You will first see an explanation of deep ocean vents on Earth, and those are never boring. Jack Cohen makes an appearance again, to speculate about similar vents in Europan seas or lakes. The vents are surrounded with walls built by bacteria that stretch upwards to form very long tubes. The speculation really gets underway when it deals with the ecosystem surrounding these tubes. There are creatures that can bite or drill through the wall of the tube, after which they gorge themselves on bacteria from within the tube. Of course there are predators out there too, preying on the 'grazers'.
Click to enlarge; copyright BBC
Here is a picture of a bacterivore; the predators have almost exactly the same shape. There is a feeding trunk on the front end of the animal, underneath the central opening. There is another opening in the front end of the animal; what is it for? Unfortunately, the documentary keeps completely silent about the body plan of these animals, which is a pity.
There is an opening right at the front, and one at the back. The one in the front is not for feeding. Perhaps these are the inlet and outlet openings of its respiratory system. After all, there is no reason to have air go in and out through the same opening, as is the case in Earth's tetrapods. Actually, using the same opening for air moving in and out is not good engineering, and is probably just a remnant of lungs starting as a sac with just one opening. In Earth's fish, waters enter the mouth and leave through its sides after having passed through the gills; a much better design! Obviously, evolution should be able to find other solutions on other worlds: air enters the lungs of Furahan hexapods through openings at the front of the trunk, and exits the body at its rear end (not that you can see that on any of the paintings on the site, but is true nevertheless).
Click to enlarge; copyright Gert van Dijk
Still, there is something else about their propulsion that makes me wonder. The animals have a set of three fins around their body, more or less like the pectoral and back fins of sharks and dolphins. This makes sense, as three such fins are useful in countering rotations around the body's front-to-aft axis. You would want such fins near the centre of the body, as they would impede movements around the other axes if placed at the front or the rear of the animal. These animals indeed have three such wings right where you would expect them, around the centre of mass. As an aside, you may well wonder why there are three. To counter rotations, two or four (or more) would work just as well. Their area may have to increase if you have fewer fins, and vice versa, but that does not seem to be an important factor. Some whales have large dorsal fins and some have no dorsal fins at all, so having two seems to work as well as having three. Why are there never four? Is this just an evolutionary accident? Perhaps it is easier to have more such fins at the bottom half of the animal than at the top half, if only to make it easier to keep the body upright.
Anyway, now have a look at the tail of Europan bacterivores: there is another, smaller, set of three fins. That only makes sense if the animal needs more to be kept on track like an arrow, but this 'triad' fin design is not optimal if you use the tail for propulsion. Suppose you wish to beat the tail in an up and down direction: with a triad set the top fin will be useless for propulsion. While moving upwards it might even start to bend sideways and then it would impair propulsion. The other two will not be perpendicular to the direction of movement and will therefore not provide optimal thrust. No, if you want a beating tail, the surfaces providing propulsion must be perpendicular to the direction of the beat, and surfaces not aiding in propulsion should not be in the way.
Whale shark / orca / orca; click to enlarge
The tails of sharks and whales provide excellent examples of this design. The pictures above were taken from the internet. The whale shark beats its tail sideways, and the 'stem' of the tail, just before the tail fin, is flattened sideways. In this way, there is room for the attachment of muscles and ligaments without impairing propulsion. The two photographs of orca's show that an orca's tail stem is flattened vertically, exactly as expected for an animal that beats its tail up and down.
Back to Europan bacterivores. Their tails suggest a mode of propulsion similar or identical to the ones I invented for Furaha. Convergent speculation once again? Possibly; remember that this type of propulsion results in linear motion without any externally visible means of propulsion. That is not what you see in the video. Instead, the predators near the end can be seen to swim with a strongly undulatory pattern, like the one you would expect for animals with sideways-beating tails.
I wonder what happened to cause this odd combination of a design plan with a movement pattern that doesn't seem to fit the plan. The people who designed these animals knew what they were doing, so the answer probably does not lie there. Perhaps the animators simply added a familiar type of movement to add some spice to the footage? That is possible: I remember from conversations with Steven Hanly that the movement of Eponan uthers in the same documentary did not come out as planned either. I doubt we will ever know.
7 comments:
"After all, there is no reason to have air go in and out through the same opening, as is the case in Earth's tetrapods. Actually, using the same opening for air moving in and out is not good engineering, and is probably just a remnant of lungs starting as a sac with just one opening. In Earth's fish, waters enter the mouth and leave through its sides after having passed through the gills; a much better design!"
I realize it is a necessary part of the creature-designer to try to make engineering judgements about the efficiency and practicality of various systems and body-forms. But i've also come to be very cautious when certain elements of living creatures are dismissed as "primitive" or "badly designed". Certainly there are trade-offs in any design, biological or otherwise. The negatives side of a trade-off may be more obvious in certain instances, and the positive side may be unclear.
But as human understanding of the functional aspects of biology is very incomplete, and our technology can only imitate only a small fraction of what biological machines can do, i'm not willing to condemn any feature that actually works in the real world as "bad design".
I didn't know that Furahan animals inhaled and exhaled through seperate openings. I think this is the first speculative animal I've heard of with a 1-way respiratory system. All real and speculative lifeforms I can think of use the same opening/2-way respiratory system(except fish obviously)
J.W.:
First, let me make it clear that 'design' is a figure of speech here, and not to be taken literally. You probably did not mean it in a literal sense either.
As a figure of speech the word 'design' leaves much to be desired, as it might imply that a complete redesign might solve the problem, and evolution does not work that way. The result often is something that works, but not as well as it might.
Birds use air sacs in a very ingenious way to get as close as possible to a one-way flow of air through their lungs. Unfortunately, the basic design still involves just one tube for air to enter as well as exit the lungs. A complete redesign would mean a new exit tube. With their improved design, the respiration of birds outclasses that of mammals. Although mammal respiration works, it is not optimal.
There are many other examples of suboptimal solutions: in our eyes the arteries and veins of the retina are situated in front of the retina instead of behind it (as in cephalopods). The result is that the vessels have to pierce the retina at one point, which is why there is a blind spot. Not an optimal solution. The crossing of our air and food pathways is not optimal either, and responsible for many cases of bronchopneumonia and hence death. Not optimal.
You are right that many features may be there for hidden reasons. But there are features where the evolutionary starting point limits what evolution can achieve. Having a blind spot seems one such effect, for which it is rather difficult to think of any benefits. Having lungs that are essentially sacs with one opening is another in my view.
Josh,
You may well be right. If no-one equipped his of her fictional beings with a superior air passage system, this is probably an example of being overly influenced by what we are used to.
I'm not sure, but the 'undulation' visible in the tri-finned tails seemed more a result of the flexible tail bending as a result of direction change from some other source, and less a source of propulsion itself. Like I said, it's a small video, and I wasn't able to watch it frame-by-frame, but I didn't see anything that conclusively points to the tail beating to locomote.
Re: bird lungs
I recently read somewhere that birds actually use their thighs to help pump their lungs, which explains why their knees don't move much. Just an interesting factoid.
Re: one way lungs
I can't bring to mind any famous aliens that have such a feature, though, i think they are out there. I've have seen a bunch of creatures on deviant art that work on that principal. Anyway it did not strike me as novel, the fish precedent seems pretty obvious.
But certainly it is relatively unexplored territory for speculative biology.
re: the swimming motion of the aliens in the video.
my best guess is that the swimming motion of terrestrial fish was used on them without much thought for how it would work with trilateral "fish".
re: crossing of airway and esophagus
In my mind, this is perhaps the best example of non-optimal design in humans (or similar creatures, but i'll use the example of human, since we are all familiar with it). And while the disadvantages are pretty obvious, i'm not sure the cost of the alternative makes the known method dramatically inferior.
To totally separate eating and breathing, lets say we adapt the nose to only breathing, and the mouth to eating. Is there anything we can take away from the mouth? I think we need it all, though the tongue wouldn't need to be so dexterous, since it is no longer the vocal organ.
But an unmodified nose that connected only to the lungs would be inferior to our set-up. It can't pass enough air extreme activity, since it's narrow convoluted passage is optimized to equalize temperature, retain moisture and filter impurities. We'd either need another direct bypass from nostrils to lungs, and/or some way to move much larger capacities of air. Additionally we'd need something to duplicate the vocal function of the tongue.
None of these things are biologically outrageous, but they would certainly add mass and thus increase somewhat the calories needed. The functionality would probably be superior, but it wouldn't necessarily justify the cost in weight and needed calories. Heh, it's someone amusing to visualize the large-snouted face of a human that had such a revision.
All throughout biology we find organs put to multiple uses. While a single purpose organ could probably perform better, a multi-purpose organs are often good enough, and more "economical".
Hello J.W.,
To start at the end: I agree that even a rather clumsy solution such as having crossed air and food ways can still have advantages. As you said, if we need lots of air the alternate / additional route through the mouth is activated. But suppose we had fully separated air and food paths as well as a need to have much more air pass through at times. Without an alternate path, I guess evolution would have developed tricks with the existing path. Look at baleen whales: when they take in enormous mouthfuls of water to eat krill, the lengthwise pleats in their throats open up. A different starting point, a different end, but with lots of convergence.
I have some news about the swimming mode of the Europan animals with three fins. Jack Cohen was kind enough to send me an email explanation. The animals were meant to swim using jets of water. That means that the fins fit the mode of propulsion, as you would expect from someone of his standing. The animation indeed does not do the animal justice. Pity.
Still, I wondered what would happen if an animal with such a trilateral tail would start to use its tail for propulsion. After a bit of thinking I found a way in which it can, not sweeping its tail up and down or left to right, but in a rounded triangle. It will take me ages to animate it, if ever, though.
You know, I was watching the video of life in the deep sea vents, and I noticed that the fish species shown has the same general body shape and fin structure as the protovertebrate Pikaia. Coincidence?
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