palaeofail-explained:

I’ve been thinking about sponges a lot lately. Yes, sponges. Those sponges. I like sponges.

Let’s channel that into writing!

(Image: A photo of a diver with a very large sponge. [Source])

Sponges are a group of animals that are some of the earliest animals to branch off of the animal family tree. They’ve traditionally been classified together in the group porifera (meaning pore-bearing), but recent evidence suggests they may not form a monophyletic clade, but instead a grade. We’ll get to that later. 

Depending on how much you know about sponges, you might be surprised to learn that sponges have a lot of structure to them. Unlike the (generally plastic) kitchen sponges you’re probably familiar with, living sponges are complex, interesting creatures that have evolved to fit specific modes of life. It’s easy to think of sponges as “primitive” because they branched off so early into animal evolution, but remember: they’ve been evolving just as long as you have.

(Image: A simplified cross-sectional diagram of the wall of a sponge, with the cell types labelled. The interior of the sponge is to the left. [Source])

Sponges don’t have true tissues - they’re some of the only animals to lack them (the others being the placozoans). However, they do have differentiated cell types, and they do different jobs. 

• The epithelial cells are also called “pinacocytes”; they form the exterior of the sponge. Other than that, they don’t seem to serve a ton of function.
• Porocytes aren’t found in all sponges. They’re shaped like donuts, or churros, or like one of those squishy finger traps you might have had as a kid. Either way they line the pores of some sponges to let water enter.
• Amoebocytes are able to actively move around the organism. They can transform into other types of cells as needed, they transport nutrients, and they can turn into egg cells for doing sexual reproduction.
• Choanocytes or “collar cells” are where it really gets interesting. They have flagella that stick off into the interior of the cell and whirl around, creating water currents that draw water in. These water currents also draw microorganisms toward the body of the cell, where they are engulfed. Sponges lack stomachs, so they do all digestion within the cells. Nutrients are then distributed by amoebocytes. Choanocytes can also transform into sperm cells, since they already have a flagellum and all.

Sponges have exterior cells and interior cells, but they don’t really have cells in between. They have more of a gelatinous zone called the mesoglea or mesohyl, in which the amoebocytes can crawl around. The mesohyl is also where the spicules are located.

(Image: Scanning electron microscope image of a bunch of sponge spicules. They form a bunch of shapes, from toothpick-shaped to jack-shaped to spherical to flowerlike. [Source])

Spicules are made by the sclerocytes, another type of cell, and they’re present in most but not all sponges. They help to give the sponges structure, and they also make it a lot more unpleasant for things to eat them. They’re typically made of silica (the stuff that makes up quartz or glass), but they’re sometimes made of calcite or aragonite (the stuff that makes up limestone or antacids).

Some sponges also get some structure from a protein called spongin, which is similar to collagen. It gives structure, but isn’t very hard.

I mentioned before that sponges pump water. It comes in through the sides, into the internal cavity, and gets shot out the top. And they’re really good at it. Seriously, watch this video if you can:

There are words to describe different levels of complexity in the pores of sponges. They don’t correspond to any evolutionary relationships, and are more descriptors of the shape of the sponge. Some people make a big stink about them, but I don’t think they’re all that big of a deal. Basically, some sponges have really simple pores, and the vast majority of them have a ton of chambers in them that give more exposure time to water in order to better filter out tasty tasty microbes.

(Image: If you’ve seen anything about sponge biology you’ve probably seen this diagram. It’s really just OK. It shows the three “body plans” of sponges: Asconoid, with simple pores leading into the interior space [”spongocoel”], syconoid, with more wiggly walls, and leuconoid, with many complex chambers and a more enclosed spongocoel. In all of them, the choanocytes are located on the insides. [Source])

Most sponges are hermaphroditic - i.e., they can produce both sperm and egg. The sperm, as mentioned, come from transformation of the choanocytes, and they get shot out of the spongocoel with the outgoing water to mix with the water like the dirty, dirty little sponge sperm they are. When they find another sponge, they get sucked in through the pores and merge with eggs inside the mesohyl. Then they develop into sponge embryos and get shot out. Yes, sponges give live birth.

(Image: Diagrams of sponge larvae. They’re kinda cute! [Source])

Sponge larvae are little balls of cells that swim around with flagella on their outside, and when they land somewhere, they turn inside out and those outside cells become their choanocytes!

Sponges can also reproduce asexually, by budding! Or, if you stick a sponge in a blender, each cell can grow into a new sponge. (Or if you do that, the cells might get back together and form new sponges that are slightly bigger.)

Sponges can also move once they’ve rooted. Very, very slowly - we’re talking milimetres per day, if that, but they can do it.

I think that’s the basics of sponge biology. Let’s get into the different types of sponges!

Calcarea

(Image: The calcarean sponge Clathrina. It’s a small (centimetre-scale) yellow sponge that looks a little amorphous. [Source])

Calcarean sponges are - as the name suggests - sponges whose spicules are mostly calcite or aragonite, but not all sponges with calcite spicules are calcareans. They are mostly found in the shallow tropics, or sometimes in the polar regions. Many of them also have exoskeletons made of calcite to give them some extra strength. They come in all different body types, but all pretty much all small. There are about 400 known species as of present. 

Demospongia

(Image: A REALLY big sponge. It’s brown in colour, but it’s on the order of 2 metres call and almost spherical. There also multiple spheres. [Source])

I don’t know where the name demosponge comes from. I hope it comes from “demon sponge”, but I kinda doubt it.

Anyway, demosponges are the most diverse sponges, both in terms of number of species and different body plans. There are about 8,000 known species, and they live everywhere from shallow tropics to polar regions to the deep ocean to freshwater lakes. If you think of a sponge, it’s probably a demosponge. It’s almost hard to write about demosponges, because they encompass so much diversity. I’ll try, though.

Demosponges can have either silica or calcite spicules, but some entirely lack spicules and are supported entirely by spongin. These are the ones that have been traditionally used by humans, because they don’t have big hunks of glass or limestone in them. Smart. 

Freshwater demosponges can form what are called gemmules, a type of asexual reproduction that helps them survive droughts. Basically, they form a ball of amoebocytes and surround it by spongin and spicules to protect it, and then when conditions are better this can grow into a new sponge.

All of the really big sponges are demosponges, but there’s also some little ones. And there’s some really weird ones too. Like this one.

(Image: A deep-sea demosponge. It doesn’t resemble a sponge at all, instead resembling a white candelabra but with five branches instead of two, like if Marty McFly came back to the future only to discover he’d ruined hanukah. [Source])

That’s a deep sea demosponge. It doesn’t resemble a sponge at all, instead resembling a…well, you get the point. It’s so weird because it’s a carnivorous sponge. No, you’re not in danger from it – it’s only about 30cm/1 foot long, and it preys on millimetre-sized arthropods. Those balls are for storing sperm, and no I will not revise this sentence.

Some other demosponges have photosynthetic bacteria living inside them that help them produce food, and some others that live near black smokers host methane-eating bacteria.

Hexactinellida (glass sponges)

(Image: A glass sponge. It’s a series of ghostly white tubes; you can see the spicules through the sides. [Source])

Glass sponges are weird. Like, really weird. REALLY REALLY WEIRD. I’m not exaggerating when I say they might be the weirdest animals in existence. If you take one thing away from this post, let it be that glass sponges are weird.

There are about 500 known species of glass sponges. They have silica spicules and tend to be pretty fragile– hence the name “glass sponges” – and are generally very pale in colour. That’s because they usually live in the deep ocean, where there’s no sunlight and thus no need to camouflage yourself or scare things off or whatever sponges do with colour. They can get pretty big - up to a few metres long.

Oh, and they are basically all one cell.

One cell with thousands of nuclei.

That’s right, glass sponges have what are called syncytia, or fused cells. These aren’t unheard of in animals - muscle cells often fuse en masse. But glass sponges take it to another level. The cell membranes between the body cells are pretty much all completely gone, and the majority of the body is one cell. Different areas are separated off by so-called “plugged junctions” which are not true cell membranes. They have currents of cytoplasm that transport nutrients, nuclei, and organelles around to make sure everything is running smoothly. They also apparently transmit electrical impulses through their cell, in a manner similar to nerves. They have a reduced or absent mesohyl (understandably), and they lack the ability to contract.

They also apparently can live at least 25,000 years.

Homoscleromorpha

(Image: A homoscleromorph sponge. It’s purple-gray and wrinkly, a little bit like a brain. [Source])

Homoscleromorphs are typically quite small and make up less than 150 known species. But don’t make any mistake, because these are one of my favourite groups of sponges.

See, even though they’re fairly “boring” sponges in terms of ecology, they are really interesting in terms of anatomy. 

No sponges have true tissues, but homoscleromorphs have cells attached to basement membranes - something no other sponges have. These are pretty similar to tissues, and they’re made of type IV collagen, like the membranes of true tissues. Their sperm also seem to be more similar to those of other animals, and genetic analysis has suggested they may be closer relatives of all other animals than to sponges. 

In other words, this would mean that sponges are not a single branch of the tree of life, but instead a paraphyletic grade that might tell us something about our own ancestry (Some studies have also suggested that calcarean sponges may also be closer relatives to the rest of animals than to sponges).

(Image: One hypothesis of sponge relationships, showing homoscleromorphs as relatives of eumetazoa (the clade containing, well, pretty much all animals that aren’t sponges.) [Source – don’t judge me, wikipedia makes pretty-looking cladograms])

If this is true, it would mean that our ancestors, and the ancestors of all other animals*, would be animals that we’d call “sponges”. Homoscleromorphs have small or absent silica spicules and possess spongin; if the above hypothesis is true, it would suggest that all other animals evolved from creatures that had silica spicules and spongin, and then secondarily lost them. (And, if I may speculate idly – perhaps the free-swimming larval form of many sponges was the launching point for more mobile animals?)

Well, I don’t know how to end this post. Here’s a type of crab that likes to grow a sponge on its head (because sponges taste bad and are full of spicules).

Here are screenshots of the ROV Okeanos Explorer finding deep sea sponges because I have them and I must get more people to see they are Fantastic  

Carnivorous sponge:

 Dr Seuss ass sponge garden

These big boys are likely thousands of years old

THERE ARE SOME SPONGES WITH FENCING OVER THEIR OPERCULUM & SHRIMP GET INSIDE THEM AS SMALL LARVAE, IN MALE/FEMALE PAIRS. THE HAPPY COUPLE GROW UP INSIDE THE SPONGE & BECOME TOO BIG TO EVER LEAVE THE SPONGE & STAY INSIDE IT FOREVER IN SAFETY: