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The wikipedia entry on the Portuguese man o' war says:

... the Portuguese man o' war is ... not actually a single multicellular organism but a colonial organism made up of many highly specialized minute individuals called zooids. These zooids are attached to one another and physiologically integrated to the extent that they are incapable of independent survival. (emphasis added)

This is contradictory. If it's got multiple cells, and those cells are highly specialized to the point of being incapable of surviving on their own, then how does that differ from a multicellular organism? That last sentence seems like it describes the cells in my body.

In fact, the entry colonial organisms says:

The difference between a multicellular organism and a colonial organism is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular life form (e.g., cells from a brain) cannot. (emphasis added)

So, is the Portuguese man o' war a colony or a multicellular organism? And if it's a colony, can its zooids survive independently, the wikipedia text notwithstanding?

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Physalia and Siphonophorans in general are multicellular Metazoans.

But the whole discussion is about modularity on the level of individuals: Siphonophorans are colonial organisms, which means they are composed of multiple individual polyps and medusae. This is in fact quite common among Hydrozoa, but in Siphonophora the degree of integrity and function division among the components is of extraordinary level.

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    $\begingroup$ I'll buy this, but it doesn't change the basic question. If they are so integrated, why aren't they simply considered a single organism like us? They derive from a single fertilized egg. They divide asexually from that egg. The individual cells take on specialized roles physiologically and structurally. Only one set of cells reproduces. Nothing about any of this suggests colony to me. Am I missing something? To me it just seems that because there are individual polypoid cells, people are hesitant to say it is one organism. $\endgroup$ – single_digit May 5 '14 at 13:41
  • $\begingroup$ I don't know, what you mean by "polypoid cells": individual modules here are zooids. No one is hesitant to call Physalia "one organism", but it's a colonial one, a very integrated modular animal. It is colonial from the morphological, developmental and evolutionary points of view. $\endgroup$ – har-wradim May 5 '14 at 16:18
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I think the Wikipedia entry on Colonies in biology is helpful. It writes:

...a colonial organism can be distinguished from a conventional multicellular organism by the looser association and repeating nature of its component subunits—perhaps with specializations, but still visibly similar. The components can also be recognized as organisms in their own right by comparison with evolutionarily related free-living species. For example, the Portuguese man o' war is a colony of four different types of polyp or related forms. These four types can be readily seen to be analogs of one another (or of immature stages), and also of related free-living cnidarians such as jellyfish.

If I interpret this correctly, the Portuguese man o' war seems to be descended from colonies of less specialized organisms, in a way that, say, jellyfish are not. Of course, we are all descended from colonies of single-celled organisms, but I think it's clear that being a colony on the level of the man o' war is an unusual thing and worth getting a special label.

I also like the label "colonial organism". I don't know how standard that is, but it seems to recognize a man o' war is a single organism in a real sense, even though it is structured as a group of specialized parts that maybe were more obviously separate organisms in the evolutionary past.

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I'm not a professional biologist, but I just did some research, because I had the same question. It's frustrating how poorly this commonly-cited fact is explained to laypeople.

The biggest difference between a man-o-war's zooids and the cells of a multicellular organism, in principle, seems to be that each zooid is itself multicellular. And one of the things that makes them intuitively seem more like organisms than cells is that they're not microscopic - my understanding is that more or less each of the knobby and tentacly things hanging down from the bottom of the man-o-war is a single zooid (the balloon on top is also one zooid).

I would think that the fact that they can't survive being separated makes them something less than individual organisms - they are analogous to cells in many ways, but another level up in complexity. But as someone else said, this kind of system is common among marine creatures, and it seems that the convention is to call them separate organisms.

Hope someone will correct me if I got anything wrong, but I'm pretty certain that what I wrote here is essentially correct.

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    $\begingroup$ Welcome. Thank you for your answer. We ask answers to be backed up by credible references. Answers ideally don't need correction from the community - they answer the question. You might benefit from taking the tour, and a close look at the page on answering questions in the help center might prove beneficial. $\endgroup$ – AliceD May 25 at 18:20

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