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When you picture a single cell, you probably imagine something super tiny that you had to look at through a microscope. But, there are some huge exceptions to this rule. And we really do mean huge.

Check out Journey to the Microcosmos here!
https://www.youtube.com/channel/UCBbnbBWJtwsf0jLGUwX5Q3g

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Sources:
Stentor coeruleus
https://pubmed.ncbi.nlm.nih.gov/25202864/
https://doi.org/10.1038/s41598-019-47701-8
https://doi.org/10.1016/j.cub.2016.12.057
https://doi.org/10.1016/S0003-9365(83)80036-0
Gromia sphaerica
https://doi.org/10.1016/S0967-0645(99)00100-9
https://doi.org/10.1016/j.cub.2008.10.028
https://doi.org/10.1186/s41200-019-0183-4
Spiculosiphon oceana
https://pubmed.ncbi.nlm.nih.gov/26312358/
https://doi.org/10.1016/j.marenvres.2018.02.015
Valonia ventricosa
https://doi.org/10.1007/978-1-4684-1116-4_19
https://doi.org/10.1098/rspb.1958.0011
https://doi.org/10.1098/rspb.1937.0011
https://doi.org/10.1038/225760a0
https://doi.org/10.1098/rspb.1932.0005
Acetabularia
https://doi.org/10.1016/S0074-7696(08)61042-6
https://pubmed.ncbi.nlm.nih.gov/25964794/
https://doi.org/10.1146/annurev.arplant.49.1.173
https://doi.org/10.1007/978-3-642-46169-9_1
https://doi.org/10.1111/j.1432-0436.1979.tb01022.x
https://doi.org/10.1038/scientificamerican1166-118
Caulerpa
https://doi.org/10.1371/journal.pgen.1004900
https://doi.org/10.2306/scienceasia1513-1874.2006.32(s1).057
https://doi.org/10.1086/702758
https://doi.org/10.2307/1311764
https://pubmed.ncbi.nlm.nih.gov/26464789/

Image Sources:
https://commons.wikimedia.org/wiki/File:Stentor_coeruleus_1.JPG
https://commons.wikimedia.org/wiki/File:Mikrofoto.de-Hydra_15.jpg
https://commons.wikimedia.org/wiki/File:Stentor_coeruleus_extended.jpg
https://en.wikipedia.org/wiki/File:Stentor_dividing.ogv
https://ars.els-cdn.com/content/image/1-s2.0-S0960982208013973-gr3_lrg.jpg
https://www.sciencedirect.com/science/article/pii/S0960982208013973#fig1
https://commons.wikimedia.org/wiki/File:Spiculosiphon_oceana_AB.png
https://mbr.biomedcentral.com/articles/10.1186/s41200-019-0183-4/figures/5
https://commons.wikimedia.org/wiki/File:Spiculosiphon_oceana_SEM.png
https://www.flickr.com/photos/oregonstateuniversity/3525931943/in/photolist-Q1Z67C-7UfZHw-oCpd8C-oCpsRe-oUBVT4-rrETCC-oCpsYt-rrETPE-6nzjJ4-2dg4Ge2-r7fTsR-X7ydNv-36v5Dy-f7XJAv-qU2EER-31eKAL-aYMcst-SFjeDD-RoLTYb-Rs2NtV-RoLNfh-7UfiDu-7UfhU5-7UcB78-7UfMu5-7Uc2VD-7UczyF-7UfSmU-7UfVod-7Ucyjg-7UfTmd-7UbPGg-7UeZMN-6DiYeS-68JK1c-28bMJXe-5QoMGv-7Tvp4L-5QoMD4-5Qt4VY-5QoME2-7UfX3C-7UfYvu-2WVJry-7VbxhJ-24utEGH-2UnLST-uPi1BV-vL9anF-uP7wsi
https://commons.wikimedia.org/wiki/File:Ventricaria_ventricosa.JPG
https://www.inaturalist.org/observations/37102959
https://commons.wikimedia.org/wiki/File:Acetabularia_acetabulum.jpg
https://commons.wikimedia.org/wiki/File:Acetabularia_sp.jpg
https://commons.wikimedia.org/wiki/File:Acetabularium_Expt_2.jpg
https://www.istockphoto.com/photo/bubble-algae-valonia-ventricosa-gm899112840-248096897
https://commons.wikimedia.org/wiki/File:Acetabularia_mediterranea_life.svg
https://www.inaturalist.org/observations/41805018
https://www.inaturalist.org/observations/19462286
{♫Intro♫} If I asked you to picture a single cell,.

I bet you’d imagine something super tiny that  you had to look at through a microscope. Which is totally valid, considering the  vast majority of cells on this planet are microscopic—especially if we’re talking whole  organisms that consist of just one cell.

But, there are some huge exceptions to  this rule. And I really do mean huge. The six big single-celled creatures we’re going to  talk about today are not your average cells.

And because of that, they’ve challenged  our ideas of how life works—at the cellular level and beyond! When scientists first spotted the trumpet-shaped Stentor coeruleus in pond water  in the late 1700s, they mistook it for a hydra —the anemone relative, not  the mythical monster. Hydras are made up of tens of  thousands of cells, though, and stentors are but one.

Each can grow to be up to two millimeters long—which, ok, is small. But, it makes it  a colossus in the unicellular world. And it’s big enough to be unwieldy.

You see, one  of the reasons cells are small is that large cells are too hard to control; it simply takes too long  to send information from one part to another. But stentors can achieve their large  sizes because they have an elongated macronucleus which puts the critter’s genetic  material everywhere it’s needed. They also have lots of small micronuclei that  help out when it’s time to make more stentors, which they do the really old fashioned  way: by simply splitting into two.

But perhaps the most remarkable thing about  them is their ability to regenerate. Most cells don’t fare so well if their membranes  are ruptured. But cut a Stentor in half, and you’ll soon have two totally fine stentors.

In fact, thanks to the shape of its macronucleus, almost any piece of Stentor can regenerate; it  just needs part of the macronucleus and a small piece of the original cell membrane. Researchers still have lots of unanswered questions about this creature's  unique healing abilities.

Like: Why don’t the cell’s insides  leak out after it’s cut open? How can multiple individuals regenerate from  different fragments? And how does each fragment know which structures they need? But the more researchers study these weird little trumpets, the more  they challenge our understanding of what cells can and can’t do.

Answering these questions will help us understand the full capabilities of  all cells—microscopic or not. In the dark depths of the  ocean, some 1200 meters down, are weird little critters called Gromia. And when they were first discovered in the early 2000s, researchers were completely puzzled.  They’d simply never seen anything like them.

It was later determined that these hard,  grape-sized cells are a kind of testate amoeba: amoebas that make themselves porous,  protective shells called tests. Which is super cool; but what’s  even cooler is that they move. Scientists believe gromias crawl ever so slowly  by extending tiny feet-like projections called pseudopods through holes in their tests.

And they leave behind noticeable tracks in the sediment. These just so happen to look very similar to fossilized tracks that date back as  far as 1.8 billion years—tracks that were said to be evidence for multicellular life. You see, many paleontologists argued that there was simply no way for a single-celled  organism to make gouges like that.

Now, of course, we know that’s not true. So the discovery of Gromia challenged the evolutionary timeline. And now, most paleontologists think multi-celled creatures didn’t come on the  scene until about 600 million years ago—just before the Cambrian, when multicellular  life as we know it really took off.

Back in 2013, scientists exploring hydrothermal  vents off the coast of Sicily found what they thought was a new species of sponge. The several-centimeter long creatures each had a stalk that penetrates into the sediment  and holds it in place, and a globe-like top with lots of thin spikes sticking out of  it—much like carnivorous sponges. And those spikes look a lot like spicules: the  small, hard structures made by sponges.

It wasn’t until later that researchers realized  the organisms weren’t animals at all. They actually belonged to a group of single-celled  amoeba-like organisms called forams. And they decided to call them  Spiculosiphon oceana.

The researchers were right to be thrown  by those spicule-like structures, though… because they are, in fact, spicules. Each Spiculosiphon collects and painstakingly arranges spicules from carnivorous  sponges to steal their look. That’s because it allows them to  capture prey, like small invertebrates, in the same way carnivorous sponges do.

So yes, these are single-celled organisms that can devour both single- and multi-celled  organisms by dressing up like a multi-celled predator. Biology is weird. These unique creatures are only found living close to hydrothermal vent systems.

And that may be because they prefer their water acidic and packed with the dissolved minerals  that spew out of these underwater volcanoes. In fact, researchers think they could tell  scientists where undiscovered vents are. Basically, if spiculosiphons are there, there must  be recently active hydrothermal vents nearby!

And they may also be a canary in the coal  mine of sorts for ocean acidification. That’s because, while they like things acidic,  they can’t tolerate super acidic conditions. Their stolen shells would dissolve.

So if they start struggling, that could indicate deeper waters are  feeling the effects of climate change. Valonia ventricosa basically looks like what  you would think a cell should look like, just super sized. They can grow to be four centimeters or larger.

They’re also known by the nickname “sailor's eyeball”, because back in the late 1800s, people  thought that they were actually eyeballs staring back at them from underwater. But I think they look much more like giant underwater grapes. And, in fact, they’re kind of like grapes… at least structurally.

The outer shell, or cell wall, contains a lot of cellulose—the same fibery  substance found in all sorts of plants, including the skin of grapes. In fact, 19th and early 20th century scientists were able to work out the structure  of cellulose by studying Valonia. They were able to look at it up close using  x-rays and fancy microscopes to see that cellulose is made up of microfibrils:  little parallel strands of cellulose bound together through hydrogen bonds.

It’s those bonds that make the cellulose rigid, which is how it gives Velonia the  toughness it needs to grow so big. This deeper understanding of cellulose  gained by studying Velonia likely played a role in the industrial development of  cellulose-based products, which includes everything from plastics and lacquers to  paper, cardboard, and more famously, film. Mermaid’s wine glasses, or Acetabularia  if you want to be scientific, can grow up to 10 centimeters tall.

And they kind of look like fungi that you might find carpeting the forest floor, with  their stems and rounded caps. But they’re actually single-celled  algae that can carpet the seafloor in subtropical shallow, rocky waters. Acetabularia has one giant nucleus located at the base of its stem.

And because of that, it helped biologists understand what the nucleus actually does. During the 1930s and 1940s, German scientist Joachim Hammerling performed a series of  experiments on Acetabularia to prove that the nucleus was the control center of a cell. He cut multiple specimens in half and found that only the ones containing the  nucleus could regenerate.

He also grafted together two species  of Acetabularia, and showed that the combined cell would take on the characteristics of  whichever species its nucleus came from. Those experiments laid the groundwork for  our modern understanding of the nucleus. It is a bit weird, though, that it’s able  to grow so large, since all of its genetic material is down at the base.

Like, those Valonia we just talked about? They have lots of nuclei. In fact, once you get larger than a Stentor, many single-celled plants have multiple nuclei  which work together to manage the huge cell.

Acetabularia gets around this by sending  some of its genetic material to the upper parts of the cell as messenger RNA; it has all the machinery it needs up there to turn those into the proteins that  that part of the cell needs. And in a surprising turn of events, it  actually does have multiple nuclei … but only when it’s about to reproduce. When a mermaid’s wine glass wants to make more wine glasses, its big nucleus goes  through several rounds of division to make little nuclei.

Then all those little copies  make their way up to the top of the cell. From there, they bud off into  numerous spore-like reproductive cysts which can float about for a ways before they  settle down and grow to their full size. At first glance, you would probably have no idea  that Caulerpa algae are individual cells.

Individuals in some species can grow  to carpet a square meter or more! And they sure don't look like your average  cells. Each has a complex structure that consists of a stem, fronds that look like leaves,  and root-like parts that anchor it in place.

The reason a Caulerpa can grow so large  is because it has loads of nuclei to keep all parts of it functioning properly. Caulerpa species also use a process called cytoplasmic streaming, which is essentially a  plant’s solution to a circulatory system. They always keep their cytoplasm  moving in an organized fashion, which allows them to quickly transfer molecules  and other structures around the cell.

Lots of scientists have studied  these algae over the decades. Researchers were particularly interested in  learning more about how they get their intricate structures, because it was long assumed that  kind of complexity required multiple cells. Now, they believe it all boils down to when  and where different genes are active.

For instance, research published in  2015 showed that different genes are active in the nuclei in different parts  of the cell, and these seem to be telling those regions what shapes to be. This has deepened our understanding of how all cells figure out their  final shape and structure. And it’s actually calling into question whether  any plant is truly multicellular.

You see, some researchers argue that all plants  should be thought of as single-celled organisms, because their cells are interconnected  and communicate with each other through channels called plasmodesmata. And if cells are super connected, are they really separate cells? It’s an  intriguing question, anyway!

While humongous cells like Caulerpa are the  exception, not the norm, because of their size, they’ve given researchers throughout history the  incredible opportunity to dig into the typically microscopic world of cells. And because of that they can help us understand the inner workings  of all cells, big and small. In fact, you may have noticed that all of  these giant cells have something in common: they are all found in water!

While it is possible for a large cell to exist on land, they are usually living in humid areas  that have recently received heavy rains. Life in the water means less stress  for cells of any size — no drying out, nutrients and waste products easily diffuse  in and out, and the fluid pressure gives some extra support to their walls. So even in their placement on this planet, these cells have taught us a lot!

Plus, they’re just unbelievably cool. Thanks for watching this episode of SciShow,  which is produced by Complexy. If you loved learning about these weird big  cells, I think you’ll love our Complexly sister channel: Journey to the Microcosmos.

Journey to the Microcosmos is a relaxing journey to the world where stentors really are fearsome  giants, with incredible microscopic footage by. James Weiss, music by Andrew Huang, and  the soothing voice of Hank Green. Plus, they’ve done whole episodes on stentors.  So you might consider checking those out next. {♫Outro♫}.