>"The major problems with life on land relate to weight and structural support
>as much as to the physiology of breathing air. A fish is buoyed up by the
>water and its body weight may be effectively zero. On land, however, the body
>as to be held up by some form of limbs, and the skeleton as all the internal
>organs have to become structurally modified in order to cope with the new
>downward pull of gravity. The backbone of a fish is adapted for the stresses
>of lateral stretching and bending during swimming, but the main forces to
>which a tetrapod is subject are caused by gravity. The vertebrae and the
>muscles around the backbone have to become modified to prevent the body from
>sagging between the limbs. The mode of locomotion of a tetrapod on land is
>generally different from that of a fish in water." [Benton, Vertebrate
>Palaeontology, Chapman & Hall 1990, pp. 46-47]
First of all a fish has the same weight whether it is immersed in the water or
if it is laying on the bottom of a boat. The only way the fish's weight can
change is if its mass changes and/or it is in a different state of
acceleration with respect to the earth's surface. The fish will only be in a
so-called weightless state if it is in a state of free fall. The free fall
trajectory can be any trajectory (including earth orbit) whose motion can be
characterized by a downward acceleration by an amount equal to the local
strength of the gravitational field at the location of the fish. If the fish
is neutrally buoyant in the water it is not weightless. Rather, the water
under the fish supports the fish against gravity. The fish is subject to
both the force of gravity and a force resulting from the hydrostatic pressure
of the water pushing inward on the fish from all sides. Since the hydrostatic
pressure in the water increases with depth (due to the water's weight), the
pressure exerted on the fish's under side by the water is greater than the
pressure exerted on the fish's upper side. The difference in the pressure
exerted on the fish at different parts of the fish's surface results in a
upward net bouyant force exerted on the fish which opposes and cancels the
gravitational force on the fish. The lack of a net total force on the fish
prevents the fish from accelerating. The fish is not weightless in this state
anymore than I am weightless as I type out this message. The downward force
of the earth's gravitational field on my body is cancelled by an upward force
due to an excess pressure exerted on my underside by a chair. The effect of
the water on the fish is to smoothly distribute the supporting force on the
fish over a large portion of the fish's surface. This minimizes localized
stresses and allows the fish's tissues to remain relatively unstrained without
a large amount of internal stiffness (provided by a land-adapted backbone and
musculature) In the water the fish is effectively laying on a perfectly
form-fitting water bed.
On the land the situation is somewhat different. In this case all of the
supporting force is provided by the ground and that force is concentrated on
the relatively small contact area between the ground and the animal. If the
animal is supported on thin limbs then all the weight is concentrated on them
resulting in a large pressure stress on them and on the joints connecting the
limbs to the rest of the animal. If the animal is laying directly on the
ground, say like a snake, then the support against the body weight is also
spread out over a large area resulting in a *reduced* stress of support. This
is why it is less uncomfortable to lay in bed all day than to stand still for
long periods of time. If the animal is not adapted for dry land its internal
stiffness may still not be sufficient to prevent unacceptable tissue strain
when placed on the ground. A jellyfish would be such an example. Since I'm
no biologist I have no idea whether or not a fish's fins would be rigid enough
to support a fish (other than a walking catfish) on dry land and to provide
a means of locomotion.
Benton states "..., but the main forces to which a tetrapod is subject are
caused by gravity.". Actually, to pick another nit, it is impossible for
gravitational forces to be felt or to cause a relative distension in an
organism (unless that organism was so enormous that the earth's gravitational
field differed significantly across the organism's body and tidal stresses
induced in the organism became significant. It's not the gravitation forces
that are the source of the stress, strain and distension; its the differential
distribution (across the organism's body) of the *non-gravitational* forces
that support the organism and prevent it from being in a state of free fall
acceleration.
David Bowman
dbowman@gtc.georgetown.ky.us