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Synchronized Swimming in Siphonophores: A Design Worth Imitating

Photo: A Portuguese man-o’war, by Volkan Yuksel, CC BY-SA 3.0 , via Wikimedia Commons.

Learning more about strange and fascinating creatures could occupy a lifetime. I had heard about siphonophores (“siphon bearers”) but knew little about them. To report on a new paper about their swimming abilities I needed to brush up on their taxonomy, anatomy, physiology, and ecology, so I read articles and watched videos of them in action. As with everything in biology, the closer one looks, the clearer the design: and this one, again, has design worth imitating.

A Floater to Avoid

Siphonophores (phylum Cnidaria) are colonial marine organisms exhibiting division of labor: some of the “zooids” (individual members of the colony) provide propulsion; others hunt and digest prey. The best-known siphonophore is the Portuguese man-o’war, known to beachgoers as a jellyfish-like floater to avoid; it has nasty stinging cells strong enough to kill a human:

But it’s not a jellyfish per se. The bell-shaped jellyfishes with which we are most familiar (phylum Cnidaria, subphylum Scyphozoa) are single individuals. The Portuguese man-o’war is classified in subphylum Hydrozoa, which includes the hydra. Like other siphonophores, it is a colony of individuals with specialized functions. Its distinctive gas-filled, sail-like bladder riding the waves like a Portuguese warship suggested the organism’s name. 

Most other siphonophores — long, rope-like organisms with hairy-looking tentacles and gelatinous bulbs arranged in rows — sit and wait underwater until prey animals like fish and plankton drift into their stinging cells. But siphonophores can swim. In fact, they travel large distances every day. If the fishing is bad, they will move to a better spot. A video taken by a remotely operated submersible for the Nautilus Ocean Exploration Trust shows one purple-colored species swimming leisurely at the bottom of the ocean:

Its odd shape defied identification at first by the puzzled scientists wondering what it was. That’s understandable, because siphonophores are barely recognizable as animals. Some species can grow to over a hundred feet long (see photo at Smithsonian Magazine).

Common but Weird and Wonderful

The common siphonophore Nanomia bijuga is very plentiful in Monterey Bay. A video by the Monterey Bay Aquarium Research Institute of this “weird and wonderful” animal shows its two main sections: a nectosome made up of 5 to 20 nectophores (zooids which do the propulsion), and a siphosome, composed of zooids that sting and digest krill:

Like other “physonect” siphonophores, N. bijuga has a third part: a “pneumatophore” at the apex of the nectosome. Filled with carbon monoxide gas, the pneumatophore helps keep the colony in a vertical orientation. So numerous and effective are these little predators, they eat more krill per day than all the whales in the bay combined! That is remarkable, considering the images we’ve seen (e.g., in Illustra’s documentary Living Waters) of humpback whales gulping big mouthfuls as they lunge with mouth agape into dense swarms of the little shrimp-like crustaceans. Another fascinating fact about N. bijuga is that it participates in the daily migration of plankton (diel vertical migration), descending to 800 meters during the daytime for protection, and up to the surface at night. That’s a lot of swimming for a little foot-long Ironman — a mile a day. 

Jet Propulsion

Like jellyfish, squid, and octopuses, siphonophores move by jet propulsion. Each nectophore looks like a bubble with a small orifice. The zooid quickly squeezes the bubble, shooting water out to provide thrust, then fills up again. Arranged in pairs along the nectosome, the nectophores cooperate like rowers in a team. One fact about their teamwork fascinated scientists led by Kevin T. Du Clos and Kelly R. Sutherland at the Oregon Institute of Marine Biology, aided by scientists at other institutions including Caltech. That fact is that N. bijuga employs both synchronized and asynchronous propulsion: sometimes the nectophores “pull” together, and sometimes they work independently. Why is that, and does it make a functional difference? They published their findings in PNAS: “Distributed propulsion enables fast and efficient swimming modes in physonect siphonophores.”

Siphonophores are colonial cnidarians that, unlike single jetters such as squids, swim using propulsion from multiple jets, produced using subunits called nectophores. Distributing propulsion spatially provides advantages in redundancy and maneuverability, and distributing propulsion over time enables context-adaptive swimming modes. We use experiments and modeling to compare swimming modes. We show that synchronous swimming produces high mean speeds and accelerations. By contrast, asynchronous swimming consumes less energy. Thus, by simple variations to the timing of thrust production, siphonophores achieve similar functionality to that of fishes, the ability to adapt swimming performance to context. A greater understanding of the benefits of multijet propulsion may also improve underwater vehicle design. [Emphasis added.]

So once again, we see nature inspiring design by imitation. These scientists found measurable benefits to the travel habits of a lowly, nondescript whatchamacallit. Its ability to get around and migrate a mile a day attracted them to wonder how, and why, with such simple equipment, this organism achieved similar performance to fish. Expecting a reason, they found one: the siphonophore can adapt its “gait” (so to speak) to the needs of the moment: pulling together to escape a predator, but breaking cadence to save energy. It’s something like we see with marching bands, sometimes moving in strict order and sometimes in a “scatter” formation to get into position with less energy. Think what the humble common siphonophore’s ingenuity could mean to energy-conscious marine vehicle design:

Providing specific advice for vehicle design is beyond the scope of this study, but experimental pulsed single jet vehicles that operate within the Reynolds number range this study (SI Appendix, Fig. S1) have been tested (e.g., Re = 1,300–2,700 for (33)), and there are general principles from this study that could be useful for vehicle research and design. Analogously to N. bijugaa single underwater vehicle with multiple propulsors could use different modes to adapt to context. Our model test cases suggest strategies for tuning the behavior of a vehicle depending on the desired performance characteristics. A propulsion pattern mimicking the asynchronous case—in which thrust is low, and asynchronous—is best if power consumption is the primary concern because it minimizes the cost of transport. If speed is more important, the asynchronous-matched case—in which thrust is high and asynchronous—is likely the best because it decreases the cost of transport with only small losses in speed when compared to the synchronous case. Interestingly, the intuitive approach of producing high thrust synchronously (as represented by the synchronous case) may be the least useful, with its primary advantage being high initial acceleration.

Our results also suggest a general approach to selecting the number of propulsors an underwater vehicle should employ. Swimming speed, efficiency, cost of transport, and synchronous acceleration all improved with increasing colony lengths in our model, but these benefits approached asymptotes for the longest colonies(Fig. 3). For underwater vehicles with few propulsors, adding propulsors may provide large performance benefits, but when the number of propulsors is high, the increase in complexity from adding propulsors may outweigh the incremental performance gains.

The multijet strategy provides flexibility in the spatial and temporal distributions of propulsion. Multijet swimmers, such as N. bijuga, take advantage of this flexibility to increase their maneuverability, redundancy, and context-specific swimming performance.

The authors were impressed enough with the animal’s skill, they used the word “design” four times, but evolution zero times. Good thing; trying to figure out the phylogeny of siphonophores is a challenge (Molecular Biology and Evolution).

The Kicker

These scientists only focused on the advantages of multijet swimming in synchronous and asynchronous modes, but there’s more. What do these abilities imply? The colony could not do these things without coordination; that implies signaling and quick response by a neural system. The know-how to go where the fishing is good implies sensing systems. The ability to hunt and digest fish implies a digestive system that benefits the community. Foresight is evident in the colony’s ability to stop adding nectophores when the optimum number is reached. The design, for sure, proceeds all the way from the whole colony down to each cell, where molecular machines, a genome, and network of parts enables the whole. A siphonophore is, using Douglas Axe’s term, a “functional whole” with design evident at every level. 

It’s quite a show. And like the design plan, the synchronization continues throughout and within every player in the colony — even in the decision to break cadence and go async when that swimming strategy makes the most sense.