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For those of you afraid of spiders, the microcosmos probably seems like a safe haven.
When you’re scanning through a microscopic ocean full of single-celled organisms,
the likelihood of a spider crawling across your screen feels comfortingly low.
The few times we’ve seen a spider under the microscope, it’s because we put it there.
But safe as the microcosmos might seem from a surprise spider attack,
it would be misleading to pretend that it’s completely free of spider-like sightings. Because
even at this small scale, you could find yourself subject to an ambush of the arachnid sort.
These are water mites, relatives of spiders in that they are also members of the Arachnid
class. And the resemblance is uncanny, especially with the eight legs waving around.
But that detail aside, there is something about this water mite that looks
more like a cartoon spider, something you might have doodled in a notebook.
For one, there’s the eyes, those simple dots. Water mite species usually have two pairs,
which act mainly to detect light. It’s much less intimidating to stare into the eyes of a
water mite than into the eight eyes of a spider. Another reason for the cartoon spider-iness of
some water mites is down to a defining aspect of their shape. Spiders have segmented bodies,
with an abdomen that is distinct from the cephalothorax that holds its head.
But water mites aren’t segmented. Instead, those different parts are all fused together.
In the case of this particular species, the final result is round and kind of adorable.
So now we’re going to disrupt that cuteness by bringing up a creature we’ve talked about before,
one that’s more closely related to water mites and one that does not evoke any sense of cuteness.
We’re talking about dust mites, which are part of the same superorder of mites called Acariformes.
If you haven’t watched our episode about dust mites before, you should check it
out after watching this video, especially if you’re looking for motivation to clean your
room. Because without spoiling too much, the main thing to know about dust mites is that their poop
is, it’s not great. I’m bringing that up now
because while water mite poop might not evoke the same concern that dust mite poop does,
it is still fascinating. Their excretory system is made of a large tube connected to a pore.
The tube takes in waste from the mite’s version of blood, called hemolymph. And in the tube,
the waste is stored as yellow or white crystals. To clear out its storage of crystals,
the mite simply moves its muscles to push everything out through its excretory pore.
From the studies we’ve found on other mites with similar systems, our best guess is that
these poops are guanine crystals, produced by the mite’s own body as it breaks down nutrients.
Now crystal poop is weird. But the weirder thing is that crystaline poop is not the
weirdest thing that water mites excrete. But before we get there, let’s talk about just how
red some of our water mites are. They’re so red that they seem like walking drops of blood.
They are not actually chock-full of blood though; the red color is likely just some kind of carotenoid
that turns the water mite crimson. And for a time, scientists believed
that the red color was a way for these species of water mites to flash
a warning signal to fish, a sort of “Do not eat me” expressed solely through threatening hue.
But like why? Why did scientists think fish would heed this warning anyway? Why would a fish
bother to listen to a water mite when it could be eating instead?
Well, many water mite species have an approach to warding off predators that
is…sticky. Water mites have little globe-shaped organs dotting their bodies called glandularia,
each with their own little opening and a little bristle-y hair called a seta. When something, say,
a fish, brushes up against that hair, it triggers the glandularia into releasing a milky fluid into
the water, where it mixes into a tackier, more viscous substance. And for predators,
the idea of having to wade through some gloppy mess to get to your food is probably unappealing.
So when the scientists saw, in the context of their own experiments, that fish seemed to quickly
stop trying to eat water mites, they decided that the red coloring was the water mites’
way of telling the fish, “We’re the sticky prey.” But while it’s possible the fish do learn not to
pick out these water mites for food, the idea that the red color exists
specifically as a warning to predators does not hold up to the basics of how these particular
species live. Red water mite species tend to be found in transient pools of water,
places that fish don’t often find themselves. So the likelihood that they would have evolved
this trait for the purpose of warning fish is low. It’s more likely that the pigments are a way of
protecting them from ultraviolet light. It just maybe happened to also protect them from fish as well.
And the sticky fluid that makes them unappealing to fish also has another use: reproduction.
In one genus called Arrenurus, the males use it to
hold the females still during copulation. This is part of a wider spectrum of weird
reproductive behaviors. On one end, there are the species like the ones we just mentioned,
which engage very directly and physically with each other.
On the other end of the spectrum, there are water mite species where the sexes don’t feel the need
to interact at all to make reproduction happen. They don’t communicate, they don’t touch. The
males simply deposit spermatophores somewhere for their female counterparts to find them and pick them up.
And then, somewhere between the directly copulating water mites and the cold shoulder
water mites, there are the dancers. They’re sort of like the second group in that the males
drop off their spermatophores. But then they actually stick around, mostly so they can draw
females in and help them find the spermatophores. And this is where dancing comes in. Sometimes this
involves vibrating their legs to mimic their favorite prey,
drawing the attention (and aggression) of a wandering female. Other times though, this means
waltzing together as a pair, moving in circles with careful pauses to pick up the spermatophores.
We obviously don’t have video of that or we would be showing it to you. But at some point,
some scientist watched this happen. Someone watched the dance of the mating water mites,
and then they carefully documented it. Just like someone saw the mite-averse fish,
and the crystal poops, and the many other strange things that make up the lives of water mites.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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The names on the screen right now, these are our Patreon patrons. They’re the people who
make it possible for us to do this thing, and I am so grateful to you because before
this channel started I knew that I would be interested in this stuff,
but I did not know anything about it yet and now I know so much. And I hope that by coming on this
journey with us, you do too. And actually the video in the background here, of these
little mites you’re seeing, that’s actually from my Microcosmos microscopes! I found some mites
in my local pond, like the same week that we were making this video. So I sent them to our editor,
and they are getting edited in. If you would like to see more
from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram.
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