Phylum: Porifera

The first installment of our first ever miniseries is here–Phylum: Porifera!

Sponges are often overlooked aside from Spongebob Squarepants and cleaning. It’s time that changed. Dive into the amazing life history of sponges!

What is the phylum? You might remember that from school: Kingdom phylum, class, order, family, genus, species? There’s a lot of ways of remembering that order. Some of them are more appropriate than others. But the point is, is that a phylum is a secondary classification of life. And in this series, we’re focusing in on nine phyla of the kingdom Animalia. Once we’re done with those nine, we’ll move on to some of the more obscure because there is more than nine. Some of them are on the tiny side. Not that they’re insignificant. They’re just tiny. Lots of tiny talk today, apparently.

Today, we begin with the phylum Porifera. A little bit of etymology about that. Porifera comes from the Latin porus for pore and ferre to bear. Hence, a pore bearing animal. For most sponges conjure up thoughts of a certain individual who lives in a pineapple under the sea, cleaning products, loofahs, or something they’ve seen in a beach gift shop (maybe cake if you’re hungry enough), but sponges are so much more fascinating than that. Their cells are clumped together and not organized in a tissue, something we’ll get into more in a moment. But this means that some species such as freshwater sponge,  Ephydatia fluviatilis, can be pushed through a cheesecloth or seive, and the individual cells will find each other come back together again and make a new sponge… like, like a Backstreet Boys reunion tour or The Sisterhood of the Traveling Pants.

Most filter feed though some are carnivorous. They don’t need a brain, a stomach, a heart or any other organ for that matter, and nothing to ask the Wizard of Oz for. They can reproduce both sexually and asexually. There are 10,000 species distributed nearly pole to pole in both marine freshwater environments. Some have glass skeletons, some have skeletons made of Tums. They don’t move as adults, but that doesn’t stop them from finding ways to get around anyway. And they’ve been around for 600 million years. 600 million!

What they lack in complexity and backbone is made up for tenfold in diversity of shape, color, and its success as an organism that has managed to survive major changes to Earth’s environment, mass explosions both literally and in the form of diversity of life, mass extinctions, mass warming’s and cooling, and all this time they’ve just been kicking it—unaware unphased persistent some individuals have even borne witness to some of these changes in their own lifetime. In Arctic glass sponges take the title of the oldest living animal with an estimated lifespan of 15,000 years. There are sponges underneath in Arctic seas that were there before humans first crossed the Bering Strait into the Americas before the end of the last ice age, before we had even domesticated pigs.

In today’s episode, we’re going to cover sponge anatomy, reproduction, diet, and classification. But first, here are eight basic things that make a sponge, a sponge, and no, none of them has to do with pineapple.

1. Sponge bodies aren’t really all that symmetrical, just blobby or vase shaped or crusty.
2. They are multicellular but don’t really have any tissues and definitely no organs.
3. Sponges as adults are seccile, which means that they don’t move.
4. No nervous system, therefore no anxiety, or depression. None of that stuff.
5. Number five, they live underwater. There’s no such thing as a land sponge. The cleaning ones are usually artificial.
6. They filter feed, mostly.
7. They reproduce both sexually and asexually.
8. No body cavity. Granted, there is a lot of cavities and a very large one that’s relatively in the center. But that’s not a body cavity. It’s just an open cavity… I’ll explain.

But let’s start with the classification. Under Porifera, there are 3 main classes: the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges). Those classes are made up by seven orders, then families, more genuses, then nearly 10,000 species, nearly twice the amount of mammal species and close to the same amount of birds. And you know how many birds there are— like tons. You could spend your whole life as an avid birdwatcher wanting to check off every bird on the list, and you could probably never do that. I mean, you’d have to be really wealthy and have really good eyes, and all kinds of stuff.

They range from tidal zones to depths of over five miles around the world. Five miles! Can you imagine being five miles underwater, especially a creature like this that can’t move that has no real skeleton? We’ll talk about that.

Sponges are tricky, but clever little things. We like to think of them as simple and they are… like, literally they belong in the category of simple animals because they only have one layer of cells to their body. The rest is just you know, clumpy stuff, which we’ll talk about that but there’s not a whole lot going on there.

There’s a reason that their body plan has persisted. It works. Sponges, like all life, have cells. The tricky bit is that unlike many organisms that have specific cells for specific jobs, like lung cells, blood cells, brain cells, etc. sponges have tons of unspecialized cells that can transform into other types and move around the body to do different jobs, sort of like a bunch of temps in college.

The body is made up of masses of these cells in a gelatinous jello matrix called the mesohyl, which basically translates to middle matter. Sponges can be classified into three major body types, asconoid, syconoid, and leuconoid. That is in order of increasing internal surface area. Here’s why that’s important.

Much like a traditional measure of adulthood in humans, adult sponges put down roots and they stay there. They are what are known as seccile. I’ve said that a couple times. They don’t move unless they found themselves attached to something like a hermit crab. So how do they eat? There’s no grub hub at the bottom of the seat. Yet, that’s also kind of exactly how they do it…if the grub hub driver was the ocean, and the food was like whatever is floating around in their body (maybe not all metaphors work). They just filter food out of the water or sometimes it’s all based on an odd symbiotic relationship with tiny organisms on its body. Anyway, they take water into their central cavity, which is called a spongocoel, at the bottom of their body. They take it in through their pores, which are all over, they’re called Ostia, which comes from the Latin for door or opening, and it flows upward out of the body like a chimney through a part called the Osculum. The water is moved through the body powered, it doesn’t just flow into the bottom and out the top. That wouldn’t make a whole lot of sense. The sponge is actually actively moving the water using flagellated cells called choanocytes, or collar cells, which engulf particles from the water, break them down. Then, little transport cells called Amoebocytes take those nutrients from the collar cells to other body cells.

Now, the less internal passageways a sponge has means less internal surface area, which means less space for cells to attach, meaning less cells for filter feeding, and less resistance for water to pass through, so it moves more quickly, meaning the cells that are there have trouble catching stuff. This means that the asconoid body structure is super limiting, meaning those sponges tend to be smaller.

The more internal passageways a sponge has means more surface area, more cells filter feeding, more resistance to waterflow. Therefore slowing the speed of the passage of the water, making leuconoid the most efficient and effective body structure that a sponge can have. That structure looks more akin to like a brain, but full of weird channels and tunnels and squiggly cells, while the syconoid looks more like a flower vase, and the leuconoid tend to be kind of crusty or small with little protrusions that you typically expect.

Now, a bit ago, I did mention that some are carnivorous, and that is an exception. Sponges of the family Cladorhizidae are especially unusual in that they feed by capturing and digesting whole animals. That’s right. These sponges are carnivores…meat-eating sponges, it’s crazy! They capture small crustaceans with these parts called spicules, that act like Velcro, and when they come into contact with the exoskeleton of a crustacean, cells migrate around that prey and start to digest the crustacean outside of their body and then slurp up all the nutritious goodness!

So more about the structure. Sponges don’t have the typical skeleton. Their body is supported by calcium carbonate, which actually is in an antacid (t’s what Tums are), or glass like silica projections called spicules, and in protein fibers called spongin. Most of what you see are these parts, they’re not cells.

So how does the sponge make more sponge? Well, one way it’s sexually, of course, and in other ways, but first, they release sperm and eggs into the water, sometimes so much that it almost looks like they’re smoking like a hydrothermal vent. It looks like a chimney, but it’s a cloud of eggs and sperm being released into the water where they fertilize each other. Then that fertilized little clump becomes a free little, little swimming sponge baby, literally. Like, like a little plankton, like a little swimming sponge baby until it settles on a place to set anchor for the remainder of its days. Sponges that reproduce asexually produce buds, or more often gemmules. If you don’t know what a gemmule is, it’s amazing. It’s like a little survival packet. It’s like it’s kind of like a pod that Kal-El (Superman) was put into except if it was multiple with different variations of powers. It’s packets of several cells of various types inside a protective covering.

So as I mentioned before, we don’t really think of sponges all that much aside from cartoons and cleaning, right. But the human use has been around for thousands of years. The weird part is is that somebody at some point decided, “hey, see this thing in the water? I bet if I beat it until it’s tenderized, and extra squishy and super absorbent, I can use this to clean my body!” Like what kind of weirdo does that? But nevertheless, it persists. And we still use them to this day. But that’s not all. That’s not their only use. Sponges have even been used to develop anti cancer drugs like eribulin mesylate, which has the brand name of Halaven. Glass sponges are even being looked at to design more efficient fiber optic cables—these sponges that are in deep, dark places, cold places being looked at to potentially enhance our internet. And of course, yeah, cleaning. But here’s the real thing about porifera that I find fascinating. While they might be simple, they’re also so complex. When you really stop to take a moment to look at them to, to understand how they work, to look at their internal structure, and you realize that they have persisted for millions of years. The first real complex multicellular animals existing all this time and not really seeing any kind of need to change. It means that they figured out the solution to the problems that they were having long ago and that solution persisted. I find something really fascinating about that. And about how we who think we’re so complex are able to look at something that we think is so simple and learn from it, because it has solved a problem. It manages in all environments from pole to pole. It manages under extreme ocean pressure, it manages and tidal zones, it manages in nutrient-dense and nutrient-poor waters, it has symbiotic relationships with tiny, tiny organisms instead of eating them or engulfing them, it works together with them. This is a body structure that, while consciously is not making these decisions, that actually might be the most fascinating part is that nature.