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Let’s get one thing out of the way: water fleas are not fleas.

For one, fleas are insects. Water fleas are a subset of small crustaceans called Cladocera.

There are over 600 known species of Cladocerans, but scientists have estimated that there might actually be around 2-4 times that number. Water fleas got their name because not only do they, you know, kind of resemble fleas, they’ve also been known to jump like them. And it’s kind of a strange experience to see something that looks fairly familiar reconfigured into the microcosmic.

If you look at Daphnia from the side, you can see the definite flea-type shape that informs their name. But if you manage to look at one from the top, well I could very easily lie to you and say that this is a completely different organism, and you would probably believe me. Now, of course, we would never do that.

That’s not the kind of show this is. Water fleas are large in microcosmos terms. They range from around 0.5 mm to 6 mm long, absolutely gigantic.

And they’ve got 10 sets of appendages distributed around their body, including 2 different sets of antennae. One of those is for sensing, the other is for swimming. There's also an array of 5-6 limbs that form a feeding and respiratory apparatus, as well as a pair of claws at the end of their abdomen, which they use as a filtering tool or for defense.

What might be especially striking is that one very large eye, something we don’t often see in the microcosmos. As embryos, Cladocerans have two brown eye spots. But in the last stage of their development, the eyespots merge together to form one large compound eye that will go on to help orient them while they swim around.

Cladocerans also another small photosensitive organ called the naupliar eye, which is located between the compound eye and the mouth. The water fleas we see most in our own journey through the microcosmos are Daphnia, a genus that includes more than a hundred species. And while yes, daphnia are found in lakes, they're also sold in aquarium stories because fish love to eat them.

You might see bags of them, filled with hundreds of daphnia. As they get packed together, the oxygen around them gets depleted. Cladocerans use hemoglobin to transport oxygen, and in situations like this, they can increase the concentration of hemoglobin in their bodies up to 20-fold to transport more oxygen.

And as that happens, you might actually see those packs of daphnia become a much more vivid red. Here, you can see the Daphnia’s heart beating. At 20 degrees Celsius, it’ll beat around 200 times a minute.

But that rate depends on the temperature, it beats slower when the daphnia are in colder conditions. The heart rate will also change in the presence of substances like alcohol and caffeine, but we’re a little bit too nervous about hurting our daphnia to demonstrate that. Our daphnia often come from those aquarium packs, rescued from their fate as fish-food to live a less hazardous life in our own fish-free pond aquarium.

They do occasionally get eaten by the hydras in the tank, but it's a better chance of survival compared with the alternative. Daphnia and their cladoceran relatives are filter feeders, gathering their food with a filtering apparatus made of their limbs. Key among these are their phyllopods, a set of flat legs that resemble leaves.

The phyllopods create a water current that flows from the front of the water flea towards the back, allowing it to collect and consume algae. Here, you can see a Daphnia filtering a rotifer, though the rotifer is quickly discarded probably because it's probably a bit too big for easy consumption. While daphnia are often transparent, you can sometimes get hints of just what food is dominating their diet by looking at the tinges of color that line their body.

With more green algae, the daphnia will start to look a little green or yellow. And when they eat more bacteria, you might see more of a white or salmon pink color. Daphnia are, of course, often eaten.

So, they employ several strategies to avoid predators, including the tried and true strategy of just getting the heck out of the way. Only when Daphnia do it, we call it diel vertical migration. During the day, when light is plentiful, they stake out lower, darker depths where predators are less able to see them.

During the night, they move upwards, able to eat without fear of looming threats. Daphnia arrange this diel vertical migration around the presence of fish kairomones. Kairomones are chemical signals that alert them to the proximity of fish.

If they don't detect these signals, the Daphnia might just spend the day hanging out towards the surface so they can gather more food. But kairomones don't just affect the Daphnia's migration, they also affect the size of their offspring. In the presence of fish kairomones, one daphnia species has been observed birthing smaller water fleas, a change in size that helps them avoid detection.

Interestingly though, the presence of the phantom midge, another predator, sets off the opposite result: their kairomones will make the Daphnia grow larger, presumably because the midge prefers a smaller prey. In response to the midge's kairomones, the Daphnia have also been observed growing a set of jagged edges on their head called "neck-teeth". You know, all the usual protective measures, growing extra large and having teeth on your neck.

Environmental cues also shape the reproduction of Daphnia by directly affecting the sexual composition of their population. An adult female daphnia will usually go through asexual reproduction, producing a clutch of parthenogenetic eggs every time she molts. The eggs go into her brood chamber, and after about a day, they hatch.

The young larvae stay inside her brood chamber for about 3 more days, emerging as young Daphnia that are genetically identical to their mother. With asexual reproduction, a female Daphnia might produce a clutch every 3-4 days until her death, producing more clones of herself and rapidly increasing their population. There are a few Daphnia species that are strictly parthenogenetic.

But most follow a pattern called cyclic parthenogenesis, interrupting their asexual cycle for sexual reproduction. This switch happens in response to environmental cues signaling that times have gotten tough. Maybe the population has become too dense and their food has become scarce.

Or perhaps the days are getting shorter, the temperatures lower, or the pond drier. When these external danger signals are sensed, some of the parthenogenetically-derived eggs will start to hatch into males. These male Daphnia are genetically identical to their female brood mates, after all they are parthenogenetically produced.

And that means that sex in daphnia is an environmentally determined trait. There isn’t a genetic difference between males and females, rather there’s some kind of external signal that takes a genetically identical population of daphnia and determines whether some of them will become male. Those males then reproduce with females from other clutches, creating an opportunity for true gene exchange.

Following sexual reproduction, the female daphnia will produce a protective saddle-like structure called an ephippium, which holds two eggs called the resting eggs. When the female molts, she will shed the ephippium too. It might sink to the bottom of the pond or disperse through the water or with the wind.

And where it lands, it might rest for years. It's only when the environment that surrounds the egg signals favorable conditions that they will hatch, producing females that will soon proceed back to their normal parthenogenetic reproduction schedule. Nature is full of change.

Light, temperature, your neighbors--these can alter dramatically over the course of days, weeks, and years. And so, the life of any organism, from the smallest microbe to the largest whale, is built on responding and adapting to the changes around them. Compared to many of their smaller neighbors, water fleas don’t look like they would be the most nimble in the face of existential threat.

In our own footage, they often stand out like awkward giants, straddling the thin line that separates the world of the micro and macro. How can an organism that doesn’t quite seem to fit in either world contend with the challenges of both? Quite well, apparently.

It’s as if they have managed to make the best of both worlds, mastering the reproductive strategies of one while processing the signals of the other. Yes, it is true, water fleas may not be the most elegant of the creatures we see in the microcosmos. But with a bag full of ingenious tricks, you can’t deny that they’ve got style.

Thanks for coming on this journey with us as we explore the unseen world that surrounds us. If you like this, well, that makes us really happy. And if you want to thank somebody, you can thank us, but you can also thank all of the people on this screen right now.

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