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Simple animals: Sponges, jellies, & octopuses

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- You and I both know, people or dogs that we don't consider particularly sophisticated. We sometimes refer to them as simple or real housewives. But when it comes to truly simple animals, we shouldn't underestimate them, because the animal phyla that we describe as being the least complex actually offer us a vivid way of understanding how animals are structured, and also how they evolved. Simple, doesn't always mean dumb. (upbeat music) Unlike those dullards that we've all meet in our lives, animals aren't considered simple because they apparently take things for granted or they think that reality TV is, reality. Their simplicity has to do with their tissue complexity. As you know, almost all animal cells are organized into tissues that perform specialized functions. The more different kinds of specialized cells an animal has, the more complex it is. And this complexity is determined in the embryonic phase. As embryos most animals ither form two layers of early tissue, called germ layers, or they form three. By exploring the very simplest phyla from animals with no layers at all, AKA sponges to the most basic of three layer animals, like mollusks. You can see how a not totally amazing sounding change in tissue results in truly fundamental, and amazing changes. So the places in the animal family tree where these transitions take place from no layers to two layers, and from two layers to three are some of the most important bench marks in animal evolution. Let's start with the very simplest of animals, and the phylum porifera, the sponges. They diverged from the protists probably 600 million years ago, and not a whole lot has changed for them since then. If you've been paying attention you've noticed by now that almost nothing that applies to other animals applies to sponges. That's because their so frickin' simple, they can't move, they just hang out and filter water for food like bacteria, while some host photosynthesizing microbes and mooch off of them. More important sponge embryos don't have any layers. They just have cells, this means that sponges don't have specialized tissues or organs, and their cells can take different forms. Some have flagella to force water into the sponge, some are more amoeba-like and wander around distributing nutrients, but these cells can transform into whatever type of cell the sponge needs. For this reason some scientists argue that sponges aren't even animals at all, they're actually colonies of cells that depend on each other to function. But for our purposes mainly because their multi-cellular eukaryotic organisms that can't make their own food, they still count, and they've managed to diversify into nearly 10,000 different species, so good for them. Things get more interesting with cnidaria, which include jellies, sea anemones, corals, and hydras. They got a couple of sweet evolutionary breaks that made them animals you do not want to mess with. First and most important break is that they developed two germ layers. You'll remember these layers are called the endoderm, or the inside derm, and the ectoderm, or the outside derm, and they form a tube that allows an animal to ingest, digest, and get rid of stuff. This makes cnidaria among the oldest living descendants of the worlds first diploblast, which is the common ancestor of all true animals, but still jellies, and anemones, and other cnidarians have only one hole that serves as both mouth and anus, and they don't have any organs so still, pretty simple. Their second evolutionary break is in there ectoderm which contains stinging cells called cnidocysts. Think Portuguese man o' war. I once stepped on a dead one. It was dead, long dead, and I wanted someone to cut my foot off it hurt so much. So now we've got two layer animals swimming around able to move, and eat, and poop, and defend themselves. The animal kingdom is just one evolutionary breakthrough away from a huge, like explosion. (explosion booms) And we can see evidence in platyhelminthes. The phylum of soft unsegmented worms that includes flatworms, planaria, tapeworms, and flukes. Not super handsome, but these guys are a big deal because they're the oldest existing phylum that is triploblastic, or has three germ layers. So in addition to an endoderm and an ectoderm. the embryos form a mesoderm. I know it's starting to sound like just another piece of toast and turkey on a club sandwich, but this development changes everything. Platyhelminthes themselves are pretty simple but a couple of phyla up the ranks, this new layer allows animals to form true organ systems. The ectoderm forming the brain and nervous system and skin. The mesoderm forming muscles, and bones, and cartilage, the heart, blood, and other very useful stuff. And the endoderm forming the digestive and respiratory systems. And this kind of complexity is only possible because one of the mesoderms key features, the coelom. A fluid filled cavity that stores and protects the major organs. It allows the internal organs to move independent of the body wall. and the fluid can provide some shock resistance. Coeloms are where all the action happens when it comes to organ systems, but not all triploblasts have them. From here on we can assess the complexity of an animal by whether it has a coelum or not, and if so how complete it is. For instance because they're the simplest of the triploblasts, platyhelminthes have their mouths and butt holes on opposite ends of their bodies, which is awesome for them. But, they're acoelomates, they don't have the ceolum which tells us that they're still on the shallow end of the pool complexity wise. To give you an idea of how simple you can cut a platyhelminthes in half and both of the pieces will happily continue on with their wormy business. That my friends is simplicity. Now you probably haven't forgotten that I mentioned an explosion a minute ago, well I'm not going to taunt you with talk of explosions without giving you one. (lively piano music) The Cambrian Explosion, not long after germ layers became a thing, say 535 million years ago. Life on Earth was undergoing some pretty terrific and rapid innovations. Over about 10 or 12 million years about half of the animal phyla that exist today started to appear. It remains the most biologically productive period in history. Think of the most creative, and vibrant, and dangerous experience, and then invite all of kingdom animalia to the party, like Burning Man and Comic-Con, and Coachella all at once. This is when animals started to look and behave as we know them today. Before the Cambrian most of the big animals were slow and soft-bodied, and ate algae or scavenged, but this explosion of diversity brought all kinds of new adaptations including predatory ones like claws, and defensive ones like spikes and armored plates. Shells and mineral skeletons made their first appearances. In fact, the adaptations were so many and so abrupt that in the 1800s the abundance of fossils from this period was used to argue against evolution. Scientists offer a lot of different theories about what caused this explosion. It was probably a combination of a few of these things. For one, oxygen levels became very high in Cambrian Seas which allowed for larger bodies and higher metabolism. It's also thought that ocean chemistry changed with more minerals becoming available for the production of shells and skeletons. And of course, with more diversity comes more competition and predation, which drove selective pressures on animals to become either better at hunting, or better at defending themselves. It's pretty near the top of my list of places I want to go once I put the finishing touches on my time machine, but for now we still have many modern animal phyla to remind us of this time of crazy awesomeness. So flukes are cool and all, but things start to get more complex with another phylum of mostly nasty parasites, nematoda, unsegmented round worms. These guys are pseudocoelomates, meaning that they have an incomplete body cavity. Unlike a true coelomate who's body cavity is contained within the mesoderm, pseudocoelomates sorta improvise one between the mesoderm and the endoderm. The vast majority of nematodes live in soil where they eat bacteria, or fungus, or parasitized plant roots, but humans host at least fifty nematode species including hookworms, which burrow into our intestines and treat us like some kind of food court. While most nematodes are very very small a single teaspoon of forest soil can have several hundred in it. Rotifera meanwhile our tiny filter feeding animals that live mostly in fresh or saltwater, though some of them can live in damp soil. They're also pseudocoelomates like nematodes and although they are way smaller than most flatworms, the big honkin' rotifera is like two millimeters long, they're anatomically more complex as they have a stomach, jaws, and a tiny little anus. My favorite fun fact about rotifera is that many of it's species are known to exist entirely of females, and they reproduce through unfertilized eggs. Fossils of rotiferas have been found as old as 35 million years and in many cases there's not a dude to be found. You go girls, okay so now for some of the big dogs, the phylum mollusca. Mollusks might be kind of simple but they're amazing, and some of them are incredibly smart. They take four different basic forms, the chitons, the snails, bivalves, and octopi and squid. Now I realize it can be hard to see how an oyster and an octopus might be related, but mollusks have some important similarities. One, they all have a Visceral Mass which is a true coelum, a body cavity completely within the mesoderm that contains most of the internal organs. Two, they also have a big muscular foot which takes different forms in each class of mollusk. Three, they have a mantle, which in some mollusks makes a shell and in others just covers the visceral mass. And four, finally all mollusks except bivalves, have a radula, or a rasping organ on their mouth that allows them to scrape up food. So chitons are these headless marine animals covered with a plated shell on one side and they use their foot to move around on rocks, scraping off algae with their radula. You know about bivalves, they have shells that are divided into hinged halves like clams and scallops. They're filter feeders so they trap particles of food in the mucus that covers their gills. Snails and slugs are the gastropods. One thing that sets them apart is a process called torsion, in which the visceral mass twists to the side during embryonic development so that by the end of it, it's anus is basically right above it's head. Most gastropods also have a single spiraled shell, and most use their radula to graze on algae and plants. And last, be certainly not least we have the cephalopods, which are the kings of the mollusks as far as I'm concerned. Cephalopods include octopi and squid, and they are obviously a lot different from other mollusks. For starters they have tentacles that they use to grab their prey, which they then bite with their beaks and immobilize with poisonous saliva. And the foot of a cephalopod has been modified into a really powerful muscle that shoots out water to help it move and steer through the water. But probably the coolest thing about cephalopods is how smart they are. While a typical mollusk might have 20,000 neurons, an octopus has half a billion. If you just do a YouTube search for octopus you'll find all kinds of videos of them opening jars and stealing peoples video cameras. They're like freakin' ocean ninjas, cephalopods got skills. So remember simple doesn't equal dumb. There is a lot to learn from our less developed cousins. Next time we'll talk about even more complex animals and what we have to learn from them.