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AstroChuck's avatar

Has nature ever produced asymmetrical animals?

Asked by AstroChuck (36375 points ) April 28th, 2009 from iPhone

If so, give me some examples. If not, why haven’t any evolved? What about other kinds of organisms? Is symmetry a law of natural genetics?

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19 Answers

Les's avatar

What about a flounder?

They’re pretty ucky looking, that’s for sure.

AstroChuck's avatar

Funny. I had just thought about both the flounder and the fiddler crab, although I don’t think of them as over the top asymmetrical, the kind I was looking for, such as having a misshapen head and having six limbs on one side and two tenticles on the other. Still, they aren’t exactly symmetric.

phoenyx's avatar

I wonder if the fact that plants and animals grow by cells dividing into two equal cells leads to the larger organisms generally being symmetrical?

phoenyx's avatar

Sponges don’t seem to try too hard to be symmetrical.

Les's avatar

Well, there are mutations. Frog mutations are some of the most common.

tonedef's avatar

Yeah, @syz pulled the link I was gonna. This got asked like two weeks ago. Which is sort of weird, for such a specific question.

The rundown from that discussion: flounders, wrybills, sponges, coral, abalone, limpet, and fiddler crabs. Very few animals exhibit internal symmetry, though.

My guess as to why this might develop is because the very nature of cell reproduction is symmetrical—mitosis creates an almost certainly symmetrical group of cells. Extrapolate that, and you have 2 very similar-looking, yet mirrored, groups of cells, stuck together in the middle.*

*I haven’t taken a biology class in 10 years and am completely unqualified to discuss this

Darwin's avatar

Amoebas are often not symmetrical, although they could be if they wanted (and technically are Protista rather than animals).

Snails in general are not symmetrical internally or externally although their ancestors were.

Oysters are typically asymetrical.

Some owls aren’t strictly symmetrical because one ear is higher than the other.

The narwhal’s tusk is a left incisor which can grow up to 10 feet in length and forms a left-handed helix so they aren’t strictly symmetrical either.

Technically, people aren’t symmetrical because of handedness, and because the left human lung is smaller than the right to make room for the asymmetrical heart.

However, because of the mechanism of cell division symmetry is the most likely result in an animal. In addition, asymmetry is an indication of illness, injury, or generally be less fit than symmetrical fellows.

RedPowerLady's avatar

Humans are asymmetric. If you cut yourself in half and then used only that half to replicate the rest of yourself you would look like your brother and not yourself.

tonedef's avatar

@Darwin amoebas are not exactly animals. I would assume that we’re talking about Kingdom Animalia here.

AstroChuck's avatar

@syz- Thanks for the link. Stupid me. I never saw that 2 week old thread. It even mentioned both the fiddler crab and the flounder. Guess I shouldn’t knock people for not using the seach function before posting questions.

El_Cadejo's avatar

I have a few corals in my tank that arent symmetrical.

Darwin's avatar

@tonedef – as I said in my post, “and technically are Protista rather than animals.”

Critter38's avatar

Also the direction towards which a species moves favours symmetry, otherwise species wouldn’t get very far (which is fine if you’re lifestyle doesn’t require it). (which also makes the point of which cross section is considered…humans are not symmetrical in all crosssections). For this reason most species limbs are relatively symmetrical to reduce the added effort of having to compensate for pull to one side.
This is to the point where for a while it was common in area of science concerned with “fluctuating assymetry” to measure the impact of stress on an individual by detecting the degree of assymetry in limbs (which is linked to our apparent attraction to symmetry in facial features as a means of detecting the genetic fitness of the individual).

This is distinct from appendages which are designed for specific functions, mouth parts in cross bills, wrybills, sexual selection (fiddler crabs), for which access to resources or mates can select for asymmetrical modifications.

Sponges are kind of neat because they fill a niche like corals between groups of cells and multicellular individuals. In other words the unit of concern (the individual) gets a bit messy, and therefore which bit to we judge for symmetry, the cell or the group.

El_Cadejo's avatar

@Critter38 well most coral are actually multiple animals all joined together since they reproduce through budding, but i mean there are some polyps of coral that arent symmetrical like that one animal.

Critter38's avatar

True. Thanks for clarifying.

CMaz's avatar

Kind of reminds me of this question

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