Meet the relatives: Sea Squirts

These are astonishingly beautiful animals sometimes confused with jellyfish. They’re not. They are actually our ancestors. (Admittedly, jellyfish are also beautiful, and if you go far enough back, they’re also our ancestors, but work with me here, okay?)

Since sea squirts are related to us, that means they’re useful as well as beautiful. Useful in medical research, that is. Pictures and fun facts below the fold.


    tunicate, with resident gobytunicate, with resident goby
    photo: Chika


    tunicate, didemnium molleTunicate
    Didemnium molle,
    beautiful, but not quite the same movie star quality.
    photo: Boogies with Fish


To be exact, only the immature stages have the famous rudimentary “backbone,” the notochord, that makes these things so interesting to us vertebrates. The adults, as happens so often, just sit on their duffs and eat crap.


What this means is that vertebrates evolved after a mutation in tunicates allowed the mutants to stay juvenile their whole lives. In other words, they could reproduce even though in other ways they were still “kids.” That’s a fascinating and recurring theme in evolution. Humans are another example, since we’re basically baby apes.

/* tangent*/
I’m rather taken with the philosophical implications of all this. I mean, compare the following:

photo: Kevin Brett

tunicate larva, ciona, showing notochordsquishy, barely visible with the naked eye
western lowland gorilla photo: Wikipedia chimp babyphoto: Tim Ellis

To me it suggests that “the meek shall inherit the Earth” is not wishful thinking. It’s just a statement of fact. Maybe we don’t have a very good understanding of what strength really is.
/*end tangent*/

Anyway. Onward and upward. Medicine. I wasn’t joking when I said the adults sit there and eat crap. They’re filter feeders, like oysters, and are often found in muddy, bacteria-laden waters.

About 80% of their genes are also found in humans and other vertebrates, but the total amount of DNA they have per cell is only about half that of vertebrates. They’re missing a lot of genes that we have, and some of them are the more advanced immune system genes. So you have a mystery. Somehow, a creature with very few immune system tools is staying healthy under conditions that would kill us, even though we’re quite similar (all appearances to the contrary notwithstanding).

Scientists started looking and found all sorts of interesting things. As you might expect from something that eats dirt, tunicates produce weird antibiotics. They’re not only new, but they have a different mode of action, which means that currently resistant bacteria are not resistant to the tunicate antibiotics at all. Unfortunately, some of these compounds are just as good at blowing up red blood cells as bacteria, so they need more work. But because of the way they’re formed (see pdf if you want a bit more of the science), genetic engineering is likely to be able to modify them to keep the good and toss the bad.

Anti-virals, immune suppressants, and anti-cancer compounds have also been found (e.g. Wikipedia), but, like the antibiotics, they’re too effective. They need work to make them less toxic to normal cells. (This isn’t unusual with cancer drugs. The Vinca alkaloids, which ultimately stopped childhood leukemia from being a death sentence, started out the same way.)

And then there’s the fascinating field of regenerative medicine, something which didn’t exist ten years ago. This includes things like re-growing spinal cords or limbs, and growing replacement organs like livers, hearts, skin, or eyes. Because tunicates are rather simplified versions of us, it’s easier to study processes of cell growth and regulation. The result is that a team of Stanford scientists published a paper (abstract) just this past May describing how they made an abnormally growing colonial tunicate grow normally again over a period of time. A tumor is nothing but an abnormally growing set of cells, so people are agog about this work. Now that they’ve found this pathway, they can study it in detail and find out exactly how the cells regulate themselves back to normality. Then they can try to modify that so it works in humans. It’ll take a few years….

So next time you happen to be snorkelling or scuba diving in a mangrove swamp, peer through the muddy water until you see bright kaleidoscope jewels loom up in front of your face and think about the cloud of invisible possibilities they represent.



Filed under 13_quixote

24 responses to “Meet the relatives: Sea Squirts

  1. Absolutely fascinating, Quixote — I just love scientists who can transmit the information AND a wild passion for the amazing discoveries and insights available in the information.

    I’m a low-level science geek (don’t tell anybody).

    Thanks for a great post.

  2. So you have a mystery. Somehow, a creature with very few immune system tools is staying healthy under conditions that would kill us, even though we’re quite similar (all appearances to the contrary notwithstanding).

    Cool article!

    What do you think about the theory that many DNA sequences in mammals may have originated from ancient retroviruses?

    It is believed that endogenous retroviruses (previously thought to be “junk” DNA) actually provide protection from infection and are involved in reproduction — such as the development of the human placenta.

    Evolution may in fact be the result of infection of organisms such as sea squirts by retroviruses. These retroviruses insert their DNA into the host cells and it is subsequently passed on to offspring.

    This could have led to the formation of an immune system, digestive organs, reproductive organs — virtually every system that has evolved over eons.

  3. Sarah

    Beautiful posting, thanks.

  4. Thank you for this, fascinating.

  5. Brynn

    Really cool post!!!

    growing replacement organs like livers, hearts, skin, or eyes.

    As an FtM, there’s another set of organs I’m really interested in. πŸ™‚ But our priorities are so far down on the list–well, they’re probably off the list–that I don’t expect anything will happen during my lifetime.

    More the pity.

  6. oddjob

    Brynn, what y’all need is a George Soros tranny! πŸ˜‰

  7. Thank you — this is so cool! 9-year-old daughter has been saying for several years that she wants to be a marine biologist. She will love the pictures and the narrative.

  8. (Nine-year-old daughter) That’s SO cool!!!!!!!

  9. Hi, all y’all (as we used to say in New Orleans). Thanks for the kind words!

    Fritz: there’s some viral DNA incorporated in the DNA, but it seems to explain less than half of the “junk” DNA. What they’re finding is that some of the “junk” is involved in gene regulation in ways that weren’t even suspected. Recently, for instance, they’ve found that some of it codes for micro-RNA, and micro-RNA exercises a type of regulation of gene expression. So, they called it “junk” because it wasn’t obviously coding for proteins, and now they find out that it’s regulating the process that eventually codes for proteins.

    Gene regulation is particularly important in the embryo and fetus, so a lot of the “junk” is essential, but turned off in adults (except when it sometimes goes crazy in cancers 😦 ). Virus DNA, if it inserts in an area important to regulation, can have huge downstream consequences. Usually those are fatal, but sometimes they’re “interesting,” or become interesting after some mutations. And then there’s stretches of DNA, viral or non-viral origin, that mutate into uselessness only to then mutate into some new and strange function.

    An example of how huge a difference one change in a regulatory gene can make: at its most basic, the ventral insect body plan is just the dorsal vertebrate body plan, except a few regulatory genes flip to one side or the other very early in embryonic development.

  10. Kathy, I am so tickled about your daughter! Tell her marine biology is the best. Really. Take her snorkelling in some nice warm place. Then it’ll be all over but the shouting.

    Where are you located, approximately? Maybe I could suggest some likely spots.

  11. Indeed it is, Nine-Year-Old-Daughter! πŸ™‚

    And Quixote, thanks for this refreshing burst of natural wonder!

  12. oddjob

    the ventral insect body plan is just the dorsal vertebrate body plan, except a few regulatory genes flip to one side or the other very early in embryonic development.

    You mean the exterior body plan, right? You weren’t referring to the interior body cavity as well, were you?

  13. Thank you for that as I can never read enough about our cousins. (Especially the research links and the’ o so pretty pictures!)

    A Sea Squirt needs to regenerate me a new shoulder!

  14. boatboy_srq

    Brilliant (and colourful) post, Q.

    And WAY TO GO, Nine-Year-Old Daughter! My neice is doing her PhD in neuroscience: I think she went that path for much the same reasons. May you do as well as she (at least so far).

    Not to throw a wet rag in here, but I’d be fascinated to learn what the Fundies think of all this research – given that Evolution is wrong and all that.

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  18. mcmillan

    You mean the exterior body plan, right? You weren’t referring to the interior body cavity as well, were you?

    If I’m remembering my developmental biology right the interior stuff gets flipped too. I know at least the nerve cord is ventral in insects, and I’m pretty sure at least some of the other stuff is flipped as well.

    One small quibble about the introduction. It wouldn’t be right right to refer to these guys or jellyfish as our ancestors, more like our cousins. It’s a peeve that one of my professors passed on to me, since that kind of language makes it easy to forget that those guys have been evolving just as long as we have and certainly have lots of changes. I doubt our common ancestor made the kind of antibiotics that we’re talking.

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  21. mcmillan: right on both counts. At that stage of development, there is no external. It’s referring to the ventral nerve cord and (I think!) dorsal blood vessel vs dorsal nerve cord and ventral blood vessel in those of us without an exoskeleton.

    And, yes, officially they’re cousins. Nice to be in amongst people who know that! Usually, it takes a small dissertation of explanations, so I just go with the flow (which is bad, I know). The only problem with the “cousin” terminology is it’s not so clear that in some important ways there really hasn’t been a lot of evolving in those lineages. They’ve maintained a steadyish state, as it were.

  22. Brynn, I meant to answer your comment the last time I visited, and then I got sidetracked, and now it’s probably too late. Piffle.

    It would be immensely difficult for a male to grow female anatomy. However, depending on how much of the male anatomy a female wanted, it could either be rather easy or equally difficult. The easy part would be growing a penis, since that’s basically an enlarged clitoris. So, in effect, they’d just need to figure out how to trigger growth in an organ that’s already there.

    Everything else, though, is way more difficult because the organs are already differentiated. Before they could be turned male > female, or female > male they’d have to be de-differentiated, and then re-differentiated along the new track. The latter is hard enough. The former may be impossible.

    Now, they might be able to take stem cells, create a new set of organs, remove the old ones, and transplant in the new ones. Just the thought of having all those sensitive tissues messed about like that kinda makes my skin crawl, but I can see where that could be theoretically possible.

    As to the priority of gender changing, I wouldn’t assume it’s actually that low. People will never admit it on a grant application, but it’s amazing how often fun takes precedence over seriousness. Just look at the money in movies versus NASA.

  23. boatboy_srq, as with Brynn, this probably comes way too late, but just in case you check back …. It’s really a giggle talking about this sort of thing to fundies. They don’t know one thousandth of what’s going on, and they say in all seriousness that some simple little thing they do happen to know about, like eyes say, or basic DNA coding, is so complicated that it has to be evidence of intelligent design. I get to come back with, “Ignorance isn’t evidence of intelligent design.”

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