Sunday, 11 September 2011

It's a bird, it's a plane, it's a... flying squid!?

A year ago images of flying squid were in the news, including two short papers in Scientific American (here and here). Somehow I missed them at the time. Perhaps they are old news to readers of this blog, but I thought they were still very interesting. After all, cephalopods (octopus, squid and the like) attract attention from just about everyone with an interest in speculative evolution. I think Dougal Dixon was the first to have them venture out on land, a concept followed so often that it has become a cliché (but which does not mean that it was not a great idea at the time). I criticised the concept of 'walking with tentacles' in a series of blog entries later, reasoning that tentacles are so poorly designed to withstand compressive forces that evolution would turn them into limbs (here are the first, second, third and fourth posts on the subject). By the way, cephalopods with jointed legs would, for me, be much more interesting than ones painfully plodding about on tentacles. Unfortunately, their renal system is probably a much larger hindrance from them leaving the water than having tentacles; but I digress.

Cephalopods have jet propulsion, also a rather interesting feature to have aboard, and one that also crops up regularly in discussions on alien animal design. Some went so far as to equip animals with fuel-burning jets, something belonging in the needs-a-lot-of-faith category.

So now it turns out that some squids can leave the water, much as flying fish do, and probably for the same reason: to escape predators. And they use jet propulsion to do so. I wonder how people would react if squid did not exist and I would invent an animal with a double set of propulsion organs, fins as well as a jet: "What, two means of propulsion? That is improbable and inefficient!" Have that followed by the remark that they can also use their fins as wings and fold up their grasping organs to have a second pair of wings: "He's lost it this time!". Facts are often stranger than fiction, and flying squid are a prime example.

Internet searches revealed more pages and photographs of flying squid,including the following two ones. I checked two books on cephalopods I already had, and one book mentioned that the family Ommastrephidae is in fact known as 'flying squids'. The other book specifically mentioned that the fins are 'not especially well modified for gliding'. It seemed I had missed all of that.

This is a large image found here; The blogger program would not let me import all of it, so I had to cut off portions not showing squid. Even so, you may have to zoom in to see them properly. Some squid trail a stream of water behind them, that appears to be breaking up into drops in some cases. The text mentions that these images were taken as a series of rapidly taken images, and that this time series allows calculation of how fast the squid moved. That is obviously true, but unfortunately the results of those calculations were not stated, which is frustrating.

Click to enlarge; from: Bartol et al, Integr. Comp. Biol. (2008) 48 (6): 720-733

Squid squeeze a jet of water out of a tube, the 'siphon'. The image above nicely shows that the siphon can be turned around allowing the squid to move in either direction. The fins at the end of the body are a normal part of squid anatomy. Squid use both their fins and their jets to move around. The principle of jet propulsion has to do with actions and opposite reactions: pushing away a mass with a certain force results in you undergoing an equal force in the opposite reaction. The force gets bigger the more mass is pushed away and the faster it is propelled. In jet engines air streams in to the engine and out of it continuously, but in squid the propulsion is 'pulsatile'. The water is held in the mantle cavity, surrounded by muscles; when these contract water is forced out. Afterwards the muscles relax, the cavity expands and sucks in water for the next cycle. On the whole squid jet propulsion is nowhere near as efficient as swimming with a tail is, as fish do. Recent calculations suggest it is not as inefficient as formerly thought, but squid still do well do use their fins as well as their jet propulsion system. In fact, they may be better off for having two propulsion systems. I found some interesting material on that subject in a free scientific paper on the subject (from which I took the diagram above as well).

Click to enlarge

This image, found here, shows one flying squid in close up. The animal is flying towards the left. The image suggests that the fins are held in a V-shape, with the tips directed upwards. Holding wings like that is a design trick to prevent rolling about the body axis: when the animal rolls to one side, the wing on that side becomes more horizontal, so it will generates more lift. The other wing becomes more vertical and generates less left. The two effects counteracts the roll and help stabilise the body. At the other end of the animal the tentacles are held in a symmetrical way in a horizontal plane, and there appears to be a membrane between at least some tentacles. This position can only mean that the tentacles act as another wing. I cannot see on the large image whether the tentacle-wings are held in a V-position as well. The close-up seems to suggest they are not. So the 'flight plan' of the flying squid consist of two pairs of wings positioned far apart, with a long body between them. Now where have I seen that before?

Click to enlarge; copyright Gert van Dijk

Actually, only here, as far as I know. The Furahan Seasoar can be found on my website. I developed it consciously in an effort to see what could be done with a four-winged body plan. I reasoned that placing the wings far apart would place relatively much mass at the ends of the animal, making it more difficult to rotate to the left and right. The design would be stable, though, good for long and energy-efficient flights. In fact, I made a paper version once that flew quite well (which gives me an interesting idea for a future post...). The front pair of wings are held in a V-shape, but the hind pair are not. In truth, I did that only because it looked good, and I never stopped to think why one pair should be held in a V-shape and the other not. That arrangement looks a lot like that of the flying squid. Perhaps it does serve a purpose besides looking good.

The large image shows trails of water behind the squid. Does that mean that the squid are actually using jet propulsion to power their flight? Yes and no. Maybe. On the one hand it is certain that the jet allowed them to accelerate enough to leave the water, where resistance against movement is very large. That same force should have a stronger propulsive effect in air, which offers much less resistance to movement than water. On the other hand, weight is not a big problem in water, but it is in the air. Any water carried into the air to serve as 'ejection mass' for jet propulsion increases the mass of the animal and will therefore impair the squid's flying ability considerably. The good part of that is that the water is squeezed out, so the mass of the squid plus its store of water decreases quickly. As the store of water is depleted the squid gets an extra boost, which helps to propel it. There must be a complex optimum in there somewhere, in which the mass of stored water, the force of propulsion and the moment the squid leaves the water are all factors that, when balanced subtly, result in the best soaring ability. But such thoughts count in the long run of evolution. For an individual squid with a predator on its heels (so to speak), getting out of the water NOW regardless of any optimisation might be the wiser choice.


Evan Black said...

I appreciate the recognition you give to the importance of imagination and creativity in the process of speculative biology you've given in this blog post. There is an all-too-common 'hyper-skepticism' in the field/community that I think can be damaging. Ideas can get dismissed out of hand without any honest scientific scrutiny, or the scientific scrutiny can be carried out without involving the original speculator and their idea can be deemed plausible/implausible without the original poster even understanding why. While I think ideas need to pass through that purifying inspection of physical, chemical, evolutionary and ecological implications, I also think that it needs to be done in a way where the speculator (who may have little or no scientific training) can understand the difference.

Take your "imaginary" squid, for example. I could scoff at your idea and wonder what kind of peyote you smoke, or I can think things, through and realize that both the jet propulsion and the muscular fins have their advantages and disadvantages; the squid could combine them to utilize the advantages of both (sudden speed from siphon, but maneuverability from the fins).

To take "flying" squids from a two-stage (launch-glide) form of aerial movement to to true flight would probably require optimizing one method at the expense of the other, however. In the water, both work together quite well, but squid siphons as they are couldn't get the same thrust from air as an 'ejection material' as they do with water and their fins lack the musculature to carry their weight in powered flight.

While I think there are some serious obstacles to squids really taking to the skies, it's a fascinating concept and those problems could probably be resolved with sufficient probability. Come to think of it, I'm reminded of my cloudrockets and sea sparrows. Just as with any "squid-bird," they could probably do with a little more attention to optimize their aerodynamics...

The seasoar has always seemed like a perfect example of speculative biology in its elegant simplicity, and to learn that its wing positioning was motivated by aesthetics rather than scientific evaluation is heartening. When that intuition can be confirmed by aerodynamic properties, it only confirms my suspicions that while science is an important component of plausible speculation it's important to develop intuitive and creative aspects as well. Too often speculative projects focus on one at the expense of the other, and their work suffers. A proper balance, as demonstrated in Furaha, is the ideal speculative biology.

Anonymous said...

Nature really needs to stop copying your designs. (the seasoar predates knowledge of the flying squid)

did Nature think that, just because there was no neck analogue in front of the front wings, that we wouldn't realize it? ;)

seriously, a great post full of useful info.

mithril said...

actually, there is another example of a four 'winged' flying (or rather, gliding), animal. Microraptor gui and it's relatives.

microraptor was a archeoptyrx like dinosaur/avian (still some debate there..)

only instead of having a set of clawed wing-arms and the long feathered tail, it also has a set of 'wing-legs' with full flight feathers.

because of the limits on the range of motion in it's skeleton, you'd have the two arms held out horizontal, and the legs held out in an upside down V shape. sort of an inverted version of the squid's form.

Sigmund Nastrazzurro said...

Evan: well said.

Rodlox: thank you!

Mithril: you have a point there. A few months ago I searched for papers on how Microraptor might have flown, and found the evidence conflicting. Some think it flew as a biplane, with its wings one above the other, and others as something novel, with wings in front as well as in back. The latter would indeed fit the squid & seasoar mold of flight. When writing the post the association did not come up; I guess the seasoar image is a lot stronger in my mind than the microraptor one.

Luke said...

This topic has fascinated me for ages, thanks for blogging about it! Imagine if you really did stumble on some aerodynamic principle with the seasoar--how cool.

In theory, any of these oceanic gliders could evolve to become fully aerial without really developing powered flight. They could become like the albatross, making use of updrafts to keep themselves aloft without flapping their wings. The fact that no flying fish or squid (to our knowledge) has done this is probably more due to ecology than anatomy; there's no food in the air, and plenty of predatory birds.

(there are prehistoric flying fish dating from the triassic that are unrelated to modern ones. )

mithril said...

Sigmund Nastrazzurro: re. microraptor: i've seen the same studies, but from what i've been able to figure out (i am not an expert mind you), the biplane approach requires the animal to dislocate it's limbs quite severly to get into the pose.

there have been a few aircraft built with such extreme stagger in their wingplans..the Rutan Quickie comes to mind here...

tandem wings are un common, but have advantages in stablility and lift over a conventional planform.