Sunday, 9 March 2014

The Creative Radiation of Cloakfish (Archives IX)

Cloakfish have not featured on this blog often (here and here); the last time was almost two years ago, so it is time to have another look, this time at their earliest evolution. Note that in the 'Archives' series of posts, 'evolution' often does not refer to the fictional biological evolution of these animals, but the evolution of the concept.

Click to enlarge; copyright Gert van Dijk
Leafing though the mouldy sketches in the damp crypts of the Museum of Furaha Biology reveals that their creative evolution started as an offshoot of Fishes. Furahan Fishes started their biological evolution not with series of paired limbs, but with an undulating membrane on either side of the body. Thinking about the movements of such membranes generated cloakfish as an offshoot. The sketch above was originally annotated in Dutch, but for this blog I overlaid them with an English translation. I hope they more or less speak for themselves. The  uppermost picture shows an undulating membrane with a central plane -a rectangle-. The second row shows the 'movement volume' of such an undulating membrane: over time, each point in this volume will be occupied by part of the membrane. In the third row I played with the idea of what would happen is this central plane would not be flat, but curved spirally itself. On the right side you can see how the membrane would undulate up and down around this central plane. The bottom row shows the movement volume of the membrane assuming such a curved central plane. The bottom right picture shows what would happen if you were to group three such volumes together, and that grouping is where Cloakfish depart from Fishes forever: we now have multiple membranes around a central axis, not one at each side of a body.

Click to enlarge; copyright Gert van Dijk
The next leaf of the sketchbook shows the evolutionary jump to a fully developed cloakfish:  the four membranes, the cloaks, surround a central rod, which I could not resist calling a 'dagger'. The body is largely a cylinder stabilised by four fins. The picture also shows an immediate variation on the theme: such a device can pull just as well as it can push. But the central plane of each membrane has reverted to a flat rectangle. I thought that undulation of the membrane around a curved surface would result in a net rotation force, so the poor animal would start rotating around its longitudinal axis. Perhaps I ought to consider the forces of that approach again, but at any rate that is how the basic cloakfish came into being.

Click to enlarge; copyright Gert van Dijk
Once there is a plan, it becomes tempting to start pulling at it to see where that leads to. The top animal here departs quite a bit from the general cloakfish, as its frontal cylinder is nowhere to be seen! It is in fact a tadpole with a cloak-and-dagger propulsion system (well, it also is not much like an earth tadpole in that it has no jaws and multiple eyes). The middle animal certainly is a generic cloakfish, although again with some twists: the front fins have rotated by 45 degrees compared to the cloak-and-dagger. The cloaks are much larger at their end than at the front or middle: this is probably as close as you can get to propulsion with a screw without continuous rotation. The bottom one has the fins and cloaks aligned, causing its four eyes to rotate as well. Whether bending the central rod as shown here would work well is dubious, is think.

Click to enlarge; copyright Gert van Dijk
Here is the result of more pushing the envelope. The left image shows a vertical cloakfish. I certainly did not spend enough attention on the cloak movement, as their shape looks rather unconvincing; then again, visualising the position of four membranes over time is not all that easy. The animal, looking suspiciously like a potted plant, could perhaps travel up and down as day makes way for night to filter plankton wherever it is most abundant. An animal with a horizontal position can do that as well, and if this animal is limited to the vertical position, that will limit its manoeuvrability severely, whereas a horizontal cloakfish could still choose to swim vertically upwards it is needs to; I like that idea. 

Click to enlarge; copyright Gert van Dijk
Of course, cloakfish can be flattened. That separates the cloaks so their movement volumes no longer all touch one another around the animal, but the membranes could still interact in pairs. How the membranes interact is explained on the main Furaha site (which is currently being redesigned). Note that this lineage has rotated its general body alignment by 45 degrees compared to the general pattern, so there no longer is a top fin, but there is a top eye. The dagger has increased in girth and now houses most of the body's internal organs; in conventional cloakfish this part of the dagger is hidden by the front cylinder.

Click to enlarge; copyright Gert van Dijk
Why not flatten the animal laterally? Here you see the result, this time with the overall rotation set to the 'top fin' mode. I doubt that such an animal, which probably has exquisite control over its cloaks, needs the four fins emanating from the front cylinder fro movement, but they do look good. The one on the right has also flattened the cylinder laterally, and is an overall exaggeration of the left one. I have this feeling that these are reef cloakfish.

Click to enlarge; copyright Gert van Dijk
Has the creative evolution of cloakfish stopped after this early burst of adaptive radiation? Not at all, but creative evolution is like biological evolution in that there may be periods of sudden intense speciation followed by slower adaptation. Dixon's recent mention of equipping his animals with a mother of pearl finish made me want to want to paint an animal with such a finish, and here is a first attempt. The result does not work well yet, but that is not surprising: painting a mother of pearl effect is difficult (if you want to see it done much better, search Google for 'Paul Quade Cambrian').

If I manage to reach that level I will certainly post the result here. Meanwhile, here is a  bioluminescent general cloakfish, another painting experiment.


Andrew Broeker said...

Force transfer is a significant hurdle with this arrangement. You're certainly correct that the central rod would need to be quite stiff in a pusher arrangement, probably to the extent that the cloakfish would not be able to bend it.

How stiff are the 'rays' that support the cloaks? I'm assuming that the lines on the cloaks are indeed anatomical features and not sketch marks. If the answer is not very, the cloaks will have sine waves propagating in two dimensions.

Have you seen knifefish rapidly switch between swimming forwards and backwards?

Nicky said...

i tend to think the cloakfish as a huge nostril. It probably needs to "breathe in" water in order to move forward. Vice versa when going backwards.

Evan Black said...

I'm sure you've mentioned this before, but what do cloakfish eat? My first impression is that they are filter feeders, but some of your images suggest that there is some variety in mouthparts. Is there variety in prey items as well?

You talked about the 'exquisite control' the cloakfish have over their cloaks, rendering the rigid fins on the front cylinder unnecessary. That certainly makes sense, but the fins could still be there for other reasons. Perhaps they're used for sexual display, or as weapons (housing poison spurs, etc.) or both.

Also, I enjoy the consideration of changing the direction of travel for the cloakfish. The fact that the bauplan would more or less work in either direction leads me to wonder why the cloakfish even choose one exclusive direction to travel at all. Why don't they move in whichever direction suits their circumstances, like cephalopods? Is this optimization really that beneficial?

Sigmund Nastrazzurro said...

Andrew Broeker: the lines were used as 'hatching', but there are certainly spines in the cloaks. In fact, i also experimented with nearly inflexible and somegat flexible spoines, to see what that might do to the volume of water that the cloaks envelop. I hoped that having elastic spines would act to store some energy and that this energy would be made useful when the spines would straighten again. I like the knifefish video. I remember having seen cuttlefish videos in which I got the impression that the beasts set different types of waves in motion at the same time (meaning different wavelength and amplitude, and perhaps even direction).

Nicky: You are right in that the unseen part of the cloakfish houses gills. in Earth fish water wil stream from the mouth over the golls towards the gill openings, but what i would have to leek up is whether gas exchange would work just as well if the water would flow the other way. But I think that that can be arranged.

Evan: they are indeed filter feeders, although I have been ready to change my mind about that several times. The main reason to do so was the lack of medium sized filter feeders on earth. I could find no argument against there being any, but am somewhat worried nevertheless.

If the fins are at one end of the animal, it makes sense to have some 'attitude control' devices at the other end, and altitude too ;-) , rather like the 'bow vanes' on submarines.

You may be right about cloakfish going both ways. I placed the eyes at one end though, and there is also food intake to consider. There must be some sieves in there, and something to scrape food particles from the sieves into the gut. I do not know whether I can make that work in both directions,- an interesting challenge!

Andrew Broeker said...

I suspect that weight savings and reduced dietary requirements would be the main benefit of flexible spines.

In my mention of wave propagation, I was referring to the wave travelling outward along the flexible spine, perpendicular to the phase offset wave of the adjacent beating spines. Somewhat like the matlab animations of cloakfish on, except that in your animations the angle of the spine at the base is constant and the structure bends uniformly across its length. I'm happy to say that you have not repeated this in your sketches. Interestingly, that's how undulating fins might work on Microplanitia. Assuming that the spine is inert and waved from a pivot at its base,the base angle should be changing and the bending should be in the opposite direction. The bend should propagate out, and if the spines are fairly flexible it should approximate part of a sinusoid wave (though that incompressible water in the way will probably muck things up a bit, providing a good modelling argument for stiffness).

As to the discussion of agility, I can't see direction changes being useful for cloakfish feeding on microscopic pelagic organisms. It could, however, be quite useful for deposit feeding or filter feeding on very small swimmers that are big enough to take evasive action. Of course, without knowing how they do their filtering I couldn't tell you how plausible radiation to such feeding strategies is. I also don't think that the cloaks would provide much useful torque given a narrow dagger stiff enough to avoid bending with compressive force, making the fins on the body the main method of turning. I also suggest that these fins should be considerably bigger if you want a cloakfish maneuvering like a cuttlefish. At low speeds you need a fairly large control surface to get considerable effect. Of course, with your cloakfish which have the cloaks anchored to sturdy, preumably muscular bodies it shouldn't be hard at all to bend the body in order to pitch, yaw or roll.

Sigmund Nastrazzurro said...

Andrew: sorry for the delay. the Matlab animations of the cloakfish were not as subtle as they could have been, and indeed treated the successive 'spines' embedded in the cloak as completely rigid structures, which they would not likely be. The cloaks were also depicted as having no thickness at all, whereas I would like them to be fairly thick near the dagger and to flare out distally. In fact, I have this image of a giant cloakfish in which the cloaks and the body/dagger merge completely; try to visualise a merger of two mantas, one horizontally and one vertically. The problem is that any such refinement quickly consumes an inordinate amount of time...

I never envisaged the cloakfish as very fast or manoeuverable animals. The opposite, in fact. Cuttlefish have an additional means of propulsion, the jet, which allows them to get out of trouble quickly. Cloakfish are slow and ponderous, but I like them too much to drop them from the catalogue ;-)

Spugpow said...

It seems like cloakfish have the potential to use their large gill (?) cavity for jet propulsion.

Another route for some cloakfish to take would be to convert their undulating membranes into flapping "wings," if that makes sense.

Petr said...

this is amazing! I really enjoy seeing more information being shared about cloakfish. I love the colored versions too, that deep-sea version with jaws(?) is especially interesting.
Seeing you take a rather simple idea and trying to come up with as many variations as possible is very inspiring :)

Anonymous said...

A 4-finned nautical probe has shown up online - perhaps it could be of some inspiration?