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:
Addison-Wesley's middle-school book Science Insights: Exploring
Living Things (1996) says it this way:
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.
Deep Breathing
William J. Bennetta
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]
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]