The evolution of symmetry in the Pterobranchia (Hemichordata)

Symmetry is an important aspect of any organism and the symmetry conditions can tell us much about relationships of taxa, but also of the life style of these. Pterobranch zooids possess a bilateral symmetry of their bodies, often modified by asymmetrical development of internal organs in these miniature organisms. A bilateral symmetry can only in part be shown for their tubaria as these colonial housing constructions appear to follow other rules of construction. A distinct left-right (L-R) asymmetry is found in the construction of the tubaria of all derived graptoloids, initially shown as antisymmetry, but in younger taxa expressed as a directional asymmetry. The encrusting stems of extant Rhabdopleura tubaria show a glide-reflection symmetry in their construction from fusellar halfrings or full rings, secreted from alternate sides. This is also seen in the tubaria of all derived graptolites, thus, differs from the basic bilateral symmetry of the pterobranch zooids. The branching patterns of the graptolite tubaria can be interpreted as glide-reflection symmetry due to the colonial organization with new thecal tubes arising alternating on the right and left sides of the stem.