|thanks to Dr. Allison Gong for the pic|
2. Are all adult echinoderms purely pentameral?
You may suddenly realize "AHA! I GOT YOU! SOME echinoderms show kind of BILATERAL SYMMETRY AS ADULTS!" Don't they???
Um. Well, yes and no.
Two notable exceptions: "irregular" urchins and sea cucumbers. Both are unusual in that most are detritivores or process sediment for food. Therefore requiring movement in one direction.
Bilateral symmetry is associated with directed movement and so, its presence is often associated with organisms which show some kind of single-directed motion.
I've written about "irregular" urchins here.. basically a bunch of skeletal modifications that are part of the morphology in sea urchins that live/feed on sediment...(again, motion in a single direction)
What happens in "irregular urchins is that yet ANOTHER "symmetry" is overlain/"added" over the radial symmetry.. So, these animals go from bilateral (as larvae)
Sea cucumbers show bilateral symmetry (right and left sides) along their worm-like bodies. Presumably, again because they have a life mode which requires them to show directed movement in order to feed.
BUT was that ALWAYS the case????
3. Not all echinoderms were pentameral...
I've mentioned in past blogs that the echinoderms of the Paleozoic were mostly NOTHING like they
Some such as this helicoplacoid (which I wrote about here) were actually asymmetrical!
and indeed some of the fossils which have been proposed as among of the oldest known echinoderms (e.g. Tribrachidium) show three-part symmetry...
One of the lesssons from paleontology though: symmetry in echinoderms might be part of a changing/evolving body form through time rather than some discrete, adaptive event.
4. A crystalographic/developmental explanation?
|from Nichols 1967|
The pentagon is the only regular polygon for which the number of sides equals the number of diagonals...In all echinoderms whose development has been studied, the first plates to form include those at the apex of the animal-that is, at the pole opposite to the mouth. These plates are required to produce a body with basically a circular cross-section, and in order to reduce the planes of potential weakness across them, the sutures between them must be as few and as short as possible. Only with a pentamerous arrangement are these requirements satisfied. from Nichols 1967, New Scientist 14, pg. 547 (italics mine).So, basically during development, Nichols arguments that the arrangement as seen above in "b"the theoretical development of these plates that this is essentially the strongest arrangement of these plates. Four or six plate arrangements (a or c) presents a clear breakage plane whereas the 5-plate arrangement does not.
He goes on to apply this structural explanation to various living echinoderms, but unfortunately, even Nichols admits, that this idea was experimentally untestable.
5. Some insight from Evo-Devo!
Some of the more intriguing clues into the "How did pentameral symmetry evolve?" are almost certainly going to be found from the field of "Evo-Devo", which is short for "Evolution & Development". A multidisciplinary field which integrates genetics and developmental biology. Which genes "turn on" or express certain characters??
One paper by Arenas-Mena et al. (2000) from Andy Cameron's lab at the California Institute of Technology in Development shows expression of the Hox cluster of genes in the purple sea urchin, Strongylocentrotus purpuratus.
There has undoubtedly been more work on this topic, but honestly, this was about all I was going to gather in the time I had and its a VERY involved field!
So, developmental perspectives give us SOME perspective into the process and its a start, but ultimately there remain a LOT of questions.
- Is pentameral symmetry evolutionarily adaptive?
- How is it relevant to the calcium carbonate skeleton? If at all?
- Under what conditions does pentameral symmetry evolve from an ancestral form with bilateral symmetry?
- How would this shift/expression be observed in early echinoderms? Like crinoids?
- Does the "5 part crystal stability" theory have any support?