Siphonophores are colonial animals. This means that they are composed of many physiologically integrated zooids. Each zooid is structurally similar to other solitary animals, but the zooids are all attached to each other rather than living independently. They do not come together to form a colony, but arise by budding from the first zooid, which itself develops from a fertilized egg.
Siphonophore zooids are of two types:
Medusae and Polyps.
Solitary medusae are better known as the true jellyfish. The most familiar solitary polyps are sea anemones.
There are other types of colonial animals which are made up of polyps, the most familiar being colonial corals.
Siphonophores differ from most other colonial animals in two fundamental respects. First, there is a high degree of specialization between the zooids. Zooids specialized 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 (which are a type of medusa) 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 specialized 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 specialized cells that are arranged into tissues and organs, siphonophores are made up of specialized zooids precisely organized 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 belong to the Cnidaria, a group of animals that includes the corals, hydroids, and true jellyfish. There are about 175 described species. Some siphonophores are the longest animals in the world, and specimens as long as 40 meters have been found. The majority of siphonophores are long and thin, consisting mostly of a clear gelatinous material. Some deep water species have dark orange or red digestive systems that can be seen inside their transparent tissues. Siphonophores are exceedingly fragile and break into many pieces under even the slightest forces. Many siphonophores are bioluminescent, glowing green or blue when disturbed. All siphonophores are predators, and use their many tentacles to capture crustaceans and small fish.
While one species of siphonophore lives at the surface of the ocean (the familiar Portuguese Man O’ War, Physalia physalis), and members of another group (the Rhodaliids) tethered themselves to the bottom with their tentacles, the vast majority of siphonophores are active swimmers and live in the water column of the open ocean. A few hardy species are sometimes found near the shore, but these are the exception.