2
$\begingroup$

Biradial symmetry occurs in ctenophores: the body plan consists of two halves which independently display radial symmetry. Spherical symmetry occurs in Volvox algae: any cut through the center of the body results in two identical halves.

Hypothetically, an organism could display spherical symmetry. For example, an organism resembling the mythical amphisbaena with a head analogue at either end of its body.

How plausible is a spherically symmetrical body plan in animals, or at least in basal animals or ancestors of derived animals that discarded this symmetry?

EDIT: Spherical doesn't have to be literal. The organisms might be worm like with a gut that opens at both ends, but the gut orifices are two way or the anus is located ventrally. I expect that as the spherical symmetry organisms grow larger and enter more niches they would undergo cephalisation.

I would think there would be vestiges of their ancestral symmetry. Similar arrangements of body parts in front and back? Vertebrates have two eyes and two gonads by chance, but maybe in this organism the eyes and gonads develop from similar embryonic structures? If the basal gut was a two-way tube, then maybe the derived organisms have a anus in the ventral region rather than the posterior?

$\endgroup$
  • 1
    $\begingroup$ Nice for you to bring up ctenophores — people under-appreciate the alien-ness of creatures we have here on Earth, pretty much equating Animal with familiar tetrapods. $\endgroup$ – JDługosz Mar 17 '17 at 7:51
  • $\begingroup$ Would these animals be in a vacuum? I know a way to maximise the amount of milk you can get from them.... $\endgroup$ – Aron Mar 20 '17 at 15:15
4
$\begingroup$

Pretty much all animal groups have undergone cephalisation - the concentration of sense organs and nervous tissue at one end of the creature. This is partly because nerves are expensive things to make, and if you shorten the distance between your eyes (or antennae or nose or tongue) and your brain, you don't need to spend as much resources on building the nerves which join them up.

Also, transmission of signals along nerves takes time. And time can cost you your life! A spherical creature 4cm in diameter with the brain in the middle, has to transmit what it sees or hears or smells 2cm to reach the brain. A 4cm long slug with all its sense organs a few millimetres from the brain, can thus recognise danger or sense food quicker than the sphere.

Spheres are not as streamlined as worm or fish shaped things, so move slower through water. If they are competing with streamlined things, they'll use up more energy per metre they travel. Natural selection will favour the streamlined.

Spheres are not really the right shape for efficiently burrowing in mud. And obviously our 4cm diameter sphere can only fit into a 4cm crack in a rock, whereas a long skinny slug or worm (which has the same volume as the sphere) can perhaps get into smaller hiding places.

So they can be spherical. But as soon as they need to do any of the things mentioned above, natural selection will be shoving them towards bilateral symmetry.

As an alternative to spherical, you might want to consider the pentameral symmetry of echinoderms. Some sea urchins are pretty 'round', others not.

$\endgroup$
  • $\begingroup$ I expect that as the spherical organisms grow larger and enter more niches they would undergo cephalisation. I would think there would be vestiges of their ancestral symmetry. For example, vertebrates have two eyes and two gonads by chance, but maybe in this organism the eyes and gonads develop from similar embryonic structures. If the basal gut was a two-way tube, then maybe the derived organisms have a anus in the ventral region rather than the posterior. Does that make sense? $\endgroup$ – Anonymous Mar 20 '17 at 12:47
  • $\begingroup$ @Anonymous. Yes it makes sense. Real world examples: Sea urchins have their mouth on the base and anus on the top. Insects have their equivalent of the 'spinal cord' running along their belly, not along their back like a vertebrate does. $\endgroup$ – DrBob Mar 21 '17 at 11:43
7
$\begingroup$

Not at all plausible. Again and again, evolution has changed the front vs back and the top vs bottom of an organism to be better suited for its role. This includes reworking parts of the body if the animal's “normal” orientation changes.

So even if the developmental process has innate symmetry, there will be ways to make them different foundv e.g. stopping growth of different tissues at different times. Or, an animal with this symmetry, unable to evolve separate front/back, will not be able to compete against those without the limit, so it will be an evolutionary dead end.

If you are thinking of aquatic creatures in particular, the shape will be properly streamlined in short order. That proper shape is round at the front and tapered at the back. Look at dolphins and mososaurs: they very quickly end up with the same shape as fish.

$\endgroup$
  • $\begingroup$ But cacaelians and worms and centipedes have body plans similar to this. Even if true symmetry is lost at some point, wouldn't its descendants still have remnants? For example, similar arrangements of body parts in front and back? $\endgroup$ – Anonymous Mar 17 '17 at 11:06
  • $\begingroup$ If the asker accepts bioengineered creatures, maybe that could be the way to go. The shoggoth comes to mind. $\endgroup$ – John Dvorak Mar 17 '17 at 19:36
  • $\begingroup$ @JanDvorak: The shoggoth doesn't have any symmetry. It changes shape. $\endgroup$ – Anonymous Mar 20 '17 at 12:38
1
$\begingroup$

The only multicellular organism I know of with such symmetry is a multicellular algae, Volvox See also Symmetry in biology: spherical symmetry

$\endgroup$
1
$\begingroup$

The more important motion in a particular direction is to a given organism, the more likely it is to form a direction in its symmetry. For fast moving animals that chase prey or flee from predators, that generally means having a head end, enabling it to move better in one particular direction.

Some aquatic animals that either crawl along the ground (sea stars), float along slowly (jellies), or attach themselves to a substrate (anemones) display radial symmetry. These animals do not move fast enough to warrant a front and back end, but they do align themselves according to gravity, so they have a top and bottom. They also eat large objects, so a mouth end is important. To justify spherical symmetry, we need an animal that has no particular inclination to move in any particular direction, and also never eats large objects that would warrant a mouth.

Some sponges are roughly spherical. They have no real digestive system or mouth, instead being covered in holes randomly placed around their body and filter out particles that pass through them. Sponges grow by attaching themselves to a substrate, though, so they aren't truly spherical.

Free-floating, planktonic sponges might be spherical, and could drift along in the current, exposing them to more food. The smaller they are, the more likely this is to occur, as it will be easier for currents to move them around. You might also have a species of sponge that gets pushed along the ground by the current, basically "rolling" to reach new areas.

Another possibility is a tiny, free-floating (spherical) colonial organism made up of (non-spherical) polyps. Like a free-floating coral. Then it's possible for the organism to have things like tentacles grabbing at food.

There is probably a reason why this doesn't seem to occur in our world. Part of it could be that free-floating animals are more vulnerable to predators, and this doesn't outweigh the benefits of extra food exposure if they can't actually chase the food in question. Maybe a species of sponge or coral that had no natural predators for whatever reason could evolve in this direction though.

However, you probably will never have a self-propelling animal with spherical symmetry. Once motion becomes a thing, animals need to get a head to get ahead.

$\endgroup$
0
$\begingroup$

Maybe in a water world where up and down don't matter too much. If there isn't a direction where light comes from and there isn't a "floor" to hide on, many of the pressures for specialized form would go away.

The one pressure against radial symmetry is that elongated forms will move faster in one direction. That could be a very helpful evolutionary trait.

Also, specialized eating equipment is easier to develop if it only needs to develop in one spot. The same goes for any natural weaponry the critter might develop.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.