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Depending on your love of horror stories or your belief in the supernatural, it might be easy to
convince you that lakes are full of ghosts. That as you plunge deeper into these lakes’ depths,
you will come across translucent bodies that come alive when nighttime sets in.
What you might not expect, however, is that the ghosts look like this:
an elongated body with its limbs all packed close to the head,
wrenching open and closed like scissors that propel our spectral friend in jarring motions.
This is Leptodora, a genus of water flea whose transparent bodies have
inspired many of the researchers who study them to call them “ghost fleas.”
Leptodora are cladocerans—the more scientific name given to water fleas. And cladocerans
are micro-sized members of the crustacean family, making the ghost flea a distant,
approximately centimeter-length relative of the much less spooky lobsters and shrimp.
The Leptodora is more closely related to Daphnia, a creature who we’ve come across
before on our journey through the microcosmos. And for Daphnia, the Leptopora family relation
is kind of an unfortunate one because frankly, Leptodora don’t care about it. They have more
important things to focus on, like for example eating Daphnia. So if you’re trying to hunt down the ghost flea,
the key is to look for lakes that are full of plankton-eating fish, because those lakes
tend to be full of planktonic cladocerans like Daphnia. For James, our master of microscopes,
there is a specific lake in Warsaw that seems to meet the criteria,
and that has reliably turned up leptodora. But even when he’s able to track them down
and watch them under the microscope, it can be difficult for him to maintain them in his own
home. They seem to be particularly sensitive to the conditions of the water they live in,
but unfortunately we don’t know exactly what it is they are sensitive to.
So we’re lucky to be able to see the ones that we can. Now despite its transparent body, there is one
feature of the leptodora that clearly stands out: the single cyclops eye that wobbles
at the front end of the flea, attached to a tube that extends into the leptodora’s body.
When you zoom in, you can see slices that look like thin petals projecting out from
the center of the eye. There are around 500 of these slices, and they are each a crystalline
cone structure called an ommatidium. Within each ommatidium are five cells that help the
leptodora see. But just as important is what the ommatidium doesn’t have: the edges are lacking
pigments, which helps the leptodora stay more transparent to avoid detection by predators.
This eye helps the leptodora move in response to the light around it, and it may even help the flea
find areas with food. But interestingly, when a group of researchers studied the leptodora’s
hunting habits in 1989, they found that sometimes the Leptopora would be swimming and swimming in
search of food only to swim right past a perfectly good meal, as if they couldn’t even see it.
And other times, they described the leptodora attacking random bits of debris
or empty cladoceran carapaces, again, as if they couldn’t even see what they were targeting.
And what that suggested to the researchers is that perhaps the ghost flea doesn’t use its eye to
hunt. Instead, it seems to be primarily a tactile hunter, using its own body like a bear trap that
springs into action upon the slightest touch. The leptodora’s thoracic legs surround its mouth,
creating a sort of trap or basket for its food. And in fact, this structure has names like
“feeding basket” or “trap basket” to describe its purpose so very precisely.
Leptodora spend the day time at the bottom of their lakes, hiding out in the low oxygen areas
that keep them safe from fish that want to eat them. And during the night time,
the leptodora will rise to the surface, deploying its trap basket body in the process.
As it swims through the waters and prey brushes up against them, the leptodora’s body responds on
instinct, pulling its abdomen forward and grasping its legs around its new meal like a
very, very dangerous hug. And from there, the leptodora uses its mandibles
to dig the tissue out of its prey and consume it, disposing of the unwanted
bits of carapace that surround their meal the way we might discard a shrimp tail.
In some instances, researchers have also observed male leptodora using their longer antennae—which
are much shorter in females—to sweep food into their feeding basket. This is both a practical
use of antennae, and a very interesting observation because for most of the year,
male leptodora don’t actually exist. You could say that they’re the most ghostly ghost flea of all.
For most of the year, leptodora reproduce parthenogenetically,
which means that they simply lay their eggs without relying on any other fertilization.
The female leptodora’s brood chamber fills with yellowish eggs that are produced
asexually. And eventually, those eggs will hatch into larvae inside the leptodora.
And for most of the year, those hatched leptodora will be female. But in the fall,
male leptodora begin to appear. Their main function is to allow for sexual reproduction,
which gives the ghost flea a chance to mix up its genes and produce a more diverse population.
And the eggs that result from sexual reproduction are different from the parthenogenetic eggs.
Instead of being kept in the leptodora herself, these transparent eggs—called resting eggs—are
released into the water, where they’ll eventually sink to the bottom of the lake.
And the eggs remain until one day they hatch, a new ghost to haunt its lake.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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