Water Fleas: Look Weird, Adapt Weirder

Summary

Water fleas, despite their name, are not insects but small crustaceans called Cladocerans, with Daphnia being the most commonly observed. Known for their flea-like appearance and jumping ability, water fleas play a crucial role in aquatic ecosystems as filter feeders. They adapt to environmental changes through behaviors like diel vertical migration and adjust their reproduction strategies based on cues from predators or environmental conditions. The video highlights their unique biology, including their large compound eye, hemoglobin-based oxygen transport, and their remarkable ability to switch between asexual and sexual reproduction when faced with survival challenges.

Takeaways

• 🦐 Water fleas, or Cladocerans, are crustaceans, not insects, with over 600 known species.
• 🐜 They got their name due to their resemblance to fleas and their ability to jump.
• 👁 Daphnia, a common genus of water fleas, have a large compound eye that develops from two eye spots in their embryonic stage.
• 🌡️ Their heart rate is temperature-dependent, beating faster in warmer conditions and slower in colder ones.
• 🦑 Water fleas use hemoglobin to transport oxygen, increasing its concentration by 20-fold when oxygen is scarce, turning them red.
• 🌊 They practice diel vertical migration to avoid predators, staying in darker, deeper waters during the day and rising at night to feed.
• 🐟 The presence of predator signals, called kairomones, can change the size of their offspring, making them smaller or larger depending on the threat.
• 🦷 Some Daphnia species can grow protective features like 'neck-teeth' to defend against predators like the phantom midge.
• 🥚 Daphnia primarily reproduce asexually but switch to sexual reproduction when environmental conditions worsen, producing males and resting eggs for survival.
• 📦 Daphnia are often sold as fish food in aquarium stores but play a crucial role in filtering algae and bacteria in aquatic ecosystems.

Q & A

What are water fleas, and how do they differ from actual fleas?

Water fleas, also known as Cladocerans, are small crustaceans, whereas fleas are insects. They share some superficial similarities like jumping, but water fleas belong to a completely different group of organisms.

Why are water fleas called 'fleas' if they are crustaceans?

Water fleas got their name because they somewhat resemble fleas and can jump in a way that reminds people of fleas, despite being crustaceans, not insects.

How large are water fleas compared to other microscopic organisms?

Water fleas are considered large in microcosmic terms, ranging from 0.5 mm to 6 mm long, making them quite large compared to many other microorganisms.

What is the function of the two sets of antennae on water fleas?

One set of antennae is used for sensing their environment, while the other set is used for swimming.

How do Daphnia's eyes develop, and what is unique about their vision?

Daphnia start with two brown eye spots as embryos, but these merge to form a single large compound eye during development. They also have a smaller photosensitive organ called the naupliar eye between their compound eye and mouth.

How do Daphnia survive in low oxygen environments?

Daphnia can increase the concentration of hemoglobin in their bodies up to 20-fold, allowing them to transport more oxygen in low-oxygen environments, which can turn their bodies a more vivid red.

What is diel vertical migration, and why do Daphnia perform it?

Diel vertical migration is a strategy where Daphnia move to lower depths during the day to avoid predators and then rise to the surface at night to feed when it's safer.

How do chemical signals called kairomones affect Daphnia's behavior and size?

Kairomones are chemical signals that alert Daphnia to the presence of predators. Fish kairomones cause them to birth smaller offspring to avoid detection, while midge kairomones cause them to grow larger and even develop defensive structures like neck-teeth.

What is the reproductive cycle of Daphnia, and how do environmental factors influence it?

Daphnia typically reproduce asexually, producing genetically identical offspring. However, under tough environmental conditions, they switch to sexual reproduction, producing males and creating opportunities for gene exchange. This ensures survival in changing environments.

How does the ephippium function in Daphnia's reproductive cycle?

The ephippium is a saddle-like structure produced after sexual reproduction that holds resting eggs. These eggs can survive harsh conditions, lying dormant for years until the environment becomes favorable for hatching.

Outlines

00:00

🦐 Water Fleas: Not Actually Fleas

Water fleas, contrary to their name, are not insects but small crustaceans known as Cladocera. With over 600 identified species (and potentially more), water fleas earned their name due to their flea-like appearance and jumping ability. A common genus of water fleas, Daphnia, has a distinctive flea-like shape when viewed from the side. They are large for microcosmic creatures, ranging from 0.5 to 6 mm, with 10 sets of appendages used for various functions like swimming, sensing, feeding, and defense. Uniquely, Daphnia have a large compound eye formed by merging two embryonic eyespots, aiding in navigation, and another photosensitive organ called the naupliar eye near the mouth.

05:03

🧬 Daphnia's Physiology and Oxygen Adaptations

Daphnia are frequently seen in both lakes and aquarium stores, as fish often feed on them. When packed together, Daphnia use hemoglobin to transport oxygen more efficiently, sometimes increasing their hemoglobin concentration up to 20-fold, which can make them appear more red. Daphnia's heart rate, typically around 200 beats per minute at 20°C, changes with temperature and exposure to substances like alcohol. The ones used in the script’s footage were rescued from aquarium packs and placed in a safer environment. Daphnia filter-feed using their limbs, notably their phyllopods, to collect food from the water.

10:08

🍽️ Daphnia's Diet and Predator Avoidance Strategies

Daphnia’s diet influences their color, with green or yellow hues appearing when they consume algae and white or pink tones when they eat bacteria. As prey, Daphnia have evolved several survival strategies, such as diel vertical migration—moving to lower depths during the day and rising at night to avoid predators. Chemical signals (kairomones) from fish alert Daphnia to the presence of predators, influencing their migration patterns and the size of their offspring. Interestingly, Daphnia produce smaller offspring in the presence of fish, but they grow larger and develop 'neck-teeth' when the predator is a phantom midge.

🌱 Daphnia's Asexual and Sexual Reproduction

Most Daphnia reproduce asexually, producing clones of themselves through parthenogenesis. Every few days, a female will generate a clutch of eggs, which hatch inside her brood chamber, and the young stay there for about three days before emerging. However, Daphnia can switch to sexual reproduction in response to environmental stressors like scarcity of food or colder conditions. During these tough times, males are produced through parthenogenesis, and sexual reproduction allows gene exchange. After mating, females produce resting eggs encased in a protective ephippium, which can remain dormant until conditions are favorable for hatching.

🌊 Adaptation in a Constantly Changing Environment

All organisms, including Daphnia, must respond to environmental changes. Despite their seemingly awkward size for the microcosmos, Daphnia have adapted well to survive in both the micro and macro worlds. Their ability to switch reproductive modes and respond to environmental cues, coupled with their various survival tactics, shows their versatility. Though not the most graceful creatures in the microscopic world, Daphnia’s clever adaptations make them remarkably resilient. The script wraps up with a nod to the creators’ patrons and a plug for more content on social platforms.

Keywords

💡Cladocera

Cladocera are a group of small crustaceans to which water fleas belong. The video highlights that there are over 600 known species, though scientists estimate the number could be 2-4 times higher. Cladocerans are a key focus of the video, as water fleas are members of this group, showcasing their unique characteristics and survival strategies.

💡Daphnia

Daphnia is a genus within the Cladocera group, often referred to as water fleas. The video specifically focuses on Daphnia, mentioning their common use in aquarium stores as fish food. They are filter feeders, and their transparent bodies often reflect the food they consume. Daphnia also exhibit interesting behavioral and reproductive strategies to avoid predators and adapt to environmental changes.

💡Kairomones

Kairomones are chemical signals that alert Daphnia to the presence of predators like fish or phantom midges. These signals play a crucial role in Daphnia's behavior, such as initiating diel vertical migration or influencing the size and physical traits of their offspring. The video explains how Daphnia respond differently to kairomones depending on the predator, showcasing their adaptability.

💡Diel Vertical Migration

Diel vertical migration refers to the behavior of Daphnia moving to lower depths during the day to avoid predators and surfacing at night to feed. This migration helps them escape from predators like fish, and the video highlights this as a key survival mechanism of Daphnia in their aquatic environments.

💡Parthenogenesis

Parthenogenesis is a form of asexual reproduction where Daphnia produce offspring that are genetically identical to the mother. The video describes how female Daphnia typically reproduce this way under normal conditions, rapidly increasing their population. It contrasts this with sexual reproduction, which occurs in response to environmental stressors.

💡Ephippium

The ephippium is a protective saddle-like structure formed by female Daphnia after sexual reproduction, containing resting eggs. These eggs can remain dormant for years, waiting for favorable environmental conditions to hatch. The video uses this concept to illustrate the Daphnia’s ability to adapt and survive in changing environments.

💡Hemoglobin

Daphnia use hemoglobin to transport oxygen in their bodies, and they can increase its concentration up to 20-fold in low-oxygen environments, like when they are packed together in aquarium bags. The video discusses this in the context of Daphnia’s physiological adaptations, noting how their bodies can turn a vivid red due to higher hemoglobin levels.

💡Compound Eye

Daphnia possess a large compound eye that helps them orient themselves while swimming. The video mentions how, during development, their two eye spots merge into a single compound eye. This eye is a distinct feature of Daphnia and contributes to their ability to navigate and sense their environment.

💡Neck-teeth

Neck-teeth are jagged edges that form on the head of Daphnia in response to the kairomones of phantom midges, another predator. The video highlights this as an example of Daphnia's physical adaptations to avoid predation. Neck-teeth offer protection, making it harder for smaller predators to eat them.

💡Filter Feeding

Daphnia are filter feeders, using their phyllopods (flat, leaf-like legs) to create water currents that help them capture food particles like algae. The video shows a Daphnia discarding a rotifer that is too large to eat, emphasizing how they selectively filter food. This feeding mechanism is central to their survival in aquatic ecosystems.

Highlights

Water fleas, despite their name, are not fleas but small crustaceans from the Cladocera group.

There are over 600 known species of Cladocerans, with estimates suggesting there could be 2-4 times that number.

Water fleas got their name because they resemble fleas and are known to jump like them.

Daphnia, a genus of water fleas, are commonly found in lakes and sold as fish food in aquarium stores.

Cladocerans have 10 sets of appendages, including two sets of antennae for sensing and swimming, and limbs for feeding and respiration.

Daphnia have a unique eye structure; they develop two brown eye spots that merge into a single compound eye in adulthood.

Daphnia use hemoglobin to transport oxygen and can increase their hemoglobin concentration 20-fold when oxygen levels are low, turning them a vivid red.

Their heart beats around 200 times per minute at 20°C, but the rate changes with temperature and in response to substances like alcohol or caffeine.

Daphnia filter feed using specialized limbs, primarily consuming algae and occasionally other microorganisms like rotifers.

They exhibit diel vertical migration, moving to deeper waters during the day to avoid predators and coming closer to the surface at night to feed.

Daphnia can sense fish predators through chemical signals called kairomones, which trigger changes in their migration and even the size of their offspring.

In response to phantom midge kairomones, Daphnia grow larger and develop neck-teeth as a defense mechanism.

Most Daphnia species reproduce asexually through parthenogenesis but switch to sexual reproduction when environmental conditions become harsh.

Male Daphnia are produced in tough conditions, not through genetic difference but as a response to environmental signals.

After sexual reproduction, Daphnia produce resting eggs enclosed in an ephippium, which can remain dormant for years until conditions are favorable for hatching.

Transcripts

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.

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