Today, we are going to talk about one of my favorite animals: corals. Yep, I said animal. Those plant-looking things are definitely animals. It’s even weirder than that, though. Let’s dive in.
Let’s quickly orient ourselves where we are on the tree of life. In the kingdom of the animals, the so-called Animalia, the Ctenophora (sea combs, also called comb-jellies, but we established they aren’t jellies) and the Porifera (sponges) branched off first, leaving us with the eumetazoans,. The boring blobs of the Placozoa branched off, leaving the Cnidaria and the Bilateria. We’ll cover the Bilateria soon because things quickly get more complex there, but for now, we are on the Cnidaria.
We already covered the cube jellies, the hydrozoans, the true jellies, and the syphozoans, along with some less-interesting clades I can’t even remember the names of, leaving us with the anthozoans, the corals and anemones. There, we arrived.
The anthozoans are typically divided into two or three subclasses. That doesn’t sound too bad, right? But before we get into that, let’s cover what they all have in common. One thing is obvious: they are all Cnidarians, so they have cnidocytes, those stinging cells we covered in the last episode on Cnidarians. When we discussed the cnidarian life stages, we established that they typically have a larval stage called a planula larva, then they develop into polyps, and finally, there’s the medusa stage. As I said before, not all Cnidaria have all stages, but that’s the default.
Anthozoans are one of those exceptions. They kinda stop at the polyp stage, never developing into a medusa. After the larval stage, they settle on the seabed or a reef and attach to the substrate with their little mouths at the top.
You know how much I like pretending that remembering names gets easier when you understand them, so let’s start there. Anthozoa means something akin to flower animal. It’s Greek, as so often, and it makes total sense to me. They do look like plants after all.
Anthozoans are exclusively marine. That means that there are no freshwater species. And, obviously, as they are water-dwellers, there are also no terrestrial corals. Anthozoans can be tiny with less than half a centimeter—that’s like a fifth of an inch—to more than a meter in diameter. Many of them live in colonies—I’ll explain what that means in a moment—but there are anthozoans that lead solitary lives, especially the anemones, which you might have seen in that viral video of an anemone jumping away from a sea star.
Most corals on the other hand prefer a community live in a coral colony. If you want to surprise people, tell them that corals are animals. If you want to freak them out just a little, tell them that a coral is many animals. Each organism you’d call a coral is made up of many small polyps and each of those polyps is one individual animal. So, one coral is actually typically many animals. There are some small coral-species that live solitary lives, but those aren’t what people have in mind. Those branchy pretty things you see in documentaries are many animals living in unison in a colony. And then there’s their symbiosis, but more on that later.
A single polyp usually looks kinda like a gooey tube. The top is the oral disc—the mouth side—and the bottom is the petal disc—the base. As you might remember from the first Cnidaria episode, these things are diploblastic, so two-layered. The inner layer of the tube is the gastrodermis. That side faces that coelenteron, the cavity with the digestive stuff. The outer wall is the epidermal layer, which is easy to remember because your own skin is the epidermis. In between, that goo stuff is called the mesoglea.
Tentacles around the mouth contain the nematocysts, those stingy things that can hurt quite a bit. Thinking of fire corals here. Ouch. With those, they filter particles or organisms out of the water and move it into the mouth, centered in the oral disc. Some anthozoans have nematocysts in other places, but let’s not make this complicated.
Inside, the mouth leads to the pharynx which reaches into that cavity, the coelenteron (meaning: inner cavity). There, you can find the stomach which digests the food. Easy enough. So, they are essentially stomachs with a tentacle-surrounded mouth.
If they are solitary, their pedal plate attaches to substrate or rock or coral (or even slow-swimming turtles). If they are colonial, the base attaches to other coral polyps of the same colony.
Their division is pretty simple (though scientists still somehow argue about things, as always). The Hexacorallia have six-fold symmetry, so you can cut them into six equal-ish pizza slices. Hexa is the Greek suffix for six, so yeah. The Octocorallia have eight-fold symmetry, so eight equal-ish pizza slices. Octo, as you probably know means eight. Yeah, easy enough.
In some literature you’ll still find the tube-dwelling anemones, the Ceriantharia, seperated out, but they are now found among the Hexacorallia.
Octocorallia: Eight-Fold Symmetry
The Octocorallia include the Alcyonacea, the soft corals and gorgonians, the Helioparacea, the blue corals, and the Pennatulacea, the sea pens, sea feathers, and sea pansies.
The Pennatulacea are mostly colonial species, and most of them look really cool with pen or feather shapes. I guess that’s why they are called sea pens and sea feathers, duh. But yeah, very pretty. And if you remind yourself that this isn’t one animal but multiple, it gets even more fascinating again.
The weird name sea pansy, I think, comes from them shrinking away when you touch them. Or maybe because they hide under the sand usually? They are roundish and can be flat or more bulbous. Some of them are actually really pretty. Here, one polyp anchors to the sediment and the rest covers it with more traditional polyp shapes. As with most anthozoans, they divide different functions among themselves. I’d still like to know how that even works. If you’ve ever tried to organize a group, that’s bad enough, but try that without a brain. They do have a simple nervous system called a nerve net, though, which starts at the mouth and covers the tentacles. I guess it helps to be able to feel the food you are trying to catch.
The blue corals, the Helioporacea are actually brown—would have been to easy, right? There’s exactly one species in this family that’s still alive today: Heliopora coerulea. They get their name from the vividly blue skeleton made from aragonite underneath.
And then there’s the Alcyonacea, the soft corals and gorgonians. Gorgonians were easily one of my favorite things to look at while diving off Catalina. They might not look like much if you are down there because of how color gets lost with depth, but shine a light on them, and they look seriously pretty.
Anyway, gorgonians and soft corals have horny skeletons—which makes it feel kinda stupid to call them soft corals. But I guess the soft, fleshy exterior gives them their name. Blue corals aren’t blue. Soft corals have hard skeletons. Seriously, who comes up with these names? Admittedly, the skeleton is rather bendy, but as I’m not in the habit of touching wild animals, I have never tried that.
As I said, these anthozoans have an eight-fold symmetry. Keep in mind that we are talking about the individual polyps here and not the colony. Nothing symmetrical about those amazing branches.
Hexacorallia: Six-Fold Symmetry
There are quite a few orders in the Hexacorallia: the Actiniaria, the sea anemones, the Anthipatharia, the black corals, the aforementioned Ceriantharia, the tube-dwelling anemones, the Corallimorpharia, the coral anemones, the Scleractinia, the stony corals or hard corals, and the Zoantharia, the zoanthids. Well, and then there’s the Ptychodactiaria with their wooping one member, an anemone-like animal that feeds on gorgonians.
But considering there are more than 4,000 species in these orders, it’s no surprise the clade is diverse. As I said, they generally have a six-fold symmetry, but nature isn’t perfect and nature likes to piss off scientists by doing things that don’t make sense, so take that with a grain of salt. The official description now seems to be that there are six or fewer axes of symmetry, so let’s go with that.
Actinaria: Sea Anemones
Sea anemones are solitary polyps, but they get comparatively big. I wonder if they are called Actinaria because they can be so active. As I mentioned before, there’s a video where an anemone evades a sea star—yep, sea stars walk pretty quickly when they want to, but we’ll get to those later down the road when we talk about the Echinodermata, prickly-skinned animals.
Sea anemones usually live attached to a hard surface. When we used to own a reef tank, we had a poor, lonely sea anemone in there. We placed it front and center because it was so pretty, but the next morning it was nowhere to be found. Turned out it was hiding at the back of the tank upside down underneath the rock plate. Yeah. It did come back to the front at some point and hosted a clown fish pair. There are some anemones that live in soft sediment or even float freely in the water-column, but most attach to rock or coral, like our “ultra-ultra green” anemone. Yep, that was the actual name at the store. It was pink, by the way, but it glowed green in the right light.
To move around, the anemones have a little less simple system of contractile fibers. Still looks pretty weird when they move.
Anthipatharia: Black Corals
The Anthipatharia, the black corals are about as black as the blue coral are blue: their black chitinous skeleton gives them their name. In some of them, you can see this skeleton between the corals.
Ceriantharia: Tube-Dwelling Anemones
The Ceriantharia are tube-dwelling anemones. They, like the sea anemones, are solitary dwellers. They have two rings of tentacles sticking out from a fibrous tube. They can be found in soft sediment all over the world, and they look damn cool. I liked seeing them around while diving around Los Angeles.
The tube-dwelling anemones might share the name with the sea anemones and kinda look like anemones stuffed into tubes, but they are in a distinct subclass and not the same thing at all.
Corallimorpharia: Coral anemones
Next are the Corallimorpharia, the anthozoans that can’t seem to decide if they are coral or anemones. They kinda look like anemones, but they are a lot shallower and look more like mushrooms to me.
Scleractinia: Stony Corals
Speaking of, we used to have a mushroom coral in our tank. It was actually a Scleractinia, a stony coral or hard coral. These come in all shapes and sizes and make up most of what people think of when they hear coral. They can be solitary or colonial, flat or branched. There’s a lot of variety here. They all share an actual stony skeleton that gives them their name. The skeleton is from calcium carbonate and protects the soft body. So, while the gorgonians had a hard skeleton, that skeleton was bendy and on the inside. Here, the skeleton is on the outside. These corals are the basis of our coral reefs. They are what forms the actual reef.
The only soft parts exposed to the water are the tentacles, which can be retracted into or onto the skeleton. Not all of them form reefs, though, just to make things more complicated. Reef-forming are those that deposit their skeleton underneath them and thus build bigger and bigger structures.
And finally, there’s the Zoantharia, which don’t really have a common name. When we got a zoanthid for our aquarium, it was just sold as a zoa. They are colonizing and colorful. Back when we had a reef tank, we got them because they are so easy to grow. They incorporate sand and other materials into their tissues to give them structure. So, while the hard corals excrete their skeleton, these things build it. They look like buttons on a flower stem. Thanks to the aquarium hobby, they now exist in any color you’d like, but even nature is pretty impressive here.
Coral Symbiosis: A Three-Way Symbiosis
Most people who have ever watched a documentary about coral know about their cool symbiosis with dinoflagellates, so microbial protists. Most people call them algae, and no, we are not getting back into that endless debate. Go to the episode on sea grasses and sea weed if you want to hear me talk about it.
Corals partner up with these Zooxanthellae, which can also be found in some sponges, jellies, and nudibranchs. We’ll talk more about nudibranchs when we reach the mollusks. Anyway, the genus is Symbiodinium, a fitting name for someone living in symbiosis. The zooxanthellae are photosynthetic, and nice enough to share the energy with the corals in exchange for nutrients and a home. While their role used to be overestimated (Wikipedia, for example, still says that they are responsible for 90% of the energy needs of the corals), they are still vastly important. Scientists have since revealed that hunting plays a much bigger role in anthozoan nutrition than previously thought.
What most people—and even most scientists—are yet unaware of is the third player in this game. Microbes play a role in communication, or so scientists think. It’s all still pretty unclear. But there is definitely more going on than a binary relationship.
No matter what other players are involved, zooxanthella play a major role in how corals sustain themselves. They need the energy from these little protists. Unfortunately, that’s exactly where climate change and rising sea temperatures cause so much trouble. When things get too hot, the zooxanthella actually turn toxic and the corals expel them. As the zooxanthella are what gives them their color, this leaves bleached corals behind. I strongly recommend watching Chasing Coral, as they document the whole progression—including a vividly colorful in-between stage—perfectly. My words won’t do the job as well.
In addition to symbiosis with dinoflagellates—and there are other examples than zooxanthella—anthozoans form partnerships with fish, shrimp, and other animals. A great example is the clown fish hosting the anemone in my reef tank. The cute little fish would always feed their home before eating themselves. It was adorable. In exchange, they get shelter, as they are immune to the stinging cells of the anemones. At some point, we’ll go deeper into symbiosis, mutualism, and commensalism, as well as the opposite: parasites.
Our next episode on Climbing the Tree of Life will introduce us to the Bilateria, so animals with bilateral symmetry. Hint, hint, you are bilaterally symmetrical, so this is where you belong as well.