Origin/fitness effect of brain coral's grooves?

[Diploria]

Hello,

Brain corals, like my avatar's Diploria, get their moniker for visually obvious reasons. I wonder whether there are informed hypotheses how come they look that way.

The answer to such a question would probably be partly a morphology/pattern formation story, and partly an adaptive story of how such a pattern can increase fitness. Feel free to speculate, but if you can point to actual research, even better :)

For the human (and similarly gyrified) brain, the evolutionary explanation is usually that the folding increases surface area and thereby 'computing power' (or some aspect of it). I suppose this is mostly of the cortex (indeed at the expense, space-wise, of the underlying white matter?) and boosts 'higher brain functions'.

But for brain corals, I read that the polyps actually sit in the valleys, not on the ridges. So with the ridges taking up space, that does not look like optimally using surface area in terms of simple number of food-gathering polyps. So, where lies the advantage? Current channeling? Predator blocking? ... ? And is the actual shape of the ridges and valleys more coincidental?bonus question: Why are walnut kernels wrinkly, too? Again, 'surface area' doesn't necessarily cut it in an obvious way. Why would a kernel 'need' surface area rather than volume? Most other nut kernels are not (very) wrinkly - they fill their shells. But walnut shells have bulkhead-like structures within the shell - perhaps to increase strength? [NON-TELEOLOGICAL EDIT: perhaps this increases strength?] - and kernel shape may be an adaptation to that...?

 

[Bystander]

My speculations parallel yours regarding the corals --- and don't consider that an endorsement or support --- life sciences aren't my department.

bonus question: Why are walnut kernels wrinkly, too?

And raisins, and dried apricots, and prunes, and shriveled grain, and dried lima beans, and ...

 

[marcus]

But for brain corals, I read that the polyps actually sit in the valleys, not on the ridges. So with the ridges taking up space, that does not look like optimally using surface area in terms of simple number of food-gathering polyps. So, where lies the advantage? Current channeling? Predator blocking? ... ? And is the actual shape of the ridges and valleys more coincidental?

Gyrified is a nice word I hadn't heard before. Some speculation parallel to yours and bystander's: I saw this in the wikipedia "brain coral" article
==quote==
Brain corals extend their tentacles to catch food at night. During the day, they use their tentacles for protection by wrapping them over the grooves on their surface. The surface is hard and offers good protection against fish or hurricanes. Branching corals, such as staghorn corals, grow more rapidly, but are more vulnerable to storm damage.
==endquote==

Somehow the coral polyps are able to build up calcium carbonate ridges and walls between the sub-colonies. Predator blocking seems a good guess and in line with what Wippy Kidium says. Also I guess you can dissipate the momentum and full force of wave currents by diverting it this way and that into random turbulence.
So the ridges/walls are like an external skeleton where nobody lives. All the life is down in the in-between and during the day when they are not catching food with their tentacles they fold their tentacles down in the trenches to form a layer of protection.

the curious thing is how the polyps down in the grooves manage to heap up walls. maybe they can extend tissue LIKE tentacle BUT ABLE TO SECRETE and they can extend secreting membrane up the side of the wall and even to the top of the ridge.

It seems to make evolutionary sense to do it for strength and protection of the colony, but it is hard to picture. I like the idea by dissipating wave energy into near random turbulence. It is like having shock absorbers. It may help trap particles of food because debris gets slowed down and loses its way in the maze, instead of rushing past.

What are the folds of the cortex called? Of the human brain? do people really say "gyrified"?
WOW! The ridges are called "gyri" (singular is gyrus)
http://en.wikipedia.org/wiki/Cerebral_cortex

Give that Diploria a like!

 

[Bystander]

Also I guess you can dissipate the momentum and full force of wave currents by diverting it this way and that into random turbulence.

Coastal engineering, marine engineering, breakwaters? Yes --- one of the "edutainment" channels at sometime in the past ten years showed an experimental effort to reduce wave action by erecting a breakwater or coastal barrier with a "sawtooth" profile --- generate interfering reflections of wave fronts and dissipate a lot of otherwise very destructive energy.

 

[Guco]

http://phys.org/news/2015-02-wrinkle-mathematical-theory-patterns-fingerprints.html

A recent article related to this topic.

As it turns out, curvature is one major determinant of whether a wrinkling surface becomes covered in hexagons or a more labyrinthine pattern: The more curved an object, the more regular its wrinkled surface. The thickness of an object's shell also plays a role: If the outer layer is very thin compared to its curvature, an object's surface will likely be convoluted, similar to a fingerprint. If the shell is a bit thicker, the surface will form a more hexagonal pattern.

 

[256bits]

Hello,

Brain corals, like my avatar's Diploria, get their moniker for visually obvious reasons. I wonder whether there are informed hypotheses how come they look that way.

There is a whole vast type of coral patterns and structures produced dependant upon the species of the animal. Most polyps will secrete the skeleton around the whole body as a tube-like structure within which they live. While feeding, though, the animal will extend their body and tentacles to attempt to catch whatever prey is available. What ones sees is a colony of polyps that have grouped together to form the magnificient structures.

Brain coral is also a colony of polyps grouped together. Rather than each individual animal on its own forming a tube-like structure, the polyps share opposite skeleton walls with one another, A valley will have several polyps living side by side as a group. The ridge is just the same as the "lip" of a tube-like skeleton, except that it has a spatial extent rather than a circular form. The length of a valley is just a product of how many polyps form the group. Of course when one group meets up with another, the ridges combine. Having many groups with many valleys and ridges meeting and combining would necessarily give a haphazard meandering pattern, similar to that of the surface of a brain (hence the name ).

Surface area maximization, I would think, comes about only by the shape of the colony, which is spherical, if one wants to go down that route. It probably is stretching it a bit much to say that is the predominate reason there is such a thing as brain coral. Other species form branches like trees, other form flat structures, pillars, mushroom shapes, etc. If maximizing surface area the only primary motive for evolutionary selection, than what does that say about all the other shapes of coral colonies? They all seem to have found their own way to build a structure that suits their needs for sunlight and food.

Perhaps you will have to look at who lives where on the reef, what depth, storm occurances and strength, tidal highs and lows, ocean currents, just to name some things that come to mind that could influence a selection of an trait for a species to survive.

Just a side note: It would be nice if someone has modeled mathematically the growth of a brain coral colony giving the similar pattern of brain-like-looking valleys and ridges.

 

[Diploria]

Thanks all for your input! Let me try to summarize and add a few more notes...

◈ So the ridge-building mechanism seems known:

the curious thing is how the polyps down in the grooves manage to heap up walls. maybe they can extend tissue LIKE tentacle BUT ABLE TO SECRETE and they can extend secreting membrane up the side of the wall and even to the top of the ridge.

Most polyps will secrete the skeleton around the whole body as a tube-like structure within which they live. [...] The ridge is just the same as the "lip" of a tube-like skeleton, except that it has a spatial extent rather than a circular form. The length of a valley is just a product of how many polyps form the group.

◈ On the evolutionary advantage (protection from water forces, predators?) we have just speculation. It seems clear that the calcium skeleton of any coral colony is protective; the question remains whether there is a particular way this works for the brain corals' (1) overall domed shape and (2) specific ridge/groove pattern.

◈ Regarding the latter specific 'brain-like' pattern :

Could be chance:

Having many groups with many valleys and ridges meeting and combining would necessarily give a haphazard meandering pattern, similar to that of the surface of a brain

And/or there might there be a bit more to say:

http://phys.org/news/2015-02-wrinkle-mathematical-theory-patterns-fingerprints.html
A recent article related to this topic.

That article (original article http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4202.html#abstract, pdf here) looks at overall curvature and outer layer thickness as determinants of wrinkling pattern, both via a theoretical model and idealized experiment.

Another recent study (news item here, original article here) does something similar, though with a different model and experimental setup (gels instead of an inflatable sphere). It specifically links the results to brain (not brain coral) surface patterns. It also marks outer layer thickness as a determinant, as well as outer layer growth rate; but not curvature:

Historically, there have been three broad ideas about how gyri and sulci develop. One idea is that some areas of the cortex simply grow more and rise above other areas, creating the gyri. Another idea is that groups of highly interconnected neurons in the cortex are mechanically pulled close to each other by the threadlike axons that make up the white matter. However, evidence suggests that neither of these two ideas is correct.
The third idea is that the gray matter grows more than the white matter, leading to a "buckling" that gives the cortex its shape, the researchers said.

(Note: I've only had a cursory look at the original articles.)

These studies are interesting, but given the brain coral skeleton growth mechanism, I'm not sure they apply to the corals, which seem essentially different from brains...

And raisins, and dried apricots, and prunes, and shriveled grain, and dried lima beans, and ...

... in that there appears no element of shrinkage/wrinkling. All these dried fruits etc get wrinkles because their inner volume decreases through dessication, meanwhile pulling on the outer layer. Brains (not brain corals) seem similar, if the other way around: it's the outer layer which grows faster than the inner volume.

That does not seem to quite correspond to how ridges form on the brain corals. That process does not immediately seem to have the inner versus outer growth rate differential thing going on, and the calcium skeleton is rigid rather than squishy. That raises the question how the global 'dome' grows. Does it grow at the bottom, lifting itself up? Seems a bit unlikely... Or does it accumulate 'onion shells', as polyps build ridges on top of ridges...? These rather pretty images of a dome section do seem to suggest that the polyps may keep building outwards, occasionally somehow creating ridge branches as space becomes available...

And then there are fingerprints... not sure if their development is a 'wrinkling' process. The finger wrinkles you get when swimming would seem to -- though apparently it's not simply a matter of the skin soaking up water:

On an interesting note, no one really knows why your fingers prune up when they get wet! It's actually a reflex! It's not that they just soak up water and buckle, like many people incorrectly believe. A recent scientific article in a respected science journal put forward the idea that the skin on your hands and feet prunes up and leads to better grip, particularly when wet. Although it's debated, this group suggested that the pruning skin acted like treads on a tire, and helped remove water while retaining better grip. However, it's just a hypothesis, and many people disagree.