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:

MORPHOSPECIES

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

“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.