The Portuguese man-o’war, Physalia physalis, is one of around 175 species in the order siphonophores, class Hydrozoa. As one that drifts on the surface, blown by winds and carried by currents, it is not a typical siphonophore; most species are active swimmers and live beneath the surface. A few – the rhodaliid family – spend their lives tethered by their tentacles to the sea bottom. Because of their habitat and their fragility, siphophores are very difficult to collect, and relatively little is known about them.

Siphonophores are colonies of polyps and medusae that function together as a single organism, and as such they are among the most complex of colonial animals. At some point in the history of the Hydrozoa, and in what can only be conceived as a quantum jump, the ancestral genome reconfigured itself to code for a structure that obliged individuals budding off from each other to transcend their individuality and become so specialised (losing many functions in the process) that they could survive only in cooperation. Each zooid – each polyp or medusa – had to depend on the others for what it could not do itself. Individuals merged their individuality into something greater. Some specialised in secreting gas for buoyancy, others in propulsion, predation, digestion and reproduction. The organisation of multicellular life, whereby cells specialise and cooperate within a single organism, was taken to a new level, in a transition engineered, it seems, to convey an expressly non-Darwinian message: the hydrozoan genome had the ability to reprogram itself. It ‘knew’ where it was going from the outset.

Siphonophores come in three basic designs: physonects, cystonects and calycophorans, and the designs consist of three basic modules: a float, a nectosome and a stem. Physonects have the full complement of modules. Cystonects lack the nectosome and thus have no capacity for independent movement. Calycophorans lack the float. In the evolutionary tree proposed by Casey Dunn et al (2005) cystonects are the most ancient of the three siphonophore groups, with calycophorans nesting within physonects.

The Portuguese man-o’-war, a cystonect, comprises four main types of polyp. A single polyp forms the large gas-filled float that sits on the ocean surface and acts as a sail. Other polyp types specialise as feeding tentacles (gastrozooids, of which there are three types), defensive or prey-capturing tentacles (dactylozooids) and reproductive organs (gonozooids). The tentacles can hang to a depth of 40 meters and deliver a powerful sting.

In complete contrast to Physalia, rhodaliids live on hydrothermal vents up to a thousand of metres beneath the surface, but they too have gas-filled floats. In most species, the float is small. Attached to the float is the nectosome, studded with medusae whose pulsations propel the colony forwards and backwards. The stem, which all siphonophores have, connects the other specialist zooids, arranged in a pattern unique to each species. As the photographs illustrate, the diversity of form produced by the variations of architecture is immense.

Different authors have attempted to deduce phylogenies from the number of zooid types in various species, with opposite results, depending on whether siphonophore evolution proceeded by a reduction or increase in functional specialisation. The most recent reconstruction suggests that the trend may have been towards zooid loss:

Our findings indicate that there has been a complex history of functional specialization in siphonophores. … We find no bias in favor of the gain of zooid types, and some evidence of a bias towards the loss of functionally specialized zooids.

C W Dunn et al. (2005), p 933.

Two types of zooid appear to have arisen at a later stage. One is the medusas of the nectosome, since the most ancient group in the evolutionary tree – the cystonects – lack a nectosome. The other is the bract, of which there are four types. They are absent in the cystonects but present in physonects and calycophorans. Bract types both increase and decrease in number across the phylogeny. The other main zooid types studied are gastrozooids and palpons. Cystonects have three types of gastrozooid, whereas most species make do with only one; the other two types were lost. Palpons are present in cystonects but absent in physonects and calycophorans. The fact that calycophorans nest within physonects indicates that the float polyp was also present in the most ancient siphonophores and subsequently lost.

The origin of the siphonophore body plan is unexplained by a scenario where novelties are gradually acquired through mutations that happen to confer an advantage, each mutation taking its place within a continually functional, complex system of interdependent parts. The body plan appears to have emerged from its hydrozoan forbear with close to its full complement of zooids. Past and present, it is a wonderful example of design.