How did torsion evolve in snails?

Darwin123

How did gastropods evolve torsion?
I have looked all over. There doesn’t seem to be any clear idea of what torsion does now, let alone what purpose it served long ago.
There was a hypothesis (Garstang’s hypothesis) that the veligar larvae developed torsion so they could pull their little heads into their little shells. However, that appears to have been disproved in 1985. This link tells why.
http://www.mbari.org/staff/peti/Pubs...Hypothesis.pdf
“in only one case was rate of predation reduced in pretorted larvae. It therefore appears that torsion does not function defensively,…”

The inheritance of torsion involves a weird delay in phenotypic expression. The gene expresses itself in the offspring of the mother. Aside from the interesting questions in development it brings up, this suggests some very twisted natural selection.
http://science.naturalis.nl/media/28...avisonnawi.pdf
“In the few species that have been characterized, chirality is determined by a single genetic locus with delayed inheritance, which means that the genotype is expressed in the mother’s offspring…Nevertheless, chiral reversal could still be a contributing factor to speciation (or to
divergence after speciation) when reproductive character displacement is involved.”

Here it is 2009, and students are still writing theses on what they don’t know about torsion. Apparently the torsion makes some snails more sensitive to ocean acidification. However, there is no evidence that it makes the snail less sensitive to anything else. So why evolve it?
http://www.escholarship.org/uc/item/39q8w7gh


It almost appears that mainstream science has given up on explaining the evolution of torsion in gastropods. Anyone have any other ideas?

Borek

This requires speculation and is on the verge of the independent research, so technically is against forum rules. That being said...

I can think of two alternatives to torsion - one is a straight cone (like orthocone), the other is a random tube (like in worm snails). The latter is present only in immobile snails, so here comes the first idea - torsion allows snails to grow the shell and stay mobile. It is much easier to carry your house in the form of a round backpack, than in the form of some shapeless, clumsy bundle.

But conical shell would be similar - it would allow growth and it would not impede mobility (too much). That's the way Turritellidae look, apparently long conical shape is not a problem (yes, the shell is twisted internally, but I am thinking just in terms of the overall shape). However - and here comes the second thought - in a twisted shell it is much easier to get out of the reach of the enemies, just retracting the body behind the first twist. Even if you meet a crab with long pincers it wont be able to get to you (unless the pincers would look like the corkscrew).

Finally, twisted shell requires less material and can be lighter, providing same defense for less effort.

Ken Natton

I happen to know someone who has done research in this very area. I don’t claim to know him well, I never actually met him, but I did have some contact with him, and I remember him talking about this very issue. My first thought, and this does not come from my contact this is my own thought, is that it is a false assumption that there has to be an evolutionary reason for it. Not all traits have a definite evolutionary purpose, some are just the result of genetic drift. It could be that snails coil because the mutation that brought it about occurred and was never selected against, until, by nothing more than chance, it had become so widespread in the species that it became fixed.

What I can tell you that did come from my contact does not answer your question and thus may be unsatisfactory to you, but I found it to be a fascinating little gem, so I’ll offer it here. The gene that controls the direction of the coil is known. Most snails, these days, are right coilers. You may see an occasional left coiler with a different allele of the gene. At one time, there was a reasonably even split of the two alleles and thus of left and right coilers. The reason that right coilers came to predominate almost certainly is a matter of nothing more than genetic drift, but having become predominant, there is now a selective advantage to being a right coiler, because it is difficult for opposite coilers to mate, and thus right coilers are far more likely to find a mate. Hence, whenever the left coiling mutation comes up, it tends to be selected against. Anyway, here’s the fascinating gem. Human beings have the exact same gene and it controls our left – right asymmetry too. Not you understand left handedness or right handedness, but the asymmetrical layout of our internal organs.

zoobyshoe

I see from the wikipedia that:

http://en.wikipedia.org/wiki/Land_snail

Following the hyperlink to logarithmic spiral I find:

And:

http://en.wikipedia.org/wiki/Logarithmic_spiral

This leads me to think there is no need for a gene, or genetic explanation, for this, that it could be a purely mechanical result of the way they add material to their shells. Does a galaxy need a 'gene' to form itself into logarithmic spirals?

Darwin123

Genetic drift doesn't do much when the inherited feature has a large effect on the fitness of the organism. When there is "genetic drift", part of the genome in a population of organisms differentiates because of random variation with no selection. Genetic drift may explain how a feature starts developing (e.g., the "half an eye"), but is doesn't explain how a specialized feature finally develops.
Your analogy with humans doesn't work for two reasons. First, bilateral asymmetry in human beings has a big effect on the chances of survival. Second, the bilateral asymmetry in snails has a large cost to it.
The bilateral asymmetry in the mammalian body has a very strong fitness value. For example, the aorta is attached to the left ventricle and the pulmonary artery to the right ventricle. For animals with lungs, a symmetric heart would not work as efficiently.
A person with a bilaterally symmetric heart would die fairly quickly. The human heart can not function symmetrically. One side of the heart has to supply blood to the lungs, and the other side has to supply blood to the rest of the body.
The asymmetry of the brain is common to all vertebrates, but is especially enhanced in the case of the human species. The asymmetry in lower vertebrates is minor, but it seems to speed up the choices made by the vertebrate. In human beings, the two hemispheres of the brain perform different functions. Each does something the other can't do. Again, this seems to speed up certain choices. Animals that throw things have to be able to distinguish left from right very rapidly. Humans apparently evolved to throw things.
The initial steps in the evolution of asymmetry in vertebrates may have started with genetic drift. As you pointed out, the dominant chirality in the extant vertebrate body was probably "selected" in those early years. However, the the asymmetry in the extant vertebrate body is very prominent because of natural selection.
The snail pays a large price in survival because of the torsion. As shown in the links that I posted, the metabolism of the snail is stressed at the stage in development just before the torsion starts. It requires energy to twist its body. It becomes more sensitive to acidity in the water just before the torsion. So one would think there must be a compensating advantage in order for natural selection to produce such a strongly asymmetric animal.
Some freshwater snails have re-evolved the symmetric shape. In fact, these snails experience two torsions in their lives. They twist one way early in their development and then they twist the opposite way later in development. Hence, the adult has a secondary bilateral symmetry.
This second torsioning does not appear consistent with the genetic drift model. First, the second torsion only occurs in freshwater species. It makes sense that a freshwater snail would face different challenges then a marine snail. However, it makes no sense that "genetic drift" would favor marine over freshwater environments. Second, if genetic drift explained the secondary bilateral symmetry then one would expect to see snails with no torsioning at all. There should be some snails that start life bilaterally symmetric and remain that way all their lives. Instead, the snails have to go through that undo the first torsioning.
Another thing is that there are genes common to all snails committed to asymmetry in the snail. How come there are no snails that just plain don't torsion?
I haven't found any paleontologist analyze torsioning. I think paleontology could probably solve this one. When does bilateral asymmetry become common among mollusks? There are animals called rostroconchia that supposedly are "intermediate" between gastropods and bivalves. The common ancestor of rostroconchia (monoplacorans?)was probably symmetric. Are there any rostroconchs that are asymmetric?
I conjecture that whatever purpose the asymmetry served in early mollusks was the reason that the asymmetry evolved. However, I haven't found an easily understandable article discussing asymmetry in fossils.

256bits

Perhaps it would help if you would back up and make a conjecture on why a shell of any kind was a necessary adaptation. Was an the shell an adaptation to ward off predators who would not care for a crunchy snack, or as camouflage - if you are cannot be distinguished from a backgrounf of pebbles you can't be eaten., or for some other reason.

To build a shell the animal has to expend energy, which in turn would mean that more time would have to be spent searching and acquiring food. If food is plentiful then any type of shape for a shell would be adequate as long as it fulfils the requirements for having a shell. Even then, as long as predation is not extensive enough to terminate continuation of the species who cares what type of shell you have.

Any stress on the population with regards to a limited food supply, or increased predation would naturally favour those offspring who expend less energy on shell buidling.

Borek alluded to what I would think is a major reason for the spiral:
and also zoobeshoe:
although mechanical in nature there would have to be a driving force, possibly a chemical gradient from inner to outer parts of the spiral, that causes the animal to continue the spiral. I doubt though if a responsible gene(s) can be ruled out.

Just my 2 cents. I am sure you have already explored these avenues.

zoobyshoe

Lets say you have a band of calcium excreting glands around the part of the snail that's just inside the shell. If the snail were strong enough to support it, each new band of calcium carbonate it secretes would add to a cone sticking up in the air. But snails aren't that strong and the cone is always too heavy so it flops over. The band of glands is therefore compressed on one side and stretched on the other. The result is that each new band of calcium it secretes is wide on the upper most side and narrow on the side nearest the body. For purely mechanical, non-genetic reasons, the shell therefore comes out as a spiral when it wants to be a right regular cone. That sort of thing.

Borek

I am not convinced - I have already mentioned Turritellidae. Their shells are even heavier than a simple cone would be (because of the additional internal structure), yet they don't flop over and some grow quite large and long. See for example http://www.gastropods.com/7/Shell_1527.shtml. Similar examples in other orders - Triseriate Auger - http://www.gastropods.com/8/Shell_1568.shtml.

While it suggests that next part is attached to the surface of the older part of the shell, it doesn't guarantee the shell to be always twisted in the same direction and symmetrically spiral - it can be as well just a clump of randomly directed twists (sorry, no idea how to better describe it in English). Something like worm snail shell growing always attached to the earlier part of the shell.

Darwin123

I was originally talking about torsion in mollusks. However, check out this example of torsion in a mammal!

http://www.huffingtonpost.com/2012/1...n_1946331.html
“Footage of Terry, a sheep that looks to have an upside-down head, has gone viral on the Internet, sparking a debate over its authenticity.
Allan McNamara, a computer technician, claims to have encountered the animal in a pasture in the north of England. He shot video of the grazing sheep, which he postulated was born with a twisted spine.
"He lives happily and has been checked by a vet to ensure he is in no pain. He can eat, sleep and do everything other sheep can," McNamara told The Daily Mail.”

Although of course I can’t be sure, I believe the story. It doesn’t really violate any law of biology. If you look at the close up shot, the bilateral asymmetry is apparent. This sheep merely has a twisted neck. It probably doesn’t have any organs out of place.

That brings up an interesting possibility. May the most recent common ancestor of all gastropods flipped upside down! For instance, a rostroconch that eats worms on the bottom of the ocean may have migrated to shallow tidal pools. Then, itmay have started floating on the surface of the water. Then, it would have been more convenient to graze with the head twisted upside down. Or the other way around.

Some freshwater snails are achiral as adults. However, they have a second torsion to counteract the first torsion. If the first torsion is a mystery, then the second torsion is even more so. If the first torsion was advantageous to the snail, or even neutral, why would natural selection undo the results of the first torsion in such an inconvenient way?

I haven’t found any studies that say a chiral shell is more mechanically stable than an achiral shell. Structural stability may explain how a coiled shell is more stable than a straight shell, hence the logarithmic curve. However, some cephalopods have lived pretty well with bilateral symmetric but achiral shells.

Maybe I should ask Terry the Sheep what the advantages are from his perspective!

zoobyshoe

These look flopped over to me! The alternative, which doesn't happen, is that the shell would be sticking up in the air, and each new band of material added would be the same width all way around: an untwisted, right regular cone.
I think I understand what you're saying.

The fact the shell always hangs to the same side in most members of a species is almost surely genetic, but that's a different issue from why it spirals as it grows. It would spiral for mechanical reasons alone in my scenario even if it were flopped straight back or straight forward. You're right: if the shell went back and forth randomly, first hanging to one side, then the other, the twists would be randomly directed, but that just proves the twisting is a separate phenomenon from the preference of a side to hang toward. The direction of the twist doesn't cause the twisting. The mutants they find that twist the "wrong" way twist just as well as the normal ones.

My idea can be checked: the number and distribution of the glands that secrete the calcium could be counted and mapped, particularly in newborn and embryonic snails (before any potential atrophy from non-use develops on the "pinched" side in older snails). If the snails are born with a uniform band of these glands all around, the spiraling is mechanical. If they develop from scratch with a non-uniform band that is narrower toward the back end and one side of the snail, then it is genetic.

Ken Natton

Please believe me Darwin123, I do not suppose to argue with you, I am not in any doubt about the superiority of your knowledge. Of course, there is every chance that I explained what was explained to me very badly. Unfortunately, I no longer have access to the text of the exact conversation I had with my contact. However, I am sure that he was a bona fide researcher at a British university, and knew what he was talking about. The following links demonstrate, I hope, the accuracy of the basis, at least, of what I said, and in point of fact, the first one might actually be a better contribution to answering your original question.

http://novelty.wikispaces.com/Asymme...+snail+shells+

http://en.wikipedia.org/wiki/PITX2

http://en.wikipedia.org/wiki/NODAL

Darwin123

[QUOTE=Ken Natton;4111000]Please believe me Darwin123, I do not suppose to argue with you, I am not in any doubt about the superiority of your knowledge./QUOTE]
Sometimes what looks like superiority of knowledge is really mediocrity of knowledge!
I don't know what is going on or I wouldn't ask! I am just an arm chair biologist. I wasn't fighting either.
I was giving my knee jerk response to the phrase "genetic drift". I am not clear what it means.
Thank you for the links.

Ken Natton

Well my understanding of the term is that it refers to changes that are the result of the same cumulative mutations that drives all change, but on which no selective pressures, either positive or negative, have acted. Like flotsam in the ocean.

256bits

The item about the sheep sparked a thought about curly hair in mammals.
and one site that popped up is this:
http://ebm.rsmjournals.com/content/223/1/1.full
in regardds to researchin molecular genetics.

Under the heading of Homeobox Genes and Curly Hair.
and heading Wavy Hair and Fibroblast Growth Factors.
they do say that some factors are unknown.

what I found interesting is that a common gene between a present day fruit fly and a mammal is active and producing certain traits in each. And how a certain gene can promote functioning of what would be considered unrelalated organs such as the tooth, hair, and the skeleton.

Certain horns in mammals have a twisted nature, if not just slightly curved versus the straight or branching aspect of other horns.

Could not all the twists and torsions be traised back to a common ancestor of mammals, insects, mollusks by a common gene.

zoobyshoe

Re-reading the first couple paragraph of this link, I see I completely misunderstood the particular torsion the term refers to. I thought it referred to the coiling of the shell alone. Instead it refers to the fact that the shells start off growing forward in the larvae, then, for some reason, become physically twisted 180° and thereafter coil backward. This means that the viscera inside are also twisted, having first formed in a 180° different orientation.

This is certainly bizarre and begs for an explanation. I apologize for going off on a tangent about logarithmic spirals, which don't really have anything to do with the torsion you want to discuss.

That said, here's a thought:

The ubiquitous land snails we have here in San Diego are extremely moisture dependent and only come out when it rains. Their shell are pretty delicate and don't seem to have any protective function against large predators (not that I know what might try to eat them). The function of their shells seems to be to retain moisture. When it's not raining they seal themselves up in them in the shadiest spot available. (It can go weeks and months here without raining.)

The ability to seal themselves up might also be important for marine gastropods as a way of retaining moisture in tidal pools at low tide, and to protect themselves from any unpleasant chemistry that develops in the water in those pools. At any rate, if land snails couldn't retract into their shells and seal them up, they would all die out fast here in this desert city. If snails can't retract and seal without the 180° torsion, then it can be seen to be vital to land snails, at least.

Darwin123

That is just it. I don't fully understand how snails retract. I know they plug up the holes with the operculum on their "tails". To do this, they have to retract totally. However, could imagine how retraction could be done using bilateral symmetric muscle sets.
The symmetric snails that I have seen seem to be able to retract fully within their shells. My father and I kept a sort of pond aquarium in our house when I was a child. One time we had a snail, which I remember as symmetric. I would usually see it retracted, hidden in its shell. I saw what I later learned was the operculum, so I think that it was fully retracted. This is all a faint childhood memory, so you should look at this as an anecdote rather than real knowledge. In any case, it was only one species of snail. I don't know how well other symmetric snails retract.
I haven't looked into the details of gastropod retraction. So I have to ask.
1) How do you know they can't retract without the 180° torsion?
2) Why can't they retract without the 180° torsion?

There is an entire order of snails, the pulmonata, that are bilaterally symmetric as adults. They torsion twice as veligar in order to come out as bilateral symmetric adults. However, I developed a reasonable (?) conjecture.
The pulmonata gastropods have a "lung" which is really a vascularized mantle. The "lung" enables them to breath air. They are mostly land snails. I conjecture that a chiral snail would have less serface area on the inner surface of their "lung." Hence, there would be a strong selection for uncoiling once the mantle got vascularized. The second torsion and the vascularization of the mantle may have evolved slowly but simultaneously.

zoobyshoe

I certainly don't understand the musculature, I just know what your link says (first paragraph under the heading "Introduction"), which is that, without torsion the tail is retracted first, and the head second, which leaves the head exposed at the unsealed opening. The operculum is on the tail, so sealing the shell means the tail must be the last thing retracted, not the first. Torsion is not required to retract, it's required to retract and seal.

So, by what's asserted in that paper, the snail you remember would have to be unusual to be able to retract the head first and tail second without torsion.

It mentions that something called "opisthobranch" veligers (I don't know what that means) can fully retract without torsion, but doesn't specifically say they can also seal the shell, so that's a loose end in my mind.

Your conjecture sounds reasonable to me. I would want to know if they can seal their shells after the second torsion, though. If yes, how? If no, why don't they dry out when it's not raining? A land snail that can't seal its shell could survive well enough if it can get to water, or at least a moist place when the weather is dry. So a species of untorted, or re-torted, snails would survive under the condition they never had the usual reasons to completely seal themselves inside. Let's say they never stray more than 4 feet from a lake or pond. OR, they seal against moisture loss in a completely different way, say by secreting slime that dries into a moisture-proof plug. One way or another land snails have to avoid dessication.

The fact so many do seal themselves up argues there's a good reason for it. It doesn't protect against tiny predators, or against much larger predators which can swallow them whole, but it certainly must protect against near equal-sized predators, whatever those might be, maybe insects, maybe other, more aggressive snails. It is also vital to the California land snail to retain moisture. In each case where they undergo torsion to form a tight seal we'd have to examine what function it serves that snail in that environment.

Understanding exactly why torsion anatomically permits reversing the order of retraction, head first, tail second, would also certainly shed some light on why it got selected. That's not clear to me at all, I just know it was asserted in the paper that it permits it. It's also asserted torsion happened all at once, that they haven't found any partially torted fossils. That makes me wonder if the operculum preceded torsion or if it's something that broke off the main shell once the torsion mutation happened, and got refined into a nicely fitting "plug". If the operculum comes into being as a by-product of torsion, then things make a great deal more sense. If not, they remain weird.