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.