What’s Inside a Xenophyophore?

Hello, friends!

So I’ve recently become obsessed with xenophyophores.  They’re these unicellular organisms found only in the deepest, darkest reaches of the ocean.  And for unicellular organisms, xenophyophores are huge.  One species (known as Syringammina fragilissima) grows as large as 20 cm in diameter, making it almost as large as a basketball!

But how large are these unicellular organisms, really?  You see, the xenophyophore “body” is composed of both living and non-living matter.  Xenophyophores collect all this sand and debris off the ocean floor and glue it together to create a special kind of shell, called a “test.”  Xenophyophores also hold on to their own waste pellets (yuck!) and incorporate that waste material into their tests as well.

So when we talk about these gigantic single-celled organisms, how much of their size is “test” and how much is the actual single cell?  Most sources I’ve looked at are a little vague on that point, but I did find one research paper that helped me understand xenophyophore anatomy a bit better.  In the paper, researchers report on the micro-CT imagining of three xenophyophore specimens.

The word “granellare” refers to the actual living portion of a xenophyophore, and as that CT imaging paper describes it, the granellare forms a “web-like system of filaments” that spreads out through the entire structure of a xenophyophore’s test.  And the micro-CT images included in the paper show exactly that: tiny filaments, spreading out everywhere, almost like blood vessels branching out throughout the human body.

So if a xenophyophore test measures 20 cm in diameter, then you can safely assume the system of web-like filaments inside the test must be 20 cm in diameter as well.  However, each filament is still very thin, and overall the total biomass of the granellare is tiny compared to the mass of waste and debris that makes up the test.  I’m sure there’s a lot of variation by species (or morphospecies), but it sounds like the granellare only takes up between 1 and 5% of the total volume of a typical xenophyophore “body.”

So when people say xenophyophores are the largest single-celled organisms on Earth, how large are they, really?  It depends on how you’re measuring them.  Measured end-to-end, the cell really is as big as it seems.  But if you’re measuring by volume, you’ll find that the living biomass of a xenophyophore is only a small percentage of the xenophyophore’s total “body.”

No matter how you measure it, though, a xenophyophore is still enormous compared to any other unicellular organism known to modern science.

P.S.: Xenophyophores are now officially my favorite unicellular organisms.  Deinococcus radiodurans (a.k.a. Conan the Bacterium) has been demoted to second favorite.

Sciency Words: Morphospecies

Hello, friends!  Welcome back to Sciency Words, a special series here on Planet Pailly where we talk about those weird and wonderful words scientists like to use.  In this week’s episode of Sciency Words, we’re talking about:


The clearest definition I’ve found for “morphospecies” comes from Wiktionary.  According to Wiktionary, a morphospecies is: “A species distinguished from others only by its morphology.”  In other words, do these two animals look alike?  If so, then they’re the same morphospecies.  This is in contrast to taxonomic or phylogenic species, which take other factors into account, like evolutionary history or reproductive compatibility.

Classifying organisms by their physical appearance alone will lead to obvious problems.  Think of caterpillars and butterflies, as an example.  Or think of all the plants and animals that have evolved to mimic other plants and animals.  As this paper from the Journal of Insect Science warns, the morphospecies concept should only be used in circumstances “where morphospecies have been assessed as reliable surrogates for taxonomic species beforehand.”

However, in some cases physical appearance may be the only thing we know about an organism or group of organisms.  I’ve been reading a lot about xenophyophores lately.   They’re my new favorite unicellular organisms (more about them later this week).   Xenophyophores live in the deepest, darkest reaches of the ocean, and marine biologists have had a very difficult time studying them.  Given how little we know about xenophyophores, classifying them by physical appearance alone may be (in some cases, at least) the best we can do.

As a science fiction writer, I wonder how useful the morphospecies concept would be for studying and categorizing life forms on some newly discovered alien world.  It would be problematic, for sure, and I’d want to read more about this topic before sticking the word “morphospecies” into a story.  But my gut feeling is that classifying alien organisms by morphospecies might be the best we could do, at least at first.

Sciency Words: Xenophyophore

Hello, friends!  Welcome to Sciency Words!  Each week, we take a closer look at some fun and interesting scientific term so we can expand our scientific vocabularies together!  This week’s Sciency Word is:


“Xenophyophore” comes from a smattering of Greek words meaning “the bearer of foreign bodies.”  The foreign bodies in question may be grains of sand, bits of debris, the broken remains of dead organisms… pretty much anything you might find at the very bottom of the ocean is fair game to a xenophyophore.

First discovered in the late 19th Century, xenophyophores are organisms that pick up all this “foreign” material and cement it together to create a special sort of shell (the shells of xenophyophores and of similar organisms are called “tests”).  Xenophyophore shells may be very simple, or they may be highly elaborate and complex, giving some xenophyophores a superficial resemblance to coral.

According to this paper from the Zoological Journal of the Linnean Society, xenophyophores were classified and reclassified and reclassified again, over and over, for almost a century.  Then in 1972, Danish zoologist Ole Secher Tendal “rescued xenophyophores from obscurity.”  They are now classified as part of the phylum Foraminifera, within the kingdom Protista.  In other words, xenophyophores are unicellular organisms.

And for unicellular organisms, xenophyophores are huge.  Some grow to be as much as 20 centimeters in diameter, making them almost as large as basketballs!  Based on what I’ve read, it sounds like most xenophyophore species are much smaller than that–maybe a couple millimeters in diameter.  Still, for a single-celled organism, a couple millimeters is huge.

This makes xenophyophores another example of abyssal gigantism: the tendency of organisms in the deepest, darkest, most abyss-like parts of the ocean to grow to gigantic sizes.

P.S.: I couldn’t find a source to back me up on this, but I think it’s safe to assume xenophyophores have started incorporating microplastics into their shells, along with all the other “foreign bodies” they were using before.

Abyssal Gigantism on Europa?

Hello, friends!

So the first time I heard about the subsurface ocean on Europa (one of Jupiter’s moons), my imagination ran wild.  Or should I say it swam wild?  I imagined all sorts of wonderful and terrifying sea creatures: krakens with lots of horrible tentacles and teeth; crab-like creatures scuttling around on the ocean floor; and perhaps even extraterrestrial merfolk with a rich and complex civilization of their own.

As I’ve learned more about space and science, though, I’ve scaled back my expectations for what we might find on Europa.  Or on Enceladus, or Dione, or Titan, or Ariel, or Pluto… there’s a growing list of planetoids in the outer Solar System where subsurface oceans of liquid water are suspected and/or confirmed to exist.

Any or all of those worlds might support alien life.  But not giant sea monsters.  When astrobiologists talk about alien life, they’re usually talking about microorganisms.  For Europa, rather than civilized merfolk and tentacle-flailing leviathans, we should imagine prokaryotic microbes clustered around hydrothermal vents, feeding on sulfur compounds and other mineral nutrients.  If we ever find evidence that these Europan microbes exists, it will come in the form of a weird amino acid residue, or something like that.

That’s the most exciting discovery we can hope for, realistically speaking.  Unless…

On Monday, I introduced you to the term “abyssal gigantism,” also known as “deep-sea gigantism.”  Abyssal gigantism refers to the tendency of deep-sea organisms to grow larger (sometimes much larger) than their shallow-water cousins.  As an example, see the giant squid.  Or if you really want to give yourself nightmares, look up the Japanese spider crab.

The more I read about abyssal gigantism, the more my thoughts turn to Europa (and Enceladus, and all the rest).  The environment beneath Europa’s icy crust shouldn’t be so different from the deepest parts of Earth’s oceans.  So shouldn’t what happens in the deepest parts of Earth’s oceans also happen on Europa?

According to this article from Hakai Magazine, yes.  Yes, it should.  The same evolutionary pressures that cause abyssal gigantism here on Earth should cause a similar kind of gigantism on Europa.  In fact, it would be strange if that didn’t happen.  One marine biologist is quoted in that article saying: “You would have to come up with a rationale why [abyssal gigantism on Europa] couldn’t happen, and I can’t do that.”

Before you or I let our imaginations swim wild, I should note that that article from Hakai Magazine was the one and only source I could find on this specific combination of topics: abyssal gigantism and life on Europa.  So maybe take all of this with a grain of salt (preferably a grain of Europan sea salt).  But… well, I’ll put it to you this way: if someone were to write a story about a NASA submarine being attacked by sea monsters, that story would seem plausible to me.

Sciency Words: Abyssal Gigantism

Hello, friends!  Welcome back to Sciency Words, a special series here on Planet Pailly where we talk about those weird and wacky terms scientists use.  This week’s Sciency Word is:


In the deepest, darkest abyss of the ocean, animals have a tendency to grow to gigantic sizes.  This tendency is known as abyssal gigantism.  It’s also known as deep-sea gigantism.

Based on what Google Ngram Viewer has to show us, it looks like these terms (both abyssal and deep-sea gigantism) first appeared in the 1950’s, but people have obviously known that giant things live in the ocean for far longer than that.  Common examples of abyssal gigantism include the giant squid, the giant oarfish, and the Japanese spider crab.  All of these animals live in the deep, deep, deeeeeep ocean, and they all grow larger—considerably larger—than their shallow-water cousins.

What causes abyssal gigantism?  That’s not entirely clear.  As you might imagine, marine biologists have a tough time studying creatures that live that far down underwater.  But based on what I’ve read about this so far, the two most common explanations seem to be:

  • Keeping warm: Bigger animals can retain more of their own body heat.  That’s important if you live in extremely cold environments, like the deep oceans.  This is related to an ecological principle known as Bergmann’s rule.
  • Being metabolically efficient: Bigger animals tend to be more metabolically efficient, as modeled by something called Kleiber’s law.  In other words, big animals need less food relative to their size than smaller animals do.  That’s important if you live in an environment where food is scarce, like the deep oceans.

I have to admit I still have a lot to learn about this topic, and some of the things I read were a little confusing to me.  For example, I’ve read contradictory things about oxygen levels in the deep ocean and how that might factor into abyssal gigantism.

But that’s not the important thing.  You see, it’s not just that animals can grow to gigantic sizes in the deep ocean; it’s that they must.  For one reason or another, there’s evolutionary pressure on deep sea animals to get bigger and bigger and bigger.  And that’s got me thinking….

Next time on Planet Pailly, let’s revisit that very deep, very dark, very cold subsurface ocean on Europa.