Sea spiders can regrow entire organs

Limb regeneration, or the ability to grow back legs, is not unheard of in the animal kingdom. Some species of salamander can regrow lost limbs, and and perhaps you’ve seen a lizard’s tail fall off and roll around on the floor as the lizard runs away to grow a new one. But the champions of regeneration are sea spiders. These look exactly as you imagine, and not only can they regenerate limbs, but also their digestive tracts and reproductive organs. Some even survive after being cut in half! So what could be behind this amazing regenerative biology? And why don’t we see such a useful adaptation more often, in us humans for instance? The University of Vienna’s Georg Brenneis...

Georg - What we know at this point is what they can do once they have regenerated. So which organ systems are growing back, what structures can be brought back. But how does this happen? That is the question that we still need to answer. And at this stage we don't know the cells, the cell types, underlying this. And we don't know anything about the genetic programs, the developmental programs that are actually driving this. We can, at this stage, just make educated guesses based on studies that have been done on other arthropods. And there have been studies on crustaceans, for example, where they could show, but just for limb regeneration, which is well known, not central body parts, that there are specific cells which may be more limited in what they can do than proper stem cells. Meaning for example, that the muscles that form in the regeneration process, they come from a specific muscle, so-called 'precursor cells'. So that is one cell type that will produce only the muscles. So, and in this case for sea spiders, we have to look potentially for more cell types because we have gut regenerating, we have reproductive organ structures, regenerating and so on.

Will - Given how old arthropods are, I mean they span back to half a billion years old, perhaps some even older, how much of a role in their continuing survival does limb regeneration play?

Georg - It definitely comes in handy, I'd say. And from what I've seen, sea spiders, particularly now we have some fossils, single fossils, that date back more than 400 million years ago and they looked almost exactly the same. So this body plan has been around for a long, long time. So they have been successful in surviving a lot of mass extinction events actually. And I also collected animals in the field after my first observations. And what you see is that more than half, in some species, are regenerating legs when you collect them. Or they have lost them, so half of the specimen. So it's a very common phenomenon and that obviously can help you to continue to stay in the game. If you lose a leg, you can then actually regrow it after a while because if you run out of legs, you cannot move around anymore. You cannot mate and so on. There are also other arthropods for which it's very obvious that this is extremely useful. We have crabs, everybody knows them and they have a preferred breakage point in legs and they basically throw them at you. So it's predator evasion. They really break very, very easily. Also some scent. Petes do that. A lot of legs are going off and then they regrow. So it's predator evasion, which obviously helps survival of the individual and gives you another chance to, to then reproduce later on.

Will - If limb regeneration was so useful and dates back such a long time, why then was this ability to regrow limbs lost in organisms that have evolved since arthropods?

Georg - Yeah, that is a very, very good question. A million dollar question. I'm not sure I can give a fully satisfying answer to that because we normally think that is so advantageous, right? But it does also have costs and evolution will over time always make, so to say, calculations about costs of what you're doing. So if you put a lot of energy into regeneration, that energy can at the same time not be put into reproduction, which is the ultimate goal if you want to do it. So if you then survive, that might help you. But there are different life strategies or life history trades across the animal kingdom where you can see there's some situations where it perhaps doesn't make that much sense. If you look at the trade-offs. For example, if you have a lifecycle where the adult is very short lived and it just reproduces and that's it. So to invest in massive regenerative abilities that then the animal won't survive because it will die anyway before that, that wouldn't make much sense. Also, mammals cannot do it. Birds, they're warm blooded, so their physiology is a little different. They invest a lot of energy into regulating their body temperature, which has many advantages because you're temperature independent or less dependent. An arthropod, for example, cold temperatures mean that you don't move as quickly. So perhaps this is, and I'm shooting out of the hip here, obviously you are more prone to be caught by a predator. But if you're all the time active, which is cost-intensive as such as mammals, then perhaps it doesn't really in the end pay off to put a lot into another very cost-intensive process. So it's multi-factor things that we look at here. I'm pretty sure.