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.