I work in a lab which is filled with people with a Neuroscience and/or Molecular Biology background. I, however, arrived late to this party, having majored in Zoology in college. While I had to do some hard studying during my PhD to catch up, it also puts me in the position of remembering odd and sometimes fun facts about animals that my colleagues are baffled by. When these moments come up I call them “this week in Zoology”, followed by a brief explanation about the animal in question. One of the first of these involved the Portuguese man o’ war, one of the most famous Hydrozoa on the planet for it’s terrible sting.
I called it a Hydrozoa, not because I want to sound smart and impressive, but because the Portuguese man o’ war is, in fact, not a jellyfish. In fact, it is not even technically a single animal.
The Portuguese man o’ war is a Siphonophore, which are colonies of individual multicellular organisms. Each tentacle is actually composed of hundreds of zooids, which are connected by tissue and all share a gas-filled sac, allowing them to float along. This is why they do not pulse, the way that jellyfish do, but rather move passively with tides and currents. This is also why a detached tentacle can sting just as badly as it can when it is attached to the organism: it is not dead tissue, as it would be if it were a detached tentacle from a true jellyfish, but a colony of stinging organisms in its own right.
However, this does not mean that each individual zooid could survive, reproduce and get along just fine on its own. The Portuguese man o war is made up of 7 types of specialized zooids, each with their own function. Siphonophores are one of those orders of animals which make you realize that evolution really did manage to create a lot of “in between” steps during the slog towards multicellularity (another great example of this is sponges, but that’s for another week in Zoology). Each zooid is made up of many specialized cells, true, but each one relies on the presence of other types of zooids to be able to get through life and reproduce. In that sense, it sort of is like one big animal, except that it’s not.
If anyone has more success than I am in coming up with an analogy to explain siphonophores a little better, please include it in the comments!
For now, I’ll just leave you with the take-home message: one of the most famous and feared jellyfish in the world is actually not a jellyfish. One of many fun facts to come to keep in your back pocket.