Why do fish bob their heads back and forth as they swim? Is that wasted movement? Is it an inescapable consequence of undulatory motions during swimming? That’s what many scientists used to think. What a team found out reminds us never to assume nature’s methods are wasteful.
A new paper in Nature Communications summarizes the find: “Fish optimize sensing and respiration during undulatory swimming.” That word optimize has design written all over it, especially when the fish optimizes three things at once:
Previous work in fishes considers undulation as a means of propulsion without addressing how it may affect other functions such as sensing and respiration. Here we show that undulation can optimize propulsion, flow sensing and respiration concurrently without any apparent tradeoffs when head movements are coupled correctly with the movements of the body. This finding challenges a long-held assumption that head movements are simply an unintended consequence of undulation, existing only because of the recoil of an oscillating tail. We use a combination of theoretical, biological and physical experiments to reveal the hydrodynamic mechanisms underlying this concerted optimization. [Emphasis added.]
Using a “bio-inspired physical model” with flow sensors, the team from Harvard and the University of Florida found that head bobbing actually improves swimming efficiency. Then they studied how the fish’s lateral line sense improves with the resulting water flow. One might think that the extra motion of the head would confuse the lateral line sense, but the opposite is true.
We discovered that the motions associated with undulation can automatically enhance lateral line sensing on the head by minimizing self-generated stimuli. Fish move their heads in a way that minimizes pressure up to 50%, establishing a twofold greater sensitivity to an external stimulus than would otherwise be possible (Fig. 3a). At swimming speeds up to 2 L s−1, we found a heightened sensitivity around the anterior region of the head, which is where the majority of the encounters related to feeding and locomotion are initiated. We propose that during swimming, fish may not have to rely as extensively on the efferent system to distinguish between external and self-generated stimuli if they rotate their head in an appropriate phase with respect to side-to-side motion.
Simultaneously, this head motion increases the flow across the gills, enhancing respiration. This is the first time the coupling of motion with respiration has been demonstrated in fish like it has been with birds, horses and humans. Scientists used to view undulation and respiration as independent processes. No longer:
Here, we discover that fishes swimming with body undulations also show respiratory-locomotor coupling. Our pressure model reveals that undulation-generated pressures around the mouth and opercula oscillate dramatically. We found that fishes exploit these pressures by timing their respiratory movements accordingly, which likely minimizes the energetic cost of pumping the dense medium of water. High-speed, high-resolution video reveals that respiratory movements are tightly synchronized with head movements (Fig. 3b). When the pressure difference between the outside and inside of the mouth reaches 0.2 mm Hg, fishes open their mouth to allow water to flow in passively. Perhaps not coincidentally, this exact pressure difference is generated by the active buccal expansion of stationary fish. In this way, we hypothesize that swimming fishes exploit self-generated pressures to circumvent the work of buccal pumping.
This is really neat. The undulatory motion of swimming with the fins moves the head back and forth in phase such that the work of breathing is reduced, and the sensitivity of the lateral line is optimized. It’s a three-for-one gain with no tradeoff in cost.
Life requires the successful, simultaneous execution of basic physiological functions. The coordination of these functions usually relies on distinct neural networks that run in parallel. Over the past several decades, a number of studies have demonstrated that the passive mechanical properties of the body can simplify individual functions, releasing them from the need for precise neural control. Here, we show that during aquatic axial undulation, head movements can allow seemingly disparate but fundamental functions to be coordinated simultaneously without tradeoffs.
Isn’t evolution smart to pull this off? Actually, the authors didn’t have much to say about evolution. Their only mention of evolution seems to falsify its expectations:
Given that the respiratory system is located in the head and the locomotory system is associated with the trunk, it is not unreasonable to assume that respiration and swimming would be decoupled. The contemporary view point is that the origin of the lung enabled respiratory-locomotor coupling to evolve in terrestrial animals.
Here, we discover that fishes swimming with body undulations also show respiratory-locomotor coupling….
But why would evolution optimize two or three things at once? Selection for traits can only act on immediate benefit from a random mutation. Like they say, “it is not unreasonable to assume” that selection for benefit in one trait would be independent of selection for other traits. The “contemporary view” may be that the lung “enabled” coupling, but if that were a useful idea, they would have said more about it. They didn’t. We know from experience, however, that when engineers succeed in optimizing multiple things at once without tradeoffs, they win prizes and promotions for intelligent work.
More evidence that this work supports intelligent design is seen in their desire to imitate it. “The power of this simple control architecture is that it can be universally applied to any size and species of undulating fish, as well as to autonomous, underwater vehicles,” they note. Yet the salmon seen bobbing their heads in Living Waters beat engineers to it. Engineers can just imitate what they see and win a design prize.
Another case of intelligent design was announced in a second paper in Nature Communications. Biologists from the College of William and Mary liked this design so much, they immediately thought of how to apply it. Notice that the design is found in birds and mammals as well as fish.
Suspension-feeding fishes such as goldfish and whale sharks retain prey without clogging their oral filters, whereas clogging is a major expense in industrial crossflow filtration of beer, dairy foods and biotechnology products. Fishes’ abilities to retain particles that are smaller than the pore size of the gill-raker filter, including extraction of particles despite large holes in the filter, also remain unexplained. Here we show that unexplored combinations of engineering structures (backward-facing steps forming d-type ribs on the porous surface of a cone) cause fluid dynamic phenomena distinct from current biological and industrial filter operations. This vortical cross-step filtration model prevents clogging and explains the transport of tiny concentrated particles to the oesophagus using a hydrodynamic tongue. Mass transfer caused by vortices along d-type ribs in crossflow is applicable to filter-feeding duck beak lamellae and whale baleen plates, as well as the fluid mechanics of ventilation at fish gill filaments.
A hydrodynamic tongue — what a concept! Fish “engineer” previously unknown flow patterns to transport the particles they need into their esophagus. Those humpback whales seen in Living Waters use this technique as they gulp krill with their huge mouths, and the small tropical fish do it with their gills. The ducks in Flight: The Genius of Birds do it with their beaks. Who taught a fish, a duck, and a whale about fluid dynamics? It must have been natural selection. Tell us, please, how that came about:
In addition to the ecological and evolutionary relevance, these problems are of substantial interest to industrial filtration engineers who seek to reduce the major operating expenses associated with clogging.
One reads with bated breath for an evolutionary explanation that never comes.
As more than 30,000 fish species possess branchial arches that may form d-type ribs, potential vortex formation in the slots between branchial arches has substantial implications for the fluid dynamics of fish feeding and ventilation throughout ontogeny and evolution. Vortical cross-step filtration could be applicable to feeding in a diversity of fish species. In addition, many filtration structures involved in vertebrate suspension feeding are composed of d-type ribs in crossflow, including fish gill rakers, tadpole gill filters, bird beak lamellae and whale baleen plates, suggesting that principles of vortical cross-step filtration could have widespread application.
And that’s it. That’s all they have to say about evolution. This beautifully designed trait, so envied by engineers, is found in all these unrelated animals. How? Because “principles of vertical cross-step filtration” work, and are found all over the animal kingdom, they must have evolved. Does that make any sense?
It should be clear to anyone that intelligent design did the heavy lifting in both papers. Evolution played no role in the experimental setup, the explanation, or the application in either case. As usual, evolution only tags along in the role of post-hoc narrative gloss.
Image source: Illustra Media.