Class S1E1: The Sponge Plunge

Life hasn’t always been here. In the brief time that it has on the cosmic timescale, it has definitely made its mark. For two billion years, life was stuck in an energetic canyon unable to get big and diversify, until two cells combined in a second genesis. In the two billion years since life has come and gone in staggering numbers and forms.

99.99% of all species that have ever lived have gone extinct.

The 8 million+ species on earth today, including our own, owe our existence to the survivors of 5 mass extinctions and life on earth’s refusal to quit.

Against all odds, we are here today.

Life itself can be found in some of the most unexpected, even unforgiving, places. From 10 stories beneath solid rock to the abyssal plains of the deep sea, from boiling pits of acid, to scalding deserts, frozen tundras, lush forests, and everywhere in between. Animals like us have only been around for the last 800 million years or so and didn’t make their way to land until just over 400 million years ago.

This series is about them. It’s an exploration of the diversity and genius of animal life. Each season looks at a new phylum of animal life. Each episode, a new class. 

I’m your host, Devon Bowker.

Welcome to Class

To begin our journey, we must travel to where life itself, probably, began.

The deep sea.

We all come from humble beginnings, but don’t let appearances fool you. Some of the most important tasks in nature are undertaken by the simplest of creatures. Semi-ethereal, delicate, persistent, otherworldly. Perhaps these aren’t the first words that come to mind when you think of sponges. Perhaps you never think of sponges at all. Aside from a certain sponge who lives in a pineapple under the sea who themselves isn’t even a sponge-sponge but a kitchen sponge, they hardly ever cross people’s minds. Even when it comes to cleaning or the occasional art project, most sponges used nowadays are a result of biomimicry.  

This season on Class, a deep dive into the amazing life history of sponges.

All sponges belong to the Phylum Porifera, a name which has its roots in the Latin porus for pore and ferre meaning bearing or to bear.

As an educator, I never want to assume what people already know, so I like to air on the side of caution. So, what is a phylum? 

You might remember this from school: kingdom, phylum, class, order, family, genus, species.

All animals belong to the Kingdom Animalia. As you move through the categories, you get more and more detailed and specific, all the way until you get to one individual species. 

I feel like an example might be helpful.

Okay, let’s take, I don’t know, the American Robin.

They, like us and every other animal, are in Kingdom Animalia. Then we have their phylum, Chordata. Lots of animals are in that one. Basically anything with a spine. For Class we have Aves, which means birds. Order, Passeriformes, or perching birds. The Family is Turdidae, which is why I picked robins. And then we’ll combine genus and species to get their full scientific name, Turdus migratorius…the migrating turd.

There are a lot of ways of remembering the order of things, but the overall point is that a phylum is a secondary classification of life. Classes are like subcategories of Phyla. 

Porifera is made up of nearly 10,000 species distributed pole to pole in both marine and freshwater environments, from the shallow intertidal zones of earth’s coasts to 5 miles beneath the ocean’s surface in the dark abyssal depths.

For 600 million years, they have persisted, virtually unchanging, outlasting the dinosaurs, carrying on through the Great Dying, and generally going about their lives oblivious to and unaffected by the world’s events. Some look at the simplest of life with little enthusiasm, but simple can be spectacular if only you take a closer look and shift your perspective. 

Some species such as the freshwater sponge,  Ephydatia fluviatilis, can be pushed through a cheesecloth, and the individual cells will find each other, come back together again, and make a new sponge. Sponges as a whole are ecosystem engineers, serving as foundations on which reefs are built, along with corals. They’re oases in the dark expanse of the ocean floor.

Arctic Glass Sponges take the title of the oldest living animal with an estimated lifespan of 15,000 years. There are sponges underneath the Arctic seas that were there thousands of years before we built the first pyramids, sea sponges that were alive when the Sahara was lush and fertile, before the world as any of us know it ever existed.

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 there—unaware, unphased, persistent.

What exactly are sponges?

For starters, they’re weird. Really weird.

They have no tissues, nor organs, and no symmetry.

Some have glass skeletons, & some have skeletons made of Tums. 

Under Porifera, there are 3 main classes: the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges). 

They also have 3 main forms: asconoid, syconoid, and leuconoid, each with more internal surface area and complexity than the last. From the vase-like shape of the asconoids to the brain-like leuconoid with its interconnected networks reminiscent of a metropolitan interstate system.

In general, the more surface area a sponge has, the more it’s able to eat and the larger it can afford to be, but this comes with a trade-off. Sponges must also strike the right balance between eating, and being eaten. 

Sponges with mushroom-like body plans, sticking out into the water from some surface, may have better odds of catching prey, but also up their odds of becoming prey. Others like encrusting or boring, lichen-like, sponges have the benefit of spreading themselves over vast areas, which comes with its own risks.

Sponges, like all life, have cells. Their body is essentially two layers of skin-like cells with a jelly filling. Within that filling are many types of highly specialized cells like the sclereocytes which secrete bone-like spies called spicules that give the sponge its structure. Sponges also have tons of unspecialized cells that can transform into other types and move around the body to do different jobs. Renaissance cells, if you will.

That jelly filling that those special cells move through is called the mesohyl, which basically means middle matter. Much like a traditional measure of adulthood in humans, adult sponges put down roots and they stay there. They are what are known as sessile. 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? 

Most filter-feed, but there is an exception to that rule. 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, that’s awesome, but like I said, most don’t do that. Most filter-feed. But how?

There’s no grub hub at the bottom of the sea. 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 near enough to their body.  

Sponges take water in through their pores, which cover their bodies. These pores are called Ostia, which comes from the Latin for door or opening. The water then enters into a central cavity, called a spongocoel. That water is then pumped up and out of the body through a large opening called the Osculum. 

It is in the middle of this process that the magic happens. Moving the water through the body are tiny flagellated cells called choanocytes, or collar cells, which engulf and break down particles from the water as they sweep the water through. Then, little transport cells called Amoebocytes take those nutrients from the collar cells to other body cells. Sponges can pump nearly 10,000 x their own volume in just one day.

This is where we get back to surface area.

The fewer internal passageways a sponge has means less internal surface area, which means less space for cells to attach, meaning fewer 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. 

All of this filtering not only helps to clean and clear the water but helps to make nutrients bioavailable for other species. Sponges, of course, don’t exist in isolation. Nothing does. Everything that happens in nature is connected to the life of something else. Sponges, while sessile, are irreplaceable fixtures of the ecosystems in which they reside. Reef-dwelling sponges may unwittingly serve as safe havens for the eggs of some fish, or even fish fully grown.

Some species, like the Sponge Crab, give sponges the adventure of a lifetime by holding them upon their backs and using them as an all-in-one shelter, disguise, shield, and weapon as the sponges of choice in these cases typically produce toxins that would-be predators of the crab find distasteful. 

Of course, a key element in the existence and persistence of animals is the ability to make more of themselves. So how does the sponge make more sponge? Well, one way is sexually. 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, but human use has been around for thousands of years. The weird part 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? 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 Halaven. Glass sponges are even being looked at to design more efficient fiber optic cables—these sponges that are in deep, dark, cold places are 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. There is a surreal complexity in their simplicity. When you really stop to take a moment to look at them, 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. It manages in all environments from pole to pole. It manages under extreme ocean pressure, it manages intertidal 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, might be one of the most fascinating in nature.

One thought on “Class S1E1: The Sponge Plunge

Leave a Reply