biological symmetry

[Jdeutsch]

As stated many times on the forum, brachiopods differ from bivalves regarding symmetry.  Brachiopods have sagittal symmetry (side to side), just like humans, birds, reptiles etc.  Bivalves have coronal symmetry (front to back).  It seems that coronal symmetry is an outlier in biology, and I'm having trouble coming up with many examples in the animal kingdom.

 

Is coronal symmetry a different branch in the evolutionary chain than sagittal symmetry?  I guess the question could also apply to radial symmetry or asymmetry.

 

Put another way, is symmetry something very basic within evolutionary chains, or do chains develop with mixed symmetry?

 

 

 

 

[Miocene_Mason]

This is a topic of much confusion I believe, maybe @doushantuo has some stuff on it.

[flyg]

Hmm. I'm not a particular expert but I'll toss out some ideas. I have always thought that bivalves have bilateral symmetry in the sagittal plane. I'm not sure why you can't define a sagittal plane in either type of organism--in bivalves the hinge is in this plane such that the valves are separated, in brachiopods the plane runs through the middle of both valves. I'm not certain here, but that is how I've always thought of it. I think the idea that brachiopods are more like vertebrates (in symmetrical terms) results because we cannot help but think of the 2 valves as a kind of "mouth" that is oriented like a vertebrate mouth. To my mind, the symmetry of bivalves is more like human (and all vertebrate) symmetry. But importantly, because they are both invertebrates, symmetry is difficult to think of in the same way that we think of vertebrate symmetry. The bivalves are more "successful" as a group in terms of extant species and habitat diversity, and I've always thought of them as more "advanced", which is an admittedly anthropomorphic way of saying they're more like us. Just thoughts.

 

G

[Mediospirifer]

Far from an expert here, just dropping in my 2 cents.

 

Consider that we're applying arbitrary labels when we call any orientation "front to back" or "side to side" with any organism that doesn't have a distinct head.

 

Looking at a diagram of the internal morphology of a bivalve, (found here) I see a mouth at one side and an anus at the other. We could easily define the dividing plane of "front to back" as a plane passing through both of these points (roughly) that divides the animal's mass equally. In the case of a bivalve, that would be along the hinge line.

 

Which would mean that bivalves flap their valves to the sides, like wings, while brachiopods flap theirs front to back.

 

[flyg]

Going back to the original question, I'm doubting the idea that bivalves have "coronal" symmetry. I admit, I'd never heard of coronal symmetry as a biological idea until I read the post. I can't think of any organisms that are symmetrical in that way, though I'm totally open to learning differently. I think symmetry is indeed very basic within "evolutionary chains". I think the difference in symmetry between brachiopods and bivalves reflects a fundamental and significant difference between their "evolutionary chains". I think all arthropods and vertebrates, for example, have bilateral symmetry (I could be wrong about the arthropods). There are plenty of evolutionary reasons why symmetry is highly adaptive.

 

That said, most organisms aren't literally symmetrical for their whole anatomy. Our hearts and livers are not symmetrical on the sagittal plane, but our kidneys and gonads are, as an example. Also, some organisms that are bilaterally symmetrical in a general classification sense are asymmetrical in reality as adults. The oysters and all the species of flatfishes being examples. The point is, symmetry is a core biological concept, and can be used to help us classify and understand organisms, but like many biological concepts it's a little messy and simply can't be made into some kind of "pure" rule like chemists and physicists are so keen on.

 

G

[doushantuo]

I was just thinking:in bivalve the viscero-pedal mass is bilaterally symmetrical,the mantle/shell complex is radially symmetrical.

In bivalves ,the mantle margin may (in some cases )split the primary ligament,and a tangential component can then be added to the (radial*) component,resulting in aberrant forms like e.g. the rudists.

*radiating from the umbones,that is

Anomiid bivalves are notoriously asymmetric,but only adult individuals

Symmetries can be secondarily disturbed in pleurothetic(flat-lying) organisms,as mentioned above. 

Fissiparous ophiuroids often show deviation of the famous pentamerous symmetry of echnoderms.And no,those aberrant tetra-and hexameral symmetries

are NOT cause by regeneration and are not teratological.

I've posted a PDF on another possible cause of asymetry,at the bottom of this box

The atrophication of organ(complexes) can cause symmetry breaks,as is the case in the adductor muscles of Tridacna 

Of course, early ontogenetic stages of animals (embryos/larvae) and plants can look pretty different,also /particularly from a standpoint of symmetry(e.g.

Gastropod larvae before torsion).

As yet there is NO solid universally applicable morphogenetic model.ALan Turing has tried in the fifties,I could post his views,but his treatment is rigorously mathematical.

(And i DO mean RIGOROUSLY)

The symmetry of the porifera is unordered,or absent(which makes organism such as mentioned below all the more exciting!) .

 

Edit: people are encouraged to contradict me,but "coronal" has found no common usage in zoology,I think(at least as a circumscriptor for 

a form of symmetry

(Btw,echinoids have coronas,of course.)

 

 

 

 

 

tronoechinodermfluctuating-asymmetryand-developmental-instabilityProtoreasternodosus.pdf

 

please note:"Succes" is hard to define in evolutionary paleobiology

 

(Last two images from Finnerty(2003/Int.J.Developm.Biol),who discusses the antiquity of Bilaterla symmetry in animals