Sunday 30 November 2008

Why there is no 'walking with tentacles'... (3)

Last time I argued that using tentacles to walk on would require wasteful amounts of energy. To withstand bending forces you would need very high pressure inside the tentacle in order to keep it stiff enough (the principle is called a muscular hydrostat by the way). The first big disadvantage that this generates is that you need a lot of power , and the second is it the high pressure puts a large strain on the circulation.

So let's do some creative evolution to work around these problems. A tentacle differs from the balloon animal in the last post in that the inside of the balloon is filled with air while the tentacle is filled with muscle cells, i.e., basically water. For walking purposes we need something well able to withstand compressive forces. The first thing I can see happening is compartmentalization. With compartments inside the tentacle, you could have high pressure in one compartment and lower in another. The next step is to have some organ inside each compartment that is built to withstand compressive forces. Evolution might start with specialised muscle cells, that no longer contract actively, but simply form an elastic blob that is fairly stiff, and less easily deformed than the cells.

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They could have a shape as shown above: a more or less cylindrical sac filled with a jelly-like substance. Perhaps its wall is non-elastic, or perhaps there are strong fibres in there, running from one side to the other. Let's call this a 'corpus gelatinosus centralis (CGC)', or, in Latin-less days, a compression blob. Whatever the solution, the blob holds its shape, and if you stack a number on top of one another they can carry weight. It will still need lots of muscles on the outside to keep the stack balanced. Three layers are shown, but that is just a rough idea. The arrangement costs less energy to keep upright, controlling it still costs lots of energy. It might just allow an animal to fumble around on shore, so perhaps this is a credible 'Walking Tentacle, Mark I'.


Click to enlarge

Evolution will not stop there, however. The next step is shown above. The CGC's have evolved, and now fit rather well together. The top half of each is spherical and fits in a depression in the bottom part of the next one. This arrangement ensures that pressures can be safely transferred down the stack of CGC's, called the 'columna corporum gelatinosorum (CCG)'. Take care here, as you can get lost easily in anatomical jargon. The spherical joints between the blobs allow movement in all directions, so from the outside the limb still has many characteristics of tentacles. The material of the blobs has evolved as well; they no longer simply keep their shape by virtue of tension fibres inside the blobs, but the gelatinous mass is now also crisscrossed by calcareous spicules that withstand compression forces directly.

Circling the blobs the various layers are still there, but with two innovations. The first is that there are now ligaments as well as muscles that attach to one blob and connect it to the next one in the line. These are crucial in fine tuning the positions of each pair of blobs, while the outer muscle layers do the brunt of the work in moving the tentacle. The second new item is that the layer of circular fibres has atrophied, as it is hardly needed anymore: it's job was mostly to generate a high tension, but those are now to a large extent taken care of by passing the forces through materials that withstand them.

I guess I should have put these refinements in the figures, and perhaps I will, at a later date. But this concludes the 'Walking Tentacle, Mark II'.


As for Mark III, I wonder if anyone sees where this is going; we are on the way to evolve a walking tentacle, and yet the thread is entitled ' Why there is no 'walking with tentacles'...


Anonymous said...

I didn't analyze it out in such detail, but i was really suspicious of the plausibility of the "megasquid" too. Thanks for working that out.

Your Mark II solutions are interesting. I wonder if the they would be made practical alternatives to endo- or exo- skeletons by decreasing the gravity, while still leaving enough gravity to hold an breathable atmosphere?

Have you seen clips of octopi "walking" on the ocean floor? It's pretty interesting. Of course that's not what you are discussing, since there is no need for weight-bearing when the creature is neutrally buoyant in water.

Sigmund Nastrazzurro said...

Hi Jwbjerk,

I think the Mark II could indeed work quite well in circumstances where high weight is not a severe problem, and in practice that either means tiny organisms or a low-gravity world.

The walking octopus are indeed amazing. In effect, water is such a low-weight environment that there is no pressure towards the Mark I or Mark II tentacle. It's a pity that there is nothing between the no-weight water environment and the full-weight air environment on Earth. More evoluntionary examples would be nice.