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from The Textbook Letter, July-August 1996

Deep Breathing

William J. Bennetta

When I was a lad, I served a term as a Paper Trooper. A Paper Trooper was a juvenile who went about with a little wagon, begged householders to give him old newspapers, magazines and books, and then hauled his gleanings to a local school. The newspapers, magazines and books were later shipped from the school to some industrial plant, which allegedly used them in some vital process.

I don't remember very much about my adventures as a Paper Trooper, but I surely can recall the day when, as I unloaded stuff from my wagon, I found a paperback book containing hundreds of items from "Believe It or Not!" -- Robert L. Ripley's syndicated catalogue of oddities that had been appearing for many years in newspapers throughout the country. I kept the paperback for myself, and during the next few days I absorbed its delights.

Using cartoons and one-line narratives that explained nothing, Ripley presented "facts" about astounding events, stupefying coincidences, marvelous objects, and the bizarre customs of peoples in distant lands. He identified the biggest this and the smallest that, he told of deformed vegetables that resembled famous persons, and he noted the feats of armless violinists and other talented freaks. And best of all, for a Paper Trooper who was interested in nature and science, he cited amazing things from the realm of natural history. Hence it was from Ripley that I first learned the "fact" that all sharks had to swim continuously to force water over their gills, and that they would suffocate if they couldn't keep moving!

Needless to say, it wasn't true. Ripley had simply reproduced a popular belief that had originated long before "Believe It or Not!" and has persisted to this day. If you require evidence of its persistence, just look at some current schoolbooks. For example, Holt's high-school book Biology: Principles and Explorations (1996) offers this comparison between sharks and bony fishes:

Movements of the opercula, shown in Figure 32-14, permit a bony fish to pump water over the gills, enabling the fish to breathe. . . . A bony fish doesn't have to swim forward with its mouth open to move water over its gills as fishes without opercula, like sharks, must do. [page 755]

Addison-Wesley's middle-school book Science Insights: Exploring Living Things (1996) says it this way:

Unlike the cartilaginous fishes [i.e., sharks and their allies], bony fishes do not have to swim to move water over their gills. They can pump water through the mouth and into the gill chamber. [page 351]

Similar comparisons appear in other books too, but all such comparisons are false -- as anyone knows who has visited a public aquarium and has seen a horn shark or a nurse shark lying quietly on the bottom and using muscles to pump water over its gills.

Now, it isn't surprising to find that schoolbook-writers are perpetuating an old story that is manifestly wrong, but this malarkey about sharks has some special significance: It reminds us that the writers of our "science" books are still devoted to the antiquated, mystical notion of "nature's ladder," even though scientists discredited that scheme long ago.

Please look again at the two passages that are quoted above. In each case, the writers are purporting to tell not about sharks per se but about bony fishes -- which, according to the lore of the ladder, are "higher" and more "advanced" than sharks are. In each case, the writers glorify the bony fishes by alleging that bony fishes can do something which sharks cannot. And in each case, the writers present this as an important distinction between the two groups of animals.

The distinction, however, is imaginary. It is an invention derived from, and intended to reinforce, the legend of the ladder. The truth about breathing in sharks and in bony fishes is much more interesting.

The Truth About Sharks -- All of the sharks evolved from ancestors that breathed by pumping water over their gills. That ancestral mode of breathing has been retained by nearly all of the sharks that exist today, including most of the pelagic species -- the ones that dwell in the open ocean and swim continuously, night and day, throughout their lives. But in a small number of pelagic species, the pumping apparatus has degenerated, this degeneration being a secondary effect of the animals' pelagic way of life. The sharks belonging to these species depend on "ram breathing": While they continuously swim forward, with mouths open, their motion causes water to be "rammed" into their mouths and through their gill chambers. Though the sharks include more than 375 living species, fewer than two dozen of these seem to be obligate ram breathers. Some examples are the white shark, the mako sharks, the threshers, the salmon shark, the porbeagle and the whale shark (Rhincodon typus).

The Truth About Bony Fishes -- All the bony fishes evolved from ancestors that breathed by pumping water over their gills. That ancestral mode of breathing has been retained by nearly all of the bony fishes that exist today, including most of the pelagic species -- the ones that live in the open ocean and swim continuously, night and day, throughout their lives. In some of the pelagic species, however, the pumping mechanism has degenerated, this degeneration being a secondary effect of the animals' pelagic way of life. These species use "ram breathing": As they continuously swim forward, with their mouths open, their motion causes water to be "rammed" through their mouths and their gill chambers. The bony fishes that rely on ram breathing include various tunas, billfishes and mackerels, as well as the bluefish (Pomatomus saltatrix).

Ram breathing, then, is not universal in sharks, is not unique to sharks, and does not distinguish sharks from bony fishes. It has arisen independently in some sharks and in some bony fishes, and it is correlated with particular conditions of life. To say this in a different way: The ram-breathing sharks and the ram-breathing bony fishes bespeak convergent evolution.

Now please look once more at the passage from the Holt book, with its claim that a bony fish pumps water by effecting "movements of the opercula." That claim is another invention -- an imaginary explanation for the imaginary fact that "fishes which lack opercula, like sharks" must swim to breathe. In other words, Holt's writers have concocted a new fiction to bolster an old one.

A bony fish does indeed move its opercula when it pumps water over its gills, just as a man moves his lips when he sings; but to say that opercular movements "permit" a bony fish to breathe is as silly as saying that labial movements enable a man to generate sounds. A typical bony fish's breathing involves coordinated movements of many bones and muscles, producing sequential changes in the volume of the fish's mouth and the volume of its gill cavities. This physiology has been well investigated and has been described in many scientific reports.

The characters who write "science" textbooks should shake off their preoccupation with "nature's ladder" and other superstitions, and they should pay more attention to the real world. Lots of interesting things are waiting to be seen out there -- believe it or not!


William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes frequently about the propagation of quackery, false "science" and false "history" in schoolbooks.

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