This episode is sponsored by Endel,   an app that creates personalized  soundscapes to help you focus,   relax and sleep.

The first 100 people to click our  description link will get a one week free trial. 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.  And thank you again to Endel  for sponsoring this episode.  Endel is an app that takes everything we know  about sound, combines it with technology,   and creates personalized soundscapes to help you  focus, relax, and sleep. Their app was named the   Apple Watch App of the Year in 2020 and they have  a brand new soundscape called Wind Down that they   made in collaboration with James Blake. The  goal of Wind Down is to help you transition   from an active day to a calmer state,  so it’s great just before bed too.  Sound has a direct impact on your  physical and mental wellbeing,   and by adapting in real-time  to things like your location,   weather, and heart rate, Endel creates simple,  pleasant sounds that can help to calm your mind.  If you’re interested in trying out Endel, just  be one of the first 100 people to download it   using the link in the description and you  will get a free week of audio experiences!

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