Jetting Animals Are Just Hearts Set Free*
    By Danna Staaf | June 22nd 2010 12:09 AM | 12 comments | Print | E-mail | Track Comments
    About Danna

    Cephalopods have been rocking my world since I was in grade school. I pursued them through a BA in marine biology at the University of California...

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    What's the best way to swim?

    If "best" is the same as "tried and recommended by the majority of fish," then the answer is easy. Think of your basic sardine beating its tail back and forth propel itself forward, and you've got it. This undulatory swimming is used by aquatic animals from the size of a pencil eraser to the size of a house, and it is by far the dominant method of locomotion in the ocean (at macroscopic scales, at least).

    But there is one whopping exception to the rule: squid, of course. Although they do have fins, and various species undulate these fins to various degrees, they are only a supplement to the real thrust: jet propulsion. The squid sucks water into its body, and squirts it out through a funnel.

    (Tolweb has more details)

    Jet propulsion is generally considered to be less efficient than undulation. It's not hard to see why. A fish tail beats left-right-left-right and each beat propels the fish forward. A squid, however, moves by jet-refill-jet-refill. Each jet propels the squid forward, but each refilling step actually pushes the squid backward, as it stops and sucks water into its mantle.

    At the moment, I'm doing a lot of reading on both the nitty-gritty details and the evolutionary implications of the efficiency of jet propulsion. That's because this week I'm writing up the last of four data chapters for my thesis, and it's all about jet propulsion in squid. My reference list is dominated by a few giants in the field. One is Ron O'Dor, who's been publishing on squid swimming for decades, and did rad experiments with oceanic squid at Dalhousie's Aquatron Laboratory (yes, that is its real name! hee!) in the eighties that are still some of the best data out there. Another is Ian Bartol, a more recent arrival who's been using some very snazz modern techinques to visualize the precise flow of water in squid jets.

    Bartol's work has shown that squid jet propulsion can actually be more efficient than we thought, with the clever use of vortex rings. It's complicated fluid dynamics that I don't really want to get into now, so here's the paper.

    But even if they use vortex rings, squid still need a huge amount of energy to fuel their jet propulsion, and O'Dor has made the intriguing argument that these high energy needs are what burn squid up so fast--why they mature so quickly and have such short lifespans.

    It's an intriguing thought, but I can't help wondering: what about octopuses? They can move by jet propulsion, but rarely do so, trusting instead to their arms to crawl around the seafloor. Intuitively, it seems their energy requirements would be much lower--and yet most still live only a year or less.

    Thoughts, anyone?

    * This fabulous title comes from a 1991 paper by O'Dor and Webber. It was a comment made in passing as the authors observed that physiologists already familiar with using Bernoulli's equations to model blood flow through a heart could apply those same equations to seawater flow through a squid.


    How efficient are side-to-side swimmers? Eels, for example, do a lot of side-to-side but don't move very fast. Tuna are very fast but have special warm-blooded muscles to provide the needed power. Fish are flattened to enhance swimming but that causes more frontal area and more drag. Does "energy" include the energy spent in building the fish -- the energy used in growing the fish from an egg? How about "evolutionary energy" -- the energy spent over the eons evolving? I am not sure how you would even measure that!



    Danna Staaf
    Okay, so I cheated. Fish swimming isn't pure undulation. There's a lot of oscillation too, and most fish fall on a continuum between undulation and oscillation. The eels that you're thinking of are undulators, whereas tuna tend to be a lot more oscillatory. Here's a video explaining the difference between undulation and oscillation in rays. (Good footage, not exactly gripping narration.)

    One correction: the "flattening" of fish actually decreases drag--that's why it enhances their swimming.

    The efficiency calculations I was discussing here only take into account the instantaneous physical energy used to move the squid forward (thrust), and the kinetic energy wasted in the water jet. When biologists talk about "energetics," that's usually what they mean. But there is certainly a cost to the animal of developing locomotory structures--the energy spend on fins could be spent on eggs or sperm, for example.

    One of the neat things about squid jet propulsion is that it serves a dual purpose--the gills are inside the mantle cavity, so every time they fill up with water, they're taking a deep breath. And the faster they swim, the harder they pump, and the more oxygen they get. Pretty neat!
    I wonder why they didn't evolve an axial-flow design? Then they would be generating forward (or reverse) propulsion throughout the cycle. You'd think a liquid piston would be fairly easy for something that basically consists of muscle and liquid under pressure.

    Danna Staaf
    Hah! That is a great question! Thing is, biology doesn't do wheels, and that includes propellers. This conspicuous absence is so fundamental to biology, in fact, that even the suggestion that an elongated shrimp might roll itself up and move kind of like a wheel is enough to get a paper in Nature.

    In the words of Richard Dawkins, "The problem of supplying a freely rotating organ with blood vessels (not to mention nerves) that don’t tie themselves in knots is too vivid to need spelling out!"
    I'm well aware that nature doesn't do wheels, but that wasn't what I was thinking of.

    Think of a peristaltic system - a tube open at both ends with muscular walls that contract in steady waves. Water is pulled in at the head end and expelled at the other. A succession of pulses, sure, but there isn't a phase where the system is working against itself.

    More funkily, and more cephalopodically, you could have a fluid-filled space around the tube through which a fluid denser than water is pumped by an internal heart-like pump, i.e. a liquid piston. That would be a closed-cycle variant of the way they do it now; the forces in both directions on the working fluid would obviously cancel, but would only work in one direction on the water.

    Danna Staaf
    Ah, gotcha. "Axial flow" made me think of propellers, but you just meant unidirectional flow along an axis.

    In fact there is one (and only one) group of jet-propelled animals with separate intake and exhaust: salps. Clearly, their jet propulsion is more economical than that of squid, and their swimming is steadier. (Lacking the muscular and nervous infrastructure, though, they can't compete for sheer speed.)

    So why didn't cephalopods go that way? I think the answer must be evolutionary inertia. Salps evolved from a sessile ancestor, probably similar to modern-day ascidians, with distinct incurrent and excurrent siphons and a pumping system for filtering food particles out of the water. When they took to the open sea, and the system was adapted for jet propulsion, they already had two openings and unidirectional flow.

    Cephalopods, meanwhile, probably evolved from shelled ancestors, and for a long time they did really well with big heavy shells that had only one opening. By the time modern coleoids ditched the shells and went all out with the jet propulsion, they were already committed to a body plan that involved a big solid sac of organs on one side, and an opening with the head hanging out on the other.

    But who knows? Squid of the future may well evolve an opening at the tail end of the mantle, and I wish I could be around to see it!
    It's an intriguing thought, but I can't help wondering: what about octopuses? They can move by jet propulsion, but rarely do so, trusting instead to their arms to crawl around the seafloor. Intuitively, it seems their energy requirements would be much lower--and yet most still live only a year or less.

    Thoughts, anyone?
    This is just speculation of my part, but octopi have considerably larger brains than squid, and a lot of that brain power is devoted to one of their primary defenses, namely camouflage--just like cuddle fish. I imagine having to expend so much energy in manipulating not only pigment cells in their skin but the texture of their skin as well to match their surroundings would require a tremendously fast metabolism, which in turn could possibly lead to a shorter lifespan.

    I would be interested to know how the lifespan of cuddle fish compares with that of octopi and squid.

    An interesting observation: It seems that jet propulsion in swimming is most optimally used for a "quick getaway" (i.e. quick acceleration from a stationary position) as octopi appear to use it, but not so great in sustained swimming as squid use it.
    Danna Staaf
    Yeah, I was wondering about the camouflage angle, too! I can't find any direct quantification of how energetically costly camouflage is, but here's a recent paper with some indirect conclusions. They argue that, because the cuttlefish in their study didn't show any preference for substrates on which they had to use fewer chromatophores to camouflage themselves, it must not be very energetically costly to use chromatophores. Of course, that doesn't address your point about changing texture, which seems like it would be a whole lot of work! I think the short answer is: nobody knows.

    Cuttlefish are pretty short-lived as well--I think 2-3 years at most, and like octopuses and squid, smaller species may live less than a year.

    Actually, both squid and octopus use jet propulsion as a fast escape, and even though squid also use it for routine locomotion, the "escape" aspect may indeed be more critical. Here's a quote I ran into while looking up papers to answer Alex's question: "Of course, hydrodynamic efficiency, however defined, may not be the major factor that led to the evolution of this mechanism of propulsion [jetting] in squid. Rapid acceleration to achieve the maximum speed during escape may be more important for their survival than the ‘efficiency’ during sustained swimming."
    All very interesting, Danna. Thanks for the links. I will have a look at the papers. : )
    That's actually an interesting sidelight. Jet engines are good at sustained maximum power. Piston engines, which do have a cycle that involves working against themselves, at least spool-up faster. This was a big problem with early jets - go-around from the approach was risky.

    "A squid, however, moves by jet-refill-jet-refill. Each jet propels the squid forward, but each refilling step actually pushes the squid backward, as it stops and sucks water into its mantle."

    Somebody should explain to squids the principle of the bagpipe. Note a bagpipe uses an air reservoir so that even though the human has to go inhale-exhale-inhale-exhale, the output to the bagpipe drones is steady. A squid similarly equipped could maintain a steady jet propulsion and not be forced to stop periodically.

    An interesting analogy, Bill. But on the other hand, the increased surface area would in effect alter the squid's streamline morphology, thus increasing drag and thus requiring more energy to overcome the extra drag. So you might not end up with that much of a net improvement in efficiency, if any at all.