Saturday, 6 April 2013

Crabs as spidrids, spidrids as crabs...

I recently realised that Earth crab locomotion resembles Furahan spidrid walking more than I expected. I may have spent too much time on spidrids or not enough on crab locomotion, as there was a lot to learn about radial walking, odd as that may sound. I found a very nice paper on walking patterns of decapod crustaceans (basically crabs and lobsters) beginning with the sentence 'Most decapod crustaceans can walk in any direction they please'. 

From: Vidal-Gadea et al. Arthropod Structure & Development 2008; 37: 95–108 (adapted)
The image above is from that paper and shows leg movements of a sideways-walking crab, a forwards walking crab, and a forwards-moving lobster. The ability to move in any direction without turning the body is one of the main features of a radial walking design, something I thought did not exist on earth. Apparently crabs, particularly forwards walking ones, are quite 'radial'. In fact, the paper uses the very word 'radial' to describe leg positions for the forwards walking crab. A peculiar convergence with spidrids is that its common name is 'spider crab' (Libinia emarginata).


While most crabs preferentially walk sideways, they can combine directions and walk diagonally if they so wish. The video above shows a crab that starts walking backwards but gradually adds a horizontal element until it ends up walking sideways only. (Click the link to see the source at a better quality).


And here is an example of a forward walking crab. Again, the original has better quality. If you look carefully you will see that the legs do not all point sideways: the front ones are angled to the front, and the hind ones point almost backwards. In short, they are almost placed and held radially around the body. Are spidrids crustaceoid or are crustacea spidridoid?

Click to enlarge; copyright Gert van Dijk

Spidrid legs, although the mere result of a thought exercise, are rather like real crab legs. The image above shows the simplified leg anatomy, say of a sideways-walking crab (or of a spidrid leg). The bottom part shows that the leg can turn forwards and backwards around a vertical hinge close to the body, movements labelled 'promotion' and 'remotion' in technical papers. Let's call that the 'body-leg joint'. The other joints, the 'intraleg joints', in spidrids have horizontal axes allowing the leg to be straightened and flexed (see the top part). There would be muscles for every joint, but I only showed them for one.

If the animal moves in the direction shown here the leg does not need  action of the promotor and remotor muscles: the power for movement comes from the intraleg joints.  If you rotate the direction of movement 90 degrees, muscle force for this leg has to come from the body-leg joint, meaning the promotor and remotor muscles.

Click to enlarge; Copyright Gert van Dijk

So what does all this mean for spidrids? Well, regardless of the direction it walks in, a spidrid has some legs parallel to the direction of movement and some at a right angle to it. The image above shows how that relates to the direction of movement and to the necessary range of motion. The legs parallel to the movement function as the legs in sideways-walking crabs, and  depend on intraleg flexion and extension, pulling and pushing the beastie. The legs at largely right angles to the movement depend on promotion and remotion. The leg in between simply make use of both sets of muscles to varying degrees. (Mind you, the word 'promotion' in crabs always refers towards the front end of the animal; in adapting it for spidrid use it must mean 'in the direction of movement', there being no front end.)

The next evolutionary spidrid twist stemmed from the idea that one of these two types of force production might be superior to the other. How would spidrids make use of that edge, while staying radially symmetrical? Before tackling that I realised I had never shown the spidrid's ability to change direction without turning. Solving that posed some interesting Matlab programming problems, but never mind, it works. I added height for fun and slanted the body a bit when the beastie is on a slope to make it look more natural.

 Copyright Gert van Dijk

Here it is! Finally, a spidrid that negotiates terrain and make a sharp turn. As you can see, the sharp turn calls for some interesting leg movements. With a shallow turn you would not see the changes well. So this is how real radial animals walk. By the way, should a rich Hollywood director wish to buy the concept for a film, I am available! Anyway, it is now time to adapt this standard spidrid walk to more energy-efficient gaits.

Copyright Gert van Dijk

The one above is built on the assumption that flexion/extension is more efficient than promotion/remotion. So, this species uses its promotion/remotion muscles to swing the legs as far parallel to the direction of movement as they will go. There are probably anatomical limits to this, so some legs still stick out at a right angle to the direction of movement regardless. The turn becomes odd, as some legs have to swing a long way to end up in their new position.

Copyright Gert van Dijk

But the existence of forwards-moving crabs shows that under given circumstances using promotion and remotion as the power house is feasible. The animation above has a spidrid moving its legs with a preference for positions at a right angle to the movement. This movement also calls for large leg swings when the animal changes direction. The legs bump into one another, which can be solved with phase changes, but I left it as it is for now. The anatomy of the animal is the same in all three variants, which may be unwise; I can see the last type having shorter legs to improve leverage, at the cost of stride length.

So there we are! Rampaging spidrids! What else is left for spidrid movement? An obvious additional adaptation would be to include slanting, but I will not provide an animation of that; what you see here was quite complex. Then again, I now have a program resulting in 3D coordinates for any part of a spidrid negotiating a 3D terrain. Perhaps I should go for a photorealistic animation? How about the 'Crown of Thorns' (Coruna spinea) making its way over rocks? Or the 'Blue Jester' (Fossor azureus) walking on the forest floor? The 'Lesser Strandsprab' (Nepa aranea) would do well on a beach, but the 'Hairstar' (Coma confusa) would be difficult to depict, with its hair cover. By the way, all of these appear in paintings I am working on...                    


Spugpow said...

This is my favorite kind of post: revelations about decapods, spidrids, and biomechanics, plus animations and intimations of new art. A+!

If you're looking for natural analogues of spidrids, I don't think you could do much better than spider crabs ( ) or pycnogonids ( )

Arachnus said...

Wonderful post! I love the animations, those changes in leg movement really make the spidrid look more natural. I would love to see a photorealistic animation.

Anonymous said...

Didn't you mention in an older post that there was a size limit to what arthropod legs can support because these posts remind me about an arthropod alien species from the old cartoon series "Roswell Conspiracies" called the Shadoen. Here's part of an episode that shows one of them ( .

Jan said...

Will there be any view on the spidrid from below?

Sigmund Nastrazzurro said...

Spugpow: thank you. The pycnogonids surely rate as some of the weirdest animals on Earth; why must the body be so thin that the digestive system is in part located in the legs?

Arachnus: thank you.

Jan: in the paintings and accompanying text (in The Book) the underside of spidrids is indeed discussed. To satisfy your curiosity: there are additional eyes, there is a mouth and there are four claws/mouth pieces.

Evan Black said...

Brilliant work as usual! I find myself wondering which of the last two animations in this post(the green one or the blue one) is more efficient. Are you able to tell from your animation calculations which requires less effort on the spidrid's part?

And perhaps they both have advantages, given certain circumstances. For example, one may be better suited for fast forward movement while the other may offer more overall maneuverability. The promotors/remotors and flexion/extension muscles may even specialize with respect to these different uses.

Could that work, or am I misunderstanding something?

In regards to your description of the spidrid's underside: are the eyes you mentioned tasked with watching what it eats? Are they more rudimentary than the, um, 'dorsal' eyes?

Sigmund Nastrazzurro said...

Evan: thank you. My personal guess is that the movement in which the legs are aligned to the direction of movement is the more efficient one. As I see it, promotor/remotor legs are now most useful for legs at a right angle to the movement direction, and the intraleg muscles for 'aligned legs'. So the use differs per leg depending on the direction.

The nether eyes are indeed meant to control eating, and are sufficient for that task. The overall anatomy of spidrids resembles that of tetropters; they must have split from a common ancestor.

Spugpow said...

Is radial symmetry the ancestral condition for Furahan animals? All the major groups you've revealed are radially symmetrical or have radially symmetrical ancestors.

Evan Black said...

They do? Well if so, then that's yet another example of convergence between Furaha and Nereus...

Sigmund Nastrazzurro said...

Spugpow: No, radial symmetry is not the ancestral plan for all Furahan animals. Tetropters and spidrids may represent common ancestry and hence just one clade. Come to think of it, I think I'll make it that way.
But Hexamera, the group to wich the Hexapoda belong, are bilaterally symmetrical. You could make a case that cloakfish are radial, but that is -probably- false for the same reason we do not think a worm is radially symmetrical.
So there are various body plans.

Evan: as you see, not all Furahan animals have radial ancestry. There are bound to be some similarities, seeing as how we all have access to similar knowledge, but there is not that much convergence between the two worlds, is there?

Evan Black said...

No, I have to admit that the convergences I see between the two are not very strong. Both have radial flyers, but there are obvious differences between the two. "Swimming with tubes" exists on Nereus as well, though it's not as sophisticated as on Furaha. Both the banana streak and the 'zummer' have wing specializations for a separation of lift and thrust, but they're hardly mirror images of each other.

So no, the convergences aren't too widespread.

"You could make a case that cloakfish are radial, but that is -probably- false for the same reason we do not think a worm is radially symmetrical."

In that debate of classification I'd certainly side with the camp that views the cloakfish as radial, or at least having a radial ancestry.

Christopher Phoenix said...

Fascinating post, Sigmund, it is amusing when speculative biology converges with real animals, shows that you must be getting SOMETHING right. :-) As I said before, specubio people should spend more time studying real Earth animals. Not only do you get ideas, but sometimes you find that something you thought didn't exist on Earth does.

Crab locomotion is interesting, I didn't realize they could move quite like that. They are rather radially symmetric in the position of their legs. I'm wondering, now, whether the one leg poking out in front of the totally-radial spidrid isn't as much help- crabs don't have them, after all. I guess it depends on whether the promotion-remotion movement or flexor-extensor movement is more efficient.

I think that radial walking has a lot of potential for mecha designs. With eight radial legs, like a crab or spidrid, a "crab mecha" could maneuver in tight spaces without needing to spin in place, or continue firing at a group of enemy craft while fleeing (if it is a military mecha). The whole story of the mecha is basically biology inspiring mechanical devices...

Please, more rampaging spidrids!! I'm looking forward to paintings, and a photorealistic animation would be so cool.

BTW, did you hear about Kepler's recent discovery of the three smallest super-Earth planets discovered in the habitable zone of their stars so far? Two in one solar system, no less, suggesting the possibility that we may find a solar system with TWO potentially habitable worlds someday. and the Centauri Dreams blog ran some info on this lately.

Petr said...

amazing post! I am excited to hear about the paintings in progress! :)

Erik M said...

I think flexion/extension is more practical as it has two advantages over the promotion/remotion gait.
Firstly, the forces are parallel to the joint motion of most of the legs, so any over-stressing would simply cause them to bend along their natural range. This is less critical for a small creature but a serious concern for a larger one.
Secondly, the legs in "front" and "back" are moving relatively parallel to each other and thus are only restricted by the length of the limb itself in how long a stride they can take.
Additionally there is less danger of them bumping or tripping over each other if coordination is at risk of being disrupted. Like when scrambling quickly over rough ground for instance.

Sigmund Nastrazzurro said...

Erik: well put! I concur with most arguments.I felt uncomfortable with the forces acting at right angles or torsional ones on the preferred direction of movements of joints. But those could be countered by small size or specific adaptations, so the arguments seemed worth of mentioning. I wonder whether anyone has studied such matters in crabs...