Intro
Cnidarians, they are not a part of the marvel universe like asgardians. I’m terribly sorry. Cnidarians are a ‘group’ (or a phylum in this case) in biology. We can break the Cnidarians down into 4 ‘groups’: Anthozoa, Hydrozoa, Scyphozoa and Cubozoa. Let’s introduce our jelly-like friends for our trip down Cnidarian lane:
Anthozoa
In my humble opinion Anthozoa contain the most beautiful Cnidarians, they include: corals, sea anemones, sea fans, and sea pens. They are so special since they lack what we call a ‘medusa stage’, they lack the life stage most of us would describe as a jellyfish (although the difference is not that clear in some cases).
Here you can see the differences between the polyp and medusa life stage, Anthozoans don’t have a medusa stage.
More about the anatomy later.
Hydrozoa
The best-known Hydrozoan will sound very contradictory if you have never heard about Hydrozoans. It is the genus Hydra, they do not have a medusa stage only a polyp stage, sorry, not sorry. But keep in mind this is more of an exception than the norm for Hydrozoans, most have a polyp and medusa stage. This group of Cnidarians are also known for their colonies like the Physalia physalis:
more commonly known as the Portuguese man o’ war. Just to be clear this is NOT a jellyfish but a siphonophore: a colony of polyps. They do have a very painful sting, but will not kill you unless you have a very severe allergic reaction, but that’s not different for wasps, is it?
So where are the ‘jellyfish’ in this group?
Let me show you!
Another ‘famous’ member of the group is Obelia, they have a medusa and polyp stadium:
Obelia: Medusa stage to the left, Polyp stage to the right. Both are tiny.
Scyphozoa
The group that contains most organisms that a lot of people identify as ‘jellyfish’; the ones you may have encountered on the beach, hopefully not too close. This group has a very short polyp stage and a long medusa stage. They range from 2 cm to 2 meters (diameter of their ‘bell’) Although the biggest ‘jellyfish’ (Cyanea capillata) also belongs to this group, the biggest specimen ever found had a bell diameter of 2.3 m or 7 ft 6 in. with tentacles that were 37 m long or 121.4 ft. This species is also found with much smaller proportions like a bell size of 50 cm or 20 in. So, don’t start to panic just yet, they like cold water so you can find them in the Artic and North Pacific Ocean, the North and Irish seas and around the coasts of Australia (where everything is trying to kill you, that’s just a myth by the way, not everything in Australia will try to kill you). They do live on the surface, no deeper than 20 meters. Sorry if I gave a fellow American, Australian or northern inhabitant of the earth another phobia. They mostly live in very cold water, so meeting them is not common, nor are fatal stings although they can be potentially fatal.
Cyanea capillata in the white sea, off coast of karelia, Russia.
A more common sight of a Scyphozoan: Aurelia aurite or commonly known as the moon jellyfish.
Cubozoa
Their name is pretty straight forward, their medusa stage is formed like a cube, they are most famous for their ‘extremely venomous’- membership card, some species in this class will kill you, others won’t, some are in no way a threat. Generally, you don’t have to be scared, but have fun figuring out which species is under your foot when you get stinged. It’s not that dramatic but still be careful if you see a cubozoan, just to be sure. They hunt instead of floating around and eating what passes by like Scyphozoans.
One of the very venomous species is: Chironex fleckeri. Now guess where this fella lives?
Australia!
Small stings will not pose a threat (good lord heavens almighty thank you, or maybe just good luck eh?) But larger stings most definitely will, their tentacles reach up to three meters which makes them less obvious and harder to avoid, on the other hand it also makes the chances of large sting less significant. Around 100 deaths have occurred in northern Australia in the past hundred years.
There are a few other deadly members in the group if you are interested in these things like: Malo kingi.
Anatomy
Since I could not find very simplified schemes of cnidarian anatomy I drew a few myself, their anatomy differs between different groups, I decided to try and explain the anatomy of Hydrozoans and the main structure of a polyp (generalised). This may not be 100% accurate, these animals are a bit complicated in my opinion especially since their anatomy differs. So, I decided on drawing my own (over)simplified drawings, sorry for my lack of artistic talent.
So far, there is not much to say about this polyp, but I’ll still break it down for you:
- Mesoglea: Jelly like substance between the gastrodermis and epidermis, acts as hydrostatic skeleton.
- Gastrodermis: Serves as a lining membrane of the gastrovascular cavity.
- Gastrovascular cavity: Digestion and nutrient transportation takes place.
- Epidermis: protective outer layer.
You can see the Hydrozoan polyp is built a little different than the ‘general’ polyp, introducing new terms such as peristome and hypostome, peristome simply means the region around the mouth and a hypostome is basically a stipe with a mouth.
- Gonads: where the sex cells are produced.
- Stomach: where digestion takes place.
- Manubrium: tubular structure that contains the mouth.
- 2 little dots above the ring channel: nerve rings.
- Velum: has contractile properties, squeezes water out of the subumbrella while swimming.
- They also have statocyst, which tells them what is up and down (triggered to make a stranger things joke now).
Reproduction
Since their methods of reproduction vary wildly I will only discuss one example of sexual reproduction and explain asexual ‘budding’.
Example:
Life cycle of the moon jelly (Scyphozoa):
To make things simple, in the polyp stage they reproduce asexually and in the medusa stage they reproduce sexually. I’ll show you a classic budding polyp later, the picture illustrates how it looks for the ‘moon jelly’. A polyp buds and makes several disc shaped structures, each disc breaks loose and matures into a medusa, this medusa finds a partner of the opposite sex and together they reproduce forming an egg that later becomes a planula, this planula attaches itself to a substrate and becomes a polyp. Everything starts again.
Asexual reproduction
‘Budding’; how to make 2 polyps out of one.
I can explain it very easily with a drawing, so here it is:
What’s for lunch?
Since I couldn’t have said it better than good old wikipedia, I’m going to use their piece on cnidarian feeding (not always good to trust but fine in this case since I knew what to look for).
Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. (Source: Wikipedia)
Where did they come from where did they go?
The first fossils we found from cnidarians are around 580 million years old. It’s hard to find medusa. Corals are easier to find than medusa because of their exoskeleton, only so called lagerstätten can preserve soft organisms. They’ve been around for a long time and I hope it will stay that way.
Saying goodbye
I hope you enjoyed this bonus article, I’ll be back next week with an article about Mollusca!
If you have any questions or comments, feel free to leave them in the comment section and I’ll get back to you as soon as I can!
Bye!
Sources used:
- For a picture: https://wonderopolis.org/wonder/are-sea-anemones-plants-or-animals
- http://www.ucmp.berkeley.edu/cnidaria/anthozoa.html
- For the correct English name: https://en.wikipedia.org/wiki/Portuguese_man_o%27_war
- For a picture: http://www.standaard.be/cnt/dmf20171027_03158036
- For a picture: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm/File_name/hydr063p/File_type/gif
- For a picture: https://fineartamerica.com/featured/lm-of-obelia-hydrozoa-polyps-garry-delong.html
- For a picture: https://www.telegraph.co.uk/comment/letters/10970975/Sherlock-Holmes-neednt-have-feared-the-jellyfish.html
- For the joke:https://www.pinterest.com/brittkneerose/science-jokes/
- For the picture of the life cycle: http://ocean.si.edu/ocean-photos/jellyfish-lifecycle-and-reproduction
- http://www.ucmp.berkeley.edu/cnidaria/Chironex.html
- For the picture of the moon jelly: http://www.beachhunter.net/thingstoknow/jellyfish/
- http://oceana.org/marine-life/corals-and-other-invertebrates/lions-mane-jellyfish
- my notes from invertebrate biology
- Biology: a global approach by Campbell and Reece
- my own memory
Geeky is beautiful 