The blue button (jellyfish) is a wonderful example of how complex evolution works. Picmybug is taking you on a thought experiment through the phylum Cnidaria.
We have a list of 11,000 species of aquatic animals infront of us, with famous members like corals, jellyfish and anemones.
I’m totally honest, the systematics of those so-called lower animals is still a mystery to me. They aren’t simple, they are highly complex! There is for example another phylum, Ctenophora, the comb jellyfish. I wonder how these two groups can be so different to separate them from Cnidaria on that level.
Lets begwith the better known Cnidaria. You might have heard that corals build two different subclasses in the phylum Cnidaria. We have stony corals and soft corals. Hexacorallia form hard colonies of six armed (or multiple of 6) polyps, with algae in their body.
Octacorallia are colonies of eight armed (or multiple of 8) polyps including algae. They use more or less soft mesoglea instead of the hard calcium carbonate which the stony corals use.
These polyps are the animals in that colony. You might tend to say the polyp has the say in this symbiosis, but I wouldn’t be too sure.
A polyp is a jellyfish-like creature, and that’s where I am heading in my thought experiment. Let’s assume all Cnidaria started their development from the most simple point: a single, floating individual.
To me polyps of a coral are kind of tiny jellyfish, with the idea to live in a community. One group decided to build houses with calcium carbonate, while the other use softer polymer. All polyps are equal, they keep their own algae, and catch their own prey from the water. It’s funny to think about them arguing about their position in the construction.
Jellyfish are the free swimmers, they are like a larger polyp with no need for a community. A jellyfish is a single animal with several body parts, and at least the adults roam around in the oceans.
So are anemones, singles, but they decided to become sessile. I want to say, in a very much simplified way, anemones are sessile jellyfish. Or a large polyp.
The blue button looks like a jellyfish, and I have to bite my tongue not to add jellyfish to the name. But they are different from all other mentioned creatures.
Unlike the jellyfish, blue buttons are not a single animal, but a colony. Unlike corals, these colonies are free swimmers, or more floaters. In opposite to corals, the members of this community aren’t all the same. They have the same DNA, but here’s the amazing story:
The so called hydrozoans are specialized to form a certain part of the colony. Somehow single creatures form body parts in order to create a button-shaped floating colony of approx. 3cm size. Those animals forming a colony are called zooids. The better known hydroid forming colonies is the Portuguese man’o war. While it has an impressive name, addressing it’s resemblance to a cool ship, the blue button is rather simple-named. I would say it deserves a more spectacular branding!
Our hydrozoa is called Porpita porpita, what seems to translate to something related to the stinging cells (cnidocysts or nematocysts) in their tentacles. It is said to be less effective than the Man’o wars stings, but no guarantee from my side, just don’t touch anything please! It belongs to the class Hydrozoa, what means nothing but water animals.
What I intentionally not mentioned is the different developmental stages of Cnidaria. Here it gets very interesting, but also complex. At different stages Cnidaria might be either sessile or floating, but there is a lot of variation.
I don’t know how many time I wrote on picmybug that the species of the post was described in Linnaeus 10th edition of the Systema Naturae. It is true in this case as well. He described over ten thousand species, so it will happen again.
The specimen in the picture was stranded with many other blue buttons, and the colonies attracted many people with their blue coloration. They did not move at all, and dried out quickly. A Google search for papers about Porpita porpita results in many new records in the Mediterranean Sea. They seem to distribute quite far, maybe they also profit from the Suez canal?
What I could not find is a number. How many zooids form a button? That’s a tough one. Researchgate has almost twice as many papers about Physalia physalia compared to Porpita porpita. Maybe I have to take it from there. The most famous hydrozoan is still considered understudied. You can find a lot about the composition of Portuguese man’o war, but I didn’t manage to find a number of individuals in one colony.
Our blue button is a floater, and so it is part of the neuston, the ecosystem in the surface of a water body. That is a gigantic surface, and one you don’t want to live in. Imagine being exposed to full blast sunshine, wind and waves, plus constant attacks from predators below.
Next time you find a blue jellyfish at the beach, you know what you are looking at!