Symmetry is a major trait in the architecture of the vast majority of animals: if we exclude sponges, a typical animal body plan will show one or more planes of symmetry. Radial symmetry is considered the ancestral state for the Eumetazoa, with body plans such as the polyps and jellyfishes within the Cnidarians, usually with benthonic or planktonic life. The acquisition of bilateral symmetry (clade Bilateria) represented a critical keystone in the evolutionary history of animals, leading to their greatest explosive radiation and the birth of most of the current phyla. The significance of this innovation is better understood if we consider that bilateral symmetry is not only about body shape: it also was linked to a process of encephalization (concentration of sense organs and increase of the complexity of the neural system in the front of the body) and a more effective locomotion and predatory behaviors.
Interestingly, many bilateral lineages have evolved secondarily and independently into a body plan that shows partially a more or less radial symmetry. Most of the times this return is linked to a re-acquisition of a benthonic life where an agile locomotion is no longer needed. Some traits of the past bilateral condition may be more or less evident in the anatomy of these animals or, at least, in their early development. So, polyp-like shapes of some annelids, bryozoans or phoronids may look radially inspired and physiologically convergent with many cnidarians, but if we look closer to their structures or explore the inner anatomy of their whole body, their bilateral condition would become evident.
Among the Bilateria, echinoderms (sea urchins, starfishes, sea cucumbers and so on) seem to have mastered the secondary return to a radial symmetry: while their larvae are always bilateral, the adults typically show an exquisite and almost perfect pentameral symmetry and even the internal organs are evenly and radially distributed. (Although it is unrelated with the rest of this post, I cannot help mentioning that some echinoderms returned to a bilateral symmetry once again).
Unlike the polypoid phoronids or bryozoans, radial echinoderms like starfishes are not expected to have any traits of their past bilateral condition and every single arm is supposed to be anatomically and physiologically equivalent to the rest. However, this is a hypothesis that has been challenged periodically by ethologists. Last year, a study 1 performed in the Agricultural University of China, used the starfish Asterias amurensis as a model to test the extent to which the radial symmetry is reflected in the behavior of this echinoderm. They chose a criterion to unequivocally identify each particular arm of a starfish and exposed hundreds of individuals to some simple behavioral experiments that would show if these animals are indeed comprised of totally equivalent parts or if they have any orientation preference.
The identification of the arms was done with the help of the position of the madreporite (a calcareous structure that controls the liquid exchange between the sea and the water vascular system), whose position is not centered, slightly breaking the symmetry of the starfish. The arm opposite to the madreporite was considered the first one in this study, while the remaining were numbered clockwise after it.
The first behavior experiment consisted on turning the starfish upside down and then check which arms did it choose to turn over itself. A starfish in this situation usually bends two adjacent arms against the seabed and pushes with the arm opposite to this pair (stamping arm), so, the researchers turned 1,034 starfishes upside down and scored which was the stamping arm for each of them. (If you have ever played with a starfish in a tidepool, you will agree that this is not exactly like watching an action movie). The other experiments were performed to test in which direction moved the echinoderm when it was allowed to crawl freely (694 starfishes) and, alternatively, when it was forced to “escape” after dripping on its center an alkaline solution (tested on 548 starfishes).
Although the starfishes were able to turn over, crawl and flee using any arm, the axis along the fifth arm received a privileged use according to the results. The authors interpreted that starfishes, and probably all the pentameral echinoderms, behave as bilateral animals despite their symmetrical anatomy, and that the direction set by their fifth arm should be considered the anterior direction. As a consequence, a higher concentration of sense organs or neural ganglia along this axis is predicted in the paper. The authors also argue that this bilateral behavior has remained within echinoderms since the Cambrian period and it is better shown in stress conditions: starfish often crawl freely in any direction, but when they want to escape they are more likely to use the “antero-posterior axis” delimited by the fifth arm. That made me think of those left-handed people that, although they were “forced” to write with their right hands since they were kids, still turn to their “strong arm” if they need to use a hammer.
In my opinion, the results of this study are interesting, but considering that starfishes are bilaterians from the ethological point of view may be going too far. The frequencies of the use of the rest of the arms are still very high in spite of the statistical prominence of the fifth arm. What I wonder (and would have liked to read about) is whether a particular starfish will consistently use a particular arm to escape or turn over repeatedly and the relative frequency of “fifth-handed” starfishes versus other possibilities. Even octopuses show preference for some particular limbs [fotnote]Byrne R.A., Kuba M.J., Meisel D.V., Griebel U. & Mather J.A. (2006). Does Octopus vulgaris have preferred arms?, Journal of Comparative Psychology, 120 (3) 198-204. DOI: 10.1037/0735-7036.120.3.198[/footnote] when they do specific tasks. I would rather think that having a “strong arm” is a consequence of specialization and proficiency after practice, but not of body symmetry.