Tuesday, 9 August 2016

Incredible Invertebrates #02: the jellyfish that can live forever

First of all, a little update on recent activities. Yesterday I joined the Phoenix group at the Humber Bridge Country Park to see if we could find any Roesel's bush crickets, which are rare in this area of Britain. This species had been sighted nearby, and they are known to travel along verges of railways to new localities, so we thought that it would be worth having a little check for the crickets on Country Park. We tried to pick up their stridulations on bat detectors, but unfortunately we found nothing - it does not mean, however, that the crickets aren't there, only that we haven't found them yet.
While we didn't discover any Roesel's bush crickets, I did find some ants. Rather too many ants, in fact, as these happened to be of the species Myrmica rubra, sometimes called the common red ant or European fire ant. The latter name is rather fitting, as this species, while unable to spray acid as many species can, delivers a very painful sting. Having shoes and trousers full of European fire ants was not a pleasant experience! I was quite surprised that the pain of the stings didn't go off for 2 hours, leaving a dull tingling sensation. During this time, it hurt every time I moved my feet. I had no idea that they had such painful stings!

Anyway, onto our incredible invertebrate. This one has always amazed me, because while some animals can live to old ages (trees even more so), one species takes this a step further: it can, theoretically, live forever.

Turritopsis dohrnii is a species of jellyfish. In its adult form, called the medusa, it is quite small: around 4.5 millimetres wide, and about the same tall. They can have up to 90 tentacles, and their large, red stomach can be seen through their transparent flesh. They feed on tiny organisms, and although they are very difficult to keep in captivity, the one person who has managed it, Shin Kubota in Japan, has cultured his on brine shrimps.

 By Bachware - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48971787

These jellyfish do not look very extraordinary, and do not stand out as something special, being rather small and not overly striking. However, it is their life cycle that is outstanding.
Jellyfish normally have fascinating life cycles, and this species is no different.
Females of the species have gonads in the walls of their stomach, of all places, and this is where they keep their eggs. Not much is known about what happens next, but is thought that the eggs are released into the water, where they are fertilized by sperm. Afterwards, the eggs sink to the seabed where they develop into the larval stage, what is known as a planula larva. They develop into colonies of polyps, called hydroids, which will eventually bud off into multiple tiny jellyfishes. Each one of these juveniles are about 1mm in size, but in a few weeks they will have grown and become sexually mature.
Once these jellyfish are adults, they are known as medusae (singular is medusa), and it is these that we know so well. The next stage in their lives will be to breed, and then . . . death? Invertebrates, after all, have famously short lifecycles, at least in some species. Most jellyfish live no more than a few months, and a few live for only a few hours. But T. dohrnii is the complete opposite. The next stage in its life is not death, but something entirely different.
The medusa's tentacles and body will begin to disappear. It will grow polyps, which will settle down on the seabed, and begin to multiply, forming a hydroid colony - and the adult jellyfish will no longer be an adult, but a sexually immature hydroid colony. It has turned its lifecycle on its head, growing back into its hydroid form - which would normally only exist just after the fertilized egg has hatched into a planula, which would begin to form the polyps. Afterwards it will form more small jellyfish.
This jellyfish has, essentially, bypassed death to revert back to a younger form. It is the equivelant of a human, once becoming old, changing back into a toddler, and then carrying on growing all over again, then repeating the process, again and again.
You will probably see what I'm getting at now. This jellyfish cannot die of old age, and is theoretically immortal. The only things that are stopping it from living forever are predators, strandings, and other causes of death. If for a moment we image that we could remove these things from their habitat, an individual of this species could live forever. Just imagine that: one individual jellyfish living for millions of years, until the end of the world. Though of course it wouldn't be just 1 jellyfish, because when it reverts back to its polyp form, it will grow into multiple young jellyfish. So it is not only immortal, but in the process it can create what is essentially lots of little versions of itself.
This is all incredible, but also speculative. Without solid evidence, we cannot be sure if this species could theoretically live forever - I say theoretically, because as already mentioned, there are predators and natural disasters which could kill the individual. Still, it has been given the rather impressive name of the immortal jellyfish.
As previously mentioned, it is very difficult to keep this species in captivity. Only a Japanese man of the Kyoto University has managed to keep the immortal jellyfish for a long period of time. He has kept them for a few years now, and he has recorded that every 2 years, an individual will have managed to revert itself to a younger form around 11 times.

Longevity seems to be a recurring theme in many marine animals. Some black corals are thought to be around 4,265 years old, and a species of clam called the ocean quahog can live for 507 years, possibly longer. The rougheye rockfish can live for up to 205 years old (although this is not the oldest fish. That goes to Hanako, a koi carp who died at 226 years old. However, if reports are to be believed, koi can live for more than 200 years.). Even marine mammals can grow to great ages: the bowhead whale can live for at least 211 years.
However, only a few creatures have the equivalent or lower mortality rates the older they get, and these are known as 'biologically immortal'. Only a few organisms are like this: Sanicula (a plant), skin beetles, the Hydra (an unusual invertebrate), and the immortal jellyfish, to name a few.
What an extraordinary creature!

Until next time, keep on the wild side!

No comments:

Post a Comment