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When is an organism not an organism? Or, when is it a bunch of different organisms living together? SciShow introduces you to the weird world of siphonophores.

Hosted by: Michael Aranda
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Sources:
http://dunnlab.org/assets/Dunn_2009.pdf
http://dunnlab.org/assets/Dunn2005_Bargmannia.pdf
http://www.siphonophores.org/index.php
http://www.wired.com/2014/08/absurd-creature-of-the-week-siphonophore/
http://dictionary.reference.com/browse/zooid
http://www.scientificamerican.com/article/how-do-jellyfish-reproduc/
This is a Portuguese Man o’ War. It looks like a jellyfish, right? Heck, it even acts like a jellyfish.    But it is not a jellyfish.   In fact… it isn’t even a single thing.   The Portuguese Man o’ War belongs to an order of extraordinarily weird animals called siphonophores.   There are about 175 species of them in the world, most of which live deep in the ocean.    Many siphonophores look like gelatinous strands of rope, and some of them can reach lengths of 50 meters. That’s longer than a blue whale!   But their sizes and shapes aren’t the only things that make them extraordinary.    The weirdest thing about siphonophores is that scientists still haven’t figured out if they’re a collection of colonial organisms -- like coral -- or a single complex animal made up of specialized tissues, like we are.   It’s kind of both of those things. And neither of them.   Because usually, a colony is defined as a group of organisms that work together, but can function independently.   But a single organism is composed of many interdependent units that can’t survive on their own.   Siphonophores have the scientific world all confused, because their structures don’t fit either of those definitions.    I mean they kinda resemble colonial organisms, in that they’re made up of little, multicellular building blocks, called zooids.    And in colonial organisms, like corals, each zooid can perform multiple tasks, like feeding and reproducing.    Since they can function just fine on their own, each zooid is considered a separate organism, and together they make up a colony.   But the zooids found in siphonophores are different.    From the outside, they look like they’re functioning as part of a single organism that’s hunting and capturing prey.    But if you take a closer look, you see that they’re so highly specialized that they almost work like separate organs and tissues.    The biggest clue here is that they’re arranged in a precise pattern, just like specialized cells are arranged to create tissues -- like how brain cells form brains, or muscle cells form muscles.    Take nectophores, for example -- they’re a type of zooid that’s in charge of movement.    Each nectophore acts like an individual propeller blade for a siphonophore, moving it through the water.    But that’s literally the only job a nectophore can do. It can’t eat, so it relies on other zooids, the gastrozooids, to kill and digest prey for it.   And the gastrozooids are what give the Man o’ War its sting -- each one has a single tentacle that contains a powerful neurotoxin capable of paralyzing prey … or making a grown-up cry.    The nutrition and energy that the gastrozooid gets from its prey can then be transferred to a nearby nectophore, through connecting tissue.    There are even special reproductive zooids, called gonophores, that produce sperm and eggs. These little zooids then mate with the gonophores on other siphonophores.     So, when you put all these things together, the whole thing seems like a giant, perfectly ordered commune, with each zooid mastering its job and relying on the other zooids to help it survive.   Which would mean that the Portuguese Man o’ War is just a single organism.    But that’s not really the case.   Even though they work as part of a whole, its zooids are often classified as individual animals, because they can move independently, without direction from a central brain.    And since each zooid is its own multicellular unit, they don’t divide and duplicate like an individual cell in a human body.    Instead, they’re born by budding, where each new zooid develops along a chain.    So each zooid is somewhat like a separate organism, but together, they also all form another organism.    And that… is just not … normal.    So we still have plenty to learn about siphonophores -- for example, scientists aren’t exactly sure how their zooids communicate with each other.   And it really doesn’t help that these animals are notoriously hard to study.    With the exception of a couple of species like the Portuguese Man o’ War, most of them live way down below the surface of the ocean, so it’s not exactly easy to observe them in action.    They’re also extremely fragile -- their gelatinous bodies often fall apart when they’re captured.   So, biologists have their work cut out for them when it comes to figuring out exactly what siphonophores are … or aren’t.   Just as a side note, if you happen to be swimming in the ocean and you see one of these things probably don’t touch it because bzzzt.    Thanks for watching this SciShow Dose, brought to you by our patrons on Patreon who help make SciShow possible. If you want to help us keep making videos like this, just go to Patreon.com/SciShow! And don’t forget to go to youtube.com/SciShow, and subscribe.