Siphonophores

Siphonophora are colonial animals that may superficially resemble jellyfish. The best known species is the Portuguese Man o' War, also known as bluebubble or bluebottle. Siphonophora are especially interesting because they are composed of a colony of functionally specialised "zooids".

Each zooid is an individual, but their integration with each other is so strong that the colony attains the character of one large individual, and most of the zooids are so specialised that they lack the ability to survive on their own. Thus it is debatable whether zooids or siphonophora are individuals. Also, because complex multicellular organisms have cells which, like zooids, are specialised and interdependent, siphonophora may provide clues regarding their evolution.

Siphonophores differ from most other colonial animals in two fundamental respects. First, there is a high degree of specialization between the zooids. Zooids specialised for one function usually have well developed features to serve that function but lack the structures associated with other functions. For instance, the nectophores that propel the colony through the water can’t eat, and the feeding polyps can’t swim. Each is dependant on the other to do what it can’t do. Second, the specialised zooids of a siphonophore are arranged in an extremely precise pattern. This pattern is the same from colony to colony of the same species, but different between species. Siphonophores, then, have become extremely complicated organisms, just as we have, but in an entirely different way. Whereas we are made up of specialised cells that are arranged into tissues and organs, siphonophores are made up of specialised zooids precisely organised at the level of the colony. Understanding how evolution has shaped siphonophores into such complex colonial organisms may tell us quite a bit about how evolution was able to generate complex multicellular organisms, including ourselves.

Siphonophores.Org

Wikipedia article on Siphonophores