Placozoa overview, or: Placs’n’pals
Welcome back to another installment of what is apparently “Hayley goes through the animal tree of life”. Today we’re gonna be looking at a fascinating group of animals called placozoans.
Great. Who here has heard of placozoans?
Anyone? Anyone? No? Right. Let’s start from the very beginning. Put on your imagination cap.
Imagine it’s 1883 and you’re a German zoologist. In between eating schnitzel and listening to bad music by Wagner, who has thankfully just died, you like to visit an aquarium in Austria. On one such occasion you notice something sticking to the glass that looks like a big (0.5mm) amoeba. Naturally, you scrape it off to look at it under a microscope, and when you do, you realise—to much shock and surprise—that it is in fact not an amoeba, but a multicellular animal that looks and lives like one.
(Image: A placozoan under the microscope. It’s an amorphous blob about half a millimetre across, with little dots all over. Those little dots are cells.[Source])
Meet Trichoplax, the only placozoan known for over 130 years.
Now, it being the only one known for 130 years, you might assume it’s known only from a very small sample, perhaps known only from a handful of individuals recovered from that one Austrian tank. Nope. Trichoplax has been found in warm oceans all over the world, from Australia to Brazil and the Mediterranean to the Caribbean.
Other species have been proposed—hundreds of them, in fact. But it wasn’t until 2017 that any of these were found to be genetically distinct, and now we’ve got a total of three known species of placozoans, with several more proposed but not yet named. It appears that there may be a particularly high diversity of them in the Mediterranean sea, but that may just be a result of sampling. One of these new species can grow giant by placozoan standards; that is, it can get up to a centimetre long. It can also form branching shapes, which is weird.
(Image: Polyplacotoma, a recently-discovered species of placozoan. It typically looks more like a normal placozoan, but here it has taken on a branching tree-like shape and is a centimetre in length. Image from Osigus et al, 2019.)
What are placozoans? Good question. It’s a question that bothered zoologists for quite some time. Not long after they were first discovered it was proposed that they were the larva of a cnidarian (jellyfish and friends), and this idea stuck around for a few decades until they were examined in more detail in the mid-1900s. Since then they’ve been agreed to be their own thing, but exactly what that thing is, no one is quite sure. They’ve been suggested to be close relatives of cnidarians, or to have branched off before or after cnidarians or sponges (all based on DNA evidence). Either way, they’re real close to the root of the animal tree.
Placozoan means “plate animal”, and it makes sense why—in form they’re kinda flat and patty-shaped, except for when they’re branching, which isn’t often (or so we think).
Placozoans have two cellular layers, one above and one below, and six types of cells. This last part is pretty important, because it implies they are truly multicellular, not just colonial, and it’s why they were considered animals in the first place. (This was before our current, cladistics-based definition of animals). They lack any true organs or tissues, though.
(Woah, this part has been getting really text-heavy, and these animals are too small and obscure for there to be any good pictures. Here’s a giraffe to break up the text. It has absolutely nothing to do with the rest of this post.)
Placozoans have a distinct top and bottom side. Most cells on both the top and bottom have cilia (tiny hair-like structures); the bottom-layer cilia help them move around. The bottom layer also has gland cells that are thought to secrete digestive enzymes.
In between the two layers of cells is a syncytium. You might remember this from a previous post, in which they were found in glass sponges. The syncytia of placozoans, however, is not the whole body, and is just a layer in the middle of the animal. The syncytium lacks cell membranes dividing compartments (obviously), but it does have some separating boundaries called septa. This syncytium also has microfibres that apparently give it some structure, and currents of calcium ions like in glass sponges. It’s been proposed that movement of this syncytial layer (called the fibre cells) is what allows the placozoans to change shape, and it’s also been suggested that it is evolutionarily related to neurons.
Placozoans lack a jelly-like matrix between cell layers, which is present in sponges (as we’ve seen) and in cnidarians and ctenophores (as we will see).
Placs can move by squishing about like amoebas, but they can also move by sort of crawling along with their lower cilia.
Just like they have two methods of moving, they also have two methods of eating. They feed upon single-celled organisms, mostly algae and bacteria; one method of feeding involves crawling over their food item and forming a digestive cavity on the underside of their body and digesting it outside of their body, presumably using their gland cells to secrete digestive fluids.
(Image: Diagram of a placozoan doing external digestion, as described above. It moves over a food particle and then forms a cavity with its body, then secretes digestive fluids to digest it. [Source])
Alternatively, using the upper layer of cells, placozoans can draw food particles in and engulf them through their cells, in a more typically amoeba-like or sponge-like fashion.
Placozoans can reproduce asexually by splitting or by budding, and cutting a placozoan in half will result in two placozoans. In fact, blending a placozoan into individual cells can lead to the cells joining back up with each other to form back into organisms. It’s also been found that, if you blend up two placozoans together, cells that formerly came from two different animals can join up into one.
It’s assumed that placozoans can reproduce sexually as well, but this has never been directly observed. What has been observed, however, is what’s presumed to be the development of egg cells. It’s weird, and complex, and you’d probably be better off just reading the wikipedia article if you’re so inclined, but the gist is that the plac floats up into the water, curls up around its bottom side, and the bottom cells undergo a magical girl transformation into an egg. No sperm have ever been observed, and it’s been proposed that they somehow entirely lack them.
No definitive fossil placozoans have ever been discovered—unsurprising when you consider that they’re a tiny hamburger patty of cells. However, there have been some mysterious and enigmatic fossils from before the Cambrian, and these have occasionally been proposed to be related to placozoans. We’ll be looking at these next time. I hope you like pancakes.
