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Is a siphonophore an individual or a colony? Siphonophores develop from a single egg, differentiating during development into a complex organism that is made up of parts that at some point in the distant past were probably free‐living polyps and medusae. How they came together and whether they are best considered as a colony or individual are questions that have piqued the interest of such luminaries as Ernst Haeckel, T. H. Huxley, E. O. Wilson, and S. J. Gould (cited in Mackie et al. 1987). However, for ease of understanding based on a lifetime of perspective, the observations of G. O. Mackie himself are the most cogent. In Mackie et al. (1987), he observed that “siphonophores … are linear in form with little branching, and are polarized, with a distinct anterior end. They are also bilaterally symmetrical. They grow by addition of modules at localized growth zones. The result is a high degree of determinacy of form.” Earlier (Mackie 1963), he offered this elegant observation.

Figure 3.32 Comparison of the life cycle of an agalmid siphonophore with that of an athecate hydroid.

Source: Reproduced from G. O. Mackie (1986), From Aggregates to Integrates: Physiological Aspects of Modularity in Colonial Animals, Philosophical Transactions of the Royal Society of London Series B, Biological Sciences, 1986, Vol 313, No. 1159, page 179, by permission of the Royal Society.

They have developed colonialism to the point where it has provided them with a means of escaping the diploblastic body plan. The higher animals escaped these limitations by becoming triploblastic using the new layer, the mesoderm, to form organs. The siphonophores have reached the organ grade of construction by a different method – that of converting whole individuals into organs.

It is important to understand the concept that siphonophores function as highly coordinated individual organisms and not as a loosely collected gaggle of different zooids. Natural selection acts on the whole individual.

Figure 3.33 Cyclic fishing behavior and optimization of feeding space. (a) The Calycophoran Muggiaea atlantica begins a feeding cycle by swimming upward with contracted stem and tentacles; (b) it releases the stem and curtain of tentacles and swims in a circle; (c) swimming stops, with fully extended stem and tentacles; the stem then sinks under its own weight creating a corkscrew shape that slowly moves downward through the water column until the stem is vertical again, where it contracts, and the cycle begins again. This deployment cycle results in the fishing array moving through a maximum volume of water.