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Pyura chilensis: the closest thing to getting blood from a stone


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Credit: Arvid Puschnig

Period Rock? You’re calling me Period Rock now? Guys, seriously, I might look like a stone, but that doesn’t mean I have the heart of one. Why doesn’t anyone ever just call me Michael?”

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Despite appearances, this is not some kind of cruelly bisected alien stone organism or a tomato thunderegg. This is Pyura chilensis, a sea creature that lives on the rocky coast of Chile and Peru. And if (like me, very recently) you’ve never seen one of these before, you’ll probably be interested to know that in Chile, they are fished commercially, and the locals eat them raw or cooked with salad and rice because apparently they’re delicious.

P. chilensis belongs to the Ascidiacea class of non-moving, sac-like marine invertebrate filter feeders that are otherwise known as sea squirts. They belong to the Tunicata subphylum, so-called because they wear thick ‘tunics’ made of tunicin, which is a hardy matrix of molecules that help the animal attach itself to a hard surface on which it will carry out its days. The insides of this tunic are lined with an epidermis and a muscular band, and inside these layers lies the main part of the animal.

P. chilensis has two siphons that connect the animal to the surrounding ocean through its tunicin – one for exhaling and one for inhaling. It eats by inhaling the water and filtering out the edible microalgae using a moving layer of mucus in its enlarged pharynx, or branchial sac, before exhaling the water back out the other siphon. The pharynx is connected to the animal’s digestive tract, which basically acts like a mouth.

Their blood is clear and, strangely, can accumulte extremely high qualities of a mysterious and rare element called vanadium. The concentration of vanadium in the blood of P. chilensis and other tunicates can be up to 10 million times that of the surrounding seawater. Just why and how these creatures are able to accumulate vanadium in such huge quantities remains unknown.

A Pyura chilensis dish from a market in Valparaiso in Chile

P. chilensis can often be found in densely packed aggregations of thousands or small handfuls of just a few, or they can be found on their own – in which case they must reproduce on their own, as there is no way of them moving to find a mate. This means P. chilensis is hermaphroditic, with the gonads of both a male and a female that can release eggs and sperm simulataneouly to meet as a fertile cloud in the surrounding water. If the sperm-egg collisions are successful, they will produce tiny tadpole-like offspring that will eventually settle onto a rock to grow into the adult form.

In 2005, biologists Patricio H. Manríquez from the Universidad Austral de Chile and Juan Carlos Castilla from the Pontificia Universidad Católica de Chile published a paper in the Marine Ecology Progress Series revealing for the first time the particulars of this creature’s reproductive habits (They also use the verb ‘selfing’ often and with glorious earnestness). They collected 30 sexually mature P. chilensis from habits in central and northern Chile and set them up in lab tanks as isolated and paired individuals. They wanted to assess the occurrence and success of fertilisation via these two types of reproduction followed by the settlement of the resulting offspring to a hard surface and their subsequent metamorphosis into adulthood.

First, the isolated individuals were placed in plastic bottles, and were left alone for 90 days, free to do all the selfing they wanted (YOLO). After this period, their body size relative to the amount of sperm in the water was measured for each spawning episode. Next, the researchers combined pairs either from the same population, or from two different populations, to see how well they would breed in comparison to the selfers. A third experiment saw them keep P. chilensis individuals in isolation for one to 16 months, to see if an extended period alone would improve the success of selfing. Finally, the researchers conducted ‘manipulated ferilisation’, which involved removing eggs from the specimens and fertilising them with extracted sperm in Petri dishes.

The results showed that P. chilensis is born male, before becoming cosexual – having both male and female gonads – in its adolescence as it increased in size. The researchers also found that given the choice – that is, if situated around other individuals – these organisms prefer to breed via cross-fertilisation, writing, “Given that more events of natural egg spawning followed by successful settlement and metamorphosis were recorded in our paired specimens and in our manipulated cross trials … it appears that cross-fertilisation predominates in this species.”

Manríquez and Castilla also found that a greater number of fertilised eggs resulted from the paired specimens, which suggests that cross-fertilisation, or reproducing with another individual, predominates because it is more effective. This assumption was strengthened by the fact that individuals that had cross-fertilised before being put in isolation took at least two months before successfully producing offspring via selfing. However, they were careful to note that while cross-fertilisation was preferred, selfing did not produce inferior offspring. “No perceptible differences in fertilisation, settlement and metamorphosis success among self and outcross progeny were found,” they reported. This suggests that when stuck alone in the ocean, selfing provides an advantageous opportunity for loner P. chilensis individuals to still pass on their genes.

Here’s a video of a German man knifing some P. chilensis with great aplomb to the tune of a handful of angry YouTube villagers.

Bec Crew About the Author: Bec Crew is a Sydney-based science writer, award-winning blogger, and science communicator at the University of Sydney. She is the author of 'Zombie Tits, Astronaut Fish and Other Weird Animals' (NewSouth Press). Follow on Twitter @BecCrew.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Cornelio at Zepponami 3:45 pm 06/27/2012

    All vanadium compounds should be considered toxic (Wikipedia). Eating these animals is safe?

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  2. 2. mhuff1294 2:20 pm 06/29/2012

    This article was too informal. I loved the information, but was the “YOLO” and mention of the verb “selfing” really necessary? It just seemed a tad immature to me.

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  3. 3. ChasCPeterson 2:26 pm 06/30/2012

    ‘tunicin’ = cellulose

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  4. 4. Becky Crew in reply to Becky Crew 11:47 pm 06/30/2012

    Hi mhuff,

    “Selfing” is the verb the scientists used in their paper. I’m sorry you don’t agree with the use of YOLO, but I think humour plays an important role in ensuring the accessibilty of scientific communication, which is what this blog, and many other science blogs, have set out to do.

    Bec

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  5. 5. Barbara3 10:50 am 07/1/2012

    Two terms that you are using as equivalent aren’t. Selfing (self fertilization) is sexual reproduction, with egg and sperm from the same individual. Asexual reproduction is, as the name suggests, not sexual reproduction. It is a form of cloning.

    Offspring produced by asexual reproduction are genetically identical to their parents. Offspring produced by selfing can be genetically different, if the parents are heterozygous at some loci. However, if a lineage selfs repeatedly, individuals become homozygous at all loci. Then offspring produced by selfing are genetically identical to their parents. That’s apparently not an issue with these tunicate, since cross fertilization prevails.

    Fascinating photo!

    Link to this
  6. 6. Gatnos 5:54 pm 07/2/2012

    I ceratinly hope that the US taxpayer is not funding this “research”.

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  7. 7. taffazull 1:30 am 07/3/2012

    tunicin=cellulose=wood=cotton;So in tunicates there is a melange of plant,animal,vertebrate and inverteberate.My zoology teacher referred to them as creatures who had undergone a devolution rather than an evolution!

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  8. 8. DrBaka 8:35 pm 07/3/2012

    Becky, pay muff no mind. Your use of humor is an excellent tool for public outreach and one I use to great effect myself. Those who want all scientific communication to be prudish and dry neither understand the best practices of the public communication of science nor have as their priority the education of people who are not like themselves (prudish and dry).

    And as for the use of taxpayer money on this kind of research, you’d be surprised what sort of applications research into selfing can have. From better understandings of the epigenetic controls on genomes to insights into hereditary disorders. Just because something may seem to you to be trivial or inconsequential doesn’t mean it doesn’t have the potential for generating downstream benefits that you may find are good uses of your tax dollar.

    Great article, Becky! Keep doing what you’re doing.

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  9. 9. Becky Crew in reply to Becky Crew 5:35 am 07/4/2012

    Thanks so much, DrBaka! That’s very kind.

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  10. 10. albertobladerunner 12:46 pm 07/4/2012

    Good article Becky.
    I just did not like the references to “cruelly bisected alien stone organism” because it is indeed a cruelly bisected alien stone-like organism.
    And then the link to a video of a “German knifing some P. Chilensis” is not the same that seeing chicken’s heads being cut but it is still cruel and is a little like talking about the science of killing others.

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  11. 11. pwcampos 9:38 pm 03/21/2013

    Biological Bulletin, Vol. 175, No. 1, Aug., 1988
    Abstract:
    Iron, titanium, and vanadium analysis were performed on the tunicates Pyura chilensis Molina, 1782, and Ascidia dispar, and the inorganic chemistry of blood was investigated. The major ionic characterization of the blood plasma and cytosolic solutions were determined. Gel chromatography was used to secure information on the possible existence of metal organic complexes. Pyura chilensis accumulates Fe and Ti, and Ascidia dispar accumulates Fe, Ti, and V in blood cells in this quantitative order. Significant levels of metals are associated with cell residues (membrane cells), although this may be, to some extent, dependent on the cell lysis technique. The elution behavior of plasma in Sephadex G-75 and LH-20 gels and the respective absorption spectra of the fractions showed evidence of organic metal complexes in the plasma of both tunicate species.

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  12. 12. doug 1 11:44 am 06/22/2013

    I seem to recall that tunicates are related ancestrally to all vertebrates…yes? Interesting article and I’d certainly give it a try if served in a dish as beautifully prepared as the illustration that accompanies this essay. cheers

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  13. 13. SethDellinger 12:03 pm 06/22/2013

    I’m fine with light humor or even pop references used to make science writing more accessible (although frankly, that’s why I also subscribe to Discover), but this opening paragraph regarding periods is completely unnecessary and off-putting. Regardless of the fact that the author is a woman and I am a man, this opening offends me deeply; is something humorous, gross, or laughable about human female sexuality? In order to make science accessible, we don’t have to make it dumb. Disclosure: I haven’t read any of the rest of the article.

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  14. 14. Voltaire242 6:01 pm 06/22/2013

    Sometime ago, I tasted this rare biche de la mer (yikes!). Bromine, iodine and salt.
    Incredible and awesome properties but awful taste.

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  15. 15. Morrissex 8:53 am 07/3/2013

    Just FYI (and if you REALLY want to order a dish of), they’re called “piure” in Chile (stress goes on “pi-”).

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  16. 16. foxxymcqueen 12:47 am 08/14/2013

    Commenters have called out bits of this article as “too informal”, 0.01% scientifically inaccurate, “unnecessary and off-putting”…

    Folks this is an article about a stone with guts. Chill, internet.

    Link to this
  17. 17. Becky Crew in reply to Becky Crew 2:37 am 08/14/2013

    Thanks Foxxy!

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  18. 18. PilFur 6:17 pm 08/17/2013

    Cool article! If I step on it, will it be squishy or hard? Oh, and YOLO, seriously? It’d be funny if it was funny, but maybe it’s an inside joke.

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  19. 19. peabody3000 2:29 am 12/2/2013

    really dunno why, as i watched them cut open this bitter vanadium-loaded otherworldly period-rock creature in the vid, i actually drooled. im prepared for a culinary disappointment but i’ll try it next chance i get.. YOLO!

    Link to this

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