Siphonophores

Bargmannia elongata, a siphonophore that is common at depths of about 600 meters off the coast of California, has proven particularly informative in a recent study of the colony level development of siphonophores. This species has a relatively large growth zone that gives rise to the feeding, reproductive, defensive and other zooids. As with most other siphonophores, these zooids are arranged in a species specific repeating pattern along the stem of the colony.

It has been found that each reiterated sequence of zooids, rather than each zooid, arises as a single bud in the growth zone. This "pro-bud" undergoes a series of well defined divisions that give rise to the different types of zooids of each repeated sequence. At first the zooids that descend from a single pro-bud are attached to the stem by a single stalk, but as they mature they spread out along the stem into the mature pattern. This mode of development has previously been suggested for another group of siphonophores, the calycophorans, but these findings indicate that it is more widespread.

In the course of this study, it was found that Bargmannia elongata is "directionally asymmetric". It is a common misconception that all cnidarians are radially symmetric- differentiated along only one body axis, but not any others. A radially symmetric animal looks the same no matter which way it is rotated along this axis. In fact, many cnidarians, including some sea anemones, coral polyps, and many siphonophores, are bilaterally symmetric. This means that they are differentiated along two axes, and can be divided in half such that one side is the mirror reflection of the other side. Humans are bilaterally symmetric at first approximation. But if you look at our internal organs, including our heart, liver, stomach, and brain, it is clear that our two halves are not perfect mirror images. They are different in well defined ways, and these differences are in the same direction from person to person. This is directional asymmetry- variations on bilateral symmetry that are consistent across individuals of a species.

The polyps and medusae of a siphonophore are attached along one side of the stem, but in Bargmannia elongata some types are displaced to the left and others to the right. The direction of this displacement is exactly the same for all Bargmannia elongata colonies. This has important evolutionary implications, as there are two possible ways of explaining the existence of directional asymmetries in a siphonophore:

Directional asymmetry evolved multiple times, once in siphonophores (or one of their cnidarian ancestors) and at least once in the group of animals that includes vertebrates, worms, and other related animals. If this is the case, then siphonophores offer an independent opportunity to investigate the origin of directional asymmetry.
Directional asymmetry is much older than previously believed, and was present in the common ancestor of cnidarians and humans. If this is the case, it means that this common ancestor was more complex than previously believed. This has important implications for understanding the history of animal evolution.

This work has been reported in the journal Developmental Dynamics:

Dunn, CW (2005) The complex colony-level organization of the deep-sea siphonophore Bargmannia elongata (Cnidaria, Hydrozoa) is directionally asymmetric and arises by the subdivision of pro-buds. Developmental Dynamics.(link to early view)