it would be a clone? Either way, jellies certainly push regeneration to extremes.

This type of regeneration is different from lizards that grow new tails or salamanders that restore their arms. These animals just replace lost parts, making them the same size. Animals, like jellies, anemones, and flatworms, that can regenerate most of their bodies from tiny bits do it differently.

We can repair minor wounds, but we can’t regrow fingers and it’s all because of sex. When animals began having sex, their ability to regenerate was diminished. There are advantages and disadvantages to the asexual and sexual reproductive methods. With budding and cloning, the animals are healthier and have an indefinite lifespan, but they have very little genetic variation, which means they have less chance of surviving environmental changes.

With sexual reproduction, you can no longer regenerate major body parts and you lose the potential for extremely long lives. With sex there’s a genetic link that also brings aging and natural death. Now there’s an age limit. On the other hand, the genetic variations of sexual animals make their species better able to survive diseases, new or more efficient predators, and other changes in their environment. They’re also able to develop complex organs and features, such as bones and brains, that can’t be regenerated, and they can grow larger. It’s our complexity and size that prevents us from regrowing our arms, as salamanders do.

One group of scientists from all around the world, but centered in Germany, have found a genetic signaling pathway that has to be turned off for major regeneration to take place, but in some species at least, it’s required for egg yolk production, among other things. This and other research may well lead to treatments that could extend our lives, reverse aging, or perhaps someday bring an end to natural death. And we’ll still be able to have sex too. Science is amazing.

 

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