Re: Why do fish have backbones?

In article <[EMAIL PROTECTED]>,
Coreleus Corneleus <[EMAIL PROTECTED]> wrote:
>
>When it comes to swimming, the organism has to create a situation
>where high friction with the surrounding water medium is favored. 
>Then it has to impart a force to the water in that environment.  Then
>it has to change its shape so that it can glide through the water with
>minimum resistance, and then remodify itself again so that it generate
>more propulsion.

Yes, that's how fish swim.  Flex, straighten and glide.

>Now if an organism were to have a fin, and that fin completely
>deformed when it went through the water, it might not be able to
>impart as much of a force near to the end of the stroke, if it were
>not rigid, in comparison with a stronger fin.

Fish fins are extremely flexible and use complex motions to steer and stop
the fish.

>If you could make an organism extend itself sideways, however, the
>sideways direction of the organism could act as a substitue for a fin.
> The organism's sideways direction could make it have increased water
>resistance at the bend.  If the muscles contracted into the bend, then
>they could thus impart force into the water.

That's how fish swim, using lengthwise horizontal sinusoidal motions of
the body.  You can see this quite clearly by watching fish swim from
above.  Check out your local pet or bait shop.  Fins are used for steering
and for hovering, compensating for the forward propulsion caused by water
pushed out through the gills.

Note that the body of a fish mostly consists of muscles arranged for this
type of propulsion.  FIsh that spend most of their time swimming forward
show this to the largest extent.

>A swimming eel could have many different bends down its vertebral
>column at the same time.  If the organism could not resist lateral
>compression at the bend, however, the limitations in its own internal
>structure might limit the amount of force that the organism might be
>able to impart into the water.

Shorter fish do the same thing.

>Thus internal structural resistance to deformation, might have
>initially been a way of increasing the capability of producing force
>output into the water, and thus increasing the potential speed of
>swimming, when needed.

Well, you'll have to express that in a less teleological form to get
the real picture, but yeah.  I've never seen amphioxus or tunicate
larvae or other primitve chordates swim, but I bet they had this method
down by then, 5-600 million years ago (or more), before they came up
with real bones.

>It was only later that this capablity became useful, toward properties
>enabling organisms to walk on land.

It's sort of a detriment on land, but if you observe salamanders and lizards,
you'll see that the horizontal flexing of the spine is still there.  In the
case of lizards, at least, it keeps them from breathing while they walk or
run.  Mammals, birds and some other reptiles have a more rigid spine, which
mostly flexes up and down (forward and back in a human).  So marine mammals
and birds (penguins) and probably ichthyosaurs swim by flexing the spine 
in the vertical plane, and the tail flukes of cetaceans and ichthyosaurs
are in the horizontal plane.  None of these animals use their limbs for
rapid propulsion, not even otters.

While swimming humans (and other terrestrial mammals if forced to swim)
use their limbs for propulsion, one of the fastest racing strokes is the
butterfly, which uses a cetacean-like flexing of the spine as well as 
limb motion.

It's certainly true that a strong flexible internal skeleton seems to be
necessary to produce a terrestrial animal of other than pretty small size 
and mass.  In that sense, a bony spine and fins was a preadaptation to 
terrestrial life for vertebrates.  But arthropods got there first, and
plants made it out of the water even earlier.