Jellyfish…or is it?

This will be the final post on Cnidaria for now, I will be returning to them later, but I got a bit side-tracked within this phylum, and want to get back to working through evolution.

So, without further ado, let me introduce you to the topics of today’s post.

Portuguese Man O’ War. Image from wikipedia

This, quite frankly, beautiful creature is in the class Hydrozoa (from Greek  ‘hudōr ‘ meaning water, and ‘zoia‘ meaning animals).  The image above is the Portuguese Man O’War (Physalia physalis), and although these are in the same class as jellyfish, and are thought of as jellyfish by many people, due to their similar appearance, I personally would say they have much more in common with the other Cnidaria we have covered, such as the corals and sea pens.

The images below show various jellyfish, and illustrate why Portuguese Man O’War are often thought to be jellyfish.

Lion’s mane Jellyfish (Cyanea capillata). One of the largest jellyfish. The longest tentacale tail recorded was 37m!  Image from wikipedia

Box jellyfish (Chironx fleckeri). One of the most poisonous jellyfish. Image from

Freshwater jellyfish (Craspedacusta sowerbii).One of the smallest jellyfish (20-25mm in diameter)  Image from

Another interesting fact is that only one of the three images above is a Hydrozoan.  The first one (Lion’s Mane Jellyfish) is a Scyphozoan, this class is the “true” jellyfish, with a dome shaped jelly above the tentacles.  The second, (the box jellyfish) is a Cubozoan, with a square jelly, and the final one is the hydrozoan.  So, as you can see, the common names for organisms can be a bit mis-leading if trying to find relationships between organisms.  This is one of the reasons those annoying Latin names come in handy! (That, and there is one Latin name for an organism, which is the same globally, whereas each country, or even region, has its own name for each organism)

Jellyfish, of one kind or another, have been around a VERY long time, which is why I am covering Cnidaria at the beginning of the evolution story.  Usually we would not expect to find any fossils of jellyfish or similar organisms, because they have no solid body structures to fossilize and so would be expected to be completely broken down by bacteria long before any sediment is able to build up on top of them, or to be crushed within the sediment before fossilization can occur.  However, the fossil record shows jellyfish-like creatures from around 600 million years ago, and the image below is of a fossil which is ca. 525 million years old. It is likely that this jellyfish was stranded on a tidal flat, and so the imprint of its body was fossilized, much like there are fossilized wave patterns from shallow tidal flats.

Fossilized jellyfish from Winsconsin. Image from UCL Berkeley 

Anyway, I said at the beginning that I consider Portuguese Man O’War to have more in common with corals than with jellyfish, and I had probably better explain myself.

As I wrote in the posts about corals and sea pens, they are colonial organisms, made up of a lot of tiny polyps which perform different functions within the colony, and together, act as one unit.  Portuguese Man O’War are also colonial, although it is not the just the polyp which is involved in the colony building.

Portuguese Man O’War zooids (the term for a single organism within a colony) are integrated within the colony to the degree that they are unable to survive individually, so if one part is damaged too much, the entire colony dies.  This is in contrast to corals, where individual polyps may die, but the reef as a whole will continue.

It has been suggested that colonies such as Portuguese Man O’War may provide an evolutionary link between colonial organisms where each individual can survive separately, and complex multi-cellular organisms such as fish, which are comprised of many different cells, all performing a function, but which are definitely one organism. Each zooid within the colony is so specialized that it can only perform one function.  The ones responsible for feeding are unable to swim, and the ones responsible for swimming are unable to feed.  In addition, each member of the colony is genetically identical.  This is in contrast to the sea pen, which is an accumulation of individual organisms.

The polyps and medusae which make up the Portuguese Man O’War (and other Siphonophores) are found as free living individuals in other Cnidarians.

Whilst researching this post, I got to thinking “If it is a genetically identical colony, how does it reproduce?”, so off to the combined knowledge of the internet I went….  I found out that they reproduce sexually, through zooids which are known as gonozoids.  These are either male or female, and so you can have a male, or female Portuguese Man O’War (this is known as dioecy, and they are therefore dioecious organisms).

So, once I found this out, I then wondered, “if it is a colony, and they are all genetically identical seperate organisms, how does this work with sexual reproduction?” Well, the answer is that once the egg has been fertilised, and larva has developed, asexual reproduction occurs.  In complex organisms, this would be similar to the way cell division occurs after fertilization to produce the different cells which have different functions.  This process is called mitosis

The balloon structure on top of the tentacles is one polyp, known as a pneumatophore, and is filled with gas.  In other Siphonophores, this gas is similar to atmospheric gases, but in the case of the Portuguese Man O’War, the gas has a higher concentration of carbon monoxide.

I apologise for the text heavy post today, I hope it was not too long or heavy to get through, I just wanted to get across that there is more to this amazing organism than being a danger on beaches in certain parts of the world.

On Monday, I will move on from Cnidaria to the next stages of the evolutionary journey.  As I said way back at the beginning of this “Life on Earth” section, I am working my way through David Attenboroughs Life on Earth series, and we are just about at the end of the first episode now.   If you get a chance, I highly recommend you watch the series, even if you know most of this stuff already, it is a really interesting series, just as good as Carl Sagans Cosmos series.


Further Reading:

These links are to sites I have used whilst writing this, so if you want to read in more depth, these are interesting starting points.