Although you may not "want to get into" it, I think you may have confused the
Reynolds number with another dimensionless quantity, the specific gravity,
here. The Reynolds number is a dimensionless measure of the relative strength
of inertial effects to viscous friction effects in the fluid as it is in
relative motion with respect to the fish. The Reynolds number is defined as:
R = [the linear extent of an object moving through the fluid] * [the mass
density of the fluid] * [the relative speed between the fluid and the
object] / [the shear viscosity of the fluid]. If R is small the flow is
laminar and dominated by viscous frictional dissipation. If R is large the
flow is inertia-dominated with a tendency to produce relatively undamped
eddies and possibly even full blown turbulence. At tiny R the drag on the
object (i.e. the fish) is proportional to the first power of the relative
speed. At larger R values the drag is proportional to the square of this
speed. The Reynolds number R has essentially no relevance to any "effective
weightlessness" effects for the fish.
OTOH, the specific gravity of an immersed object measures it's buoyant
tendency. The specific gravity is the ratio of the object's (fish's) average
mass density to the mass density of the surrounding water. If the specific
gravity is greater than 1 the object tends to sink (in the absence of any
propulsive motion inducing a "lift" on the object). If the specific gravity
is less than 1 the object tends to float. If it is exactly 1 the object is
neutrally buoyant and tends to stay where it is put in the water. It is
possible that Benton considered the case of neutral buoyancy to be "effective
weightlessness". If so, it is a peculiar kind of weightlessness as it only
counts as weight a supporting force exerted on the fish by the ocean/lake
bottom and not by the water itself.
>On land, as you point out, the situation is different. You state:
>
><< Actually, to pick another nit, it is impossible for
>gravitational forces to be felt or to cause a relative distension in an
>organism>>
>
>According to Kardong, however:
>
>"Gravity's persistent attempt to accelerate a terrestrial animal and PULL IT
>DOWN constitutes the animal's weight." [pg. 135, emphasis added]
I don't know why you made this quote from Kardong in response to my statement
above. This quote does not address the issue I brought up above. It does
give (loosely) a common definition of weight though. This definition says
that the weight of an object is the the gravitational force acting on the
object. This definition of weight, however, does not correspond to our usual
sensation of weight though. According to this definition objects in free fall
(e.g. in orbit) are not weightless, but have the same weight they would have if
they were not accelerating. I prefer a second more sensible (sensible in the
sense of "can be sensed") definiton of weight. According to this second
definition weight is the magnitude of the supporting *non-gravitational*
forces acting on a body which prevents it from being in a free-fall state.
According to this definition orbiting astronauts and occupants of a falling
elevator whose cable has broken *are* weightless. (An advantage of the second
definition of weight is that it is most compatible with Einstein's Principle
of Equivalence and General Relativity, according to which the gravitational
*force* acting on a body has no objective frame independent meaning.) It
should be noted that according to *both* of these definitions of weight a fish
immersed in water is *not* weightless.
Getting back to the topic of walking catfish, I think I may have seen some of
these creatures just last week (although I'm not sure of this). Over
Christmas break my family spent some time over New Year's Day in Florida. One
day we visited Everglades National Park. Some of the boardwalk trails go over
the shallow (and surprisingly clear) waters of the everglades where many sorts
of wildlife can be seen. On one such trail we could see a (to me) strangely
behaving school of catfish in the clear water. There was a cluster of about a
couple of dozen tightly packed catfish standing vertically on their tails on a
small mound on the marsh bottom. These fish were packed so tightly that they
touched their neighbors while standing there. They sort of looked like a
colony of anemones or coral polyps. Every 15 seconds or so one of the catfish
would gently swim straight up to the surface and take (what looked like) a
gulp of air. Then it would lower itself back down into its previous spot in
the colony. About 15 seconds later another one of these fish would "come up
for air". It may be that these were not "walking" catfish but some weird sort
of amphibian than I had never seen before, but they sure looked just like
catfish in the still, clear, shallow water. Since they seemed to "breathe
air" I wonder if they were the famous Florida walking catfish. Since they
were in plenty of water they had no need to leave the pond and "walk" over
land to find another pond. Would any biologically knowlegeable list member
please venture an opinion of what was this weird sight that my family and I
saw in the everglades?
David Bowman
dbowman@gtc.georgetown.ky.us