Re: Why do fish have backbones?

On 20 Sep 2003 20:58:45 -0700, [EMAIL PROTECTED] (Coreleus
Corneleus) wrote:

>Skeletons are generally needed for structural support for land
>organisms in order for their bodies to overcome gravity and aid in
>locomotion.
>
>What are the underlying structural and physical reasons for the
>evolution of the endoskeleton in vertebrate fish, however?
>
>In water there generally tends to be no need to generate a structural
>support for the purpose of overcoming gravity alone.  The density of
>biological tissue might be slightly greater than water, but swim
>bladders or the like tend to work relatively well as a counterbalance
>to those types of forces.
>
>For locomotion, you need to have low drag or resistance to movement as
>you are swimming through the water, but at the same time an organism
>also needs to impart a backward force to the water itself in order to
>propel that organism forward.  The generation of 'lift' would not
>necessesarily be as critical to a water organism as it would be to an
>air organism, again, because of the lesser density differential
>between the organism itself and its surrounding medium, when an
>organism is floating in water rather than when it is flying in air.
>
>You generally tend to need a situation where the organism can generate
>a high water resistance, impart a force, then generate a lower water
>resistance in order to be impelled forward while simultaneously
>extending itself back to its initial conditions so that it can impart
>a new force, all without producing an opposite backward force of the
>same magnitude, which would, ultimately, make it go nowhere if were
>not minimised.
>
>Now it just seems to me that fish tend to swim faster than jellyfish,
>although I can't say for certain.
>
>The early chordates were supposed to have a sessile form and a younger
>moving form, that gradually became the fishes.
>
>It would seem to me that a notochord or backbone would enable a
>sessile organism to thrive in rapidly flowing waters withoug being
>torn apart or rupturing.  Nonetheless, the notochord tends to be a
>structure for the larval rather than the adult state for at least some
>of the most primitave chordates, although I am not sure of the
>relative extents.
>
>Are there any structural reasons with underlying physics involved, as
>to why a backbone or notochord might be advantageous for the purpose
>of locomotion speeds in the early fishes, rather than simply having
>opposing muscles resist contraction on the opposite side through
>closed water chambers, or the like?
>
>What advantage does a fish have, in comparison to a swimming worm, an
>octopus, or a starfish?

The flexible but incompressible rod called the notochord allowed early
chordates to swim by flapping their tails from side to side.  That was
a very effective and efficient method of locomotion.  There are other
ways of swimming rapidly, but that turned out to be a very good one.

If you want to move efficiently, you have to contract muscles and
still control the form and shape of the body.  Without some type of
skeleton, contracting longitudinal muscles on the side of the body
will simply shorten the body, not bend it.  Hydrostatic skeletons have
severe limitations so the best way to maintain body shape is with some
form of skeleton, internal or external.  There other limitations on
external skeletons.  You find an enormous range of aquatic animals
alive today using all three strategies -- hydrostatic skeleton in the
jellyfish, a wide variety of "worms", and molluscs like squid and
octopus, external skeletons in the crustacea, and internal skeletons
in the vertebrates.  The vertebrates "won" in being able to produce
very large very fast moving organsms but the alternatives are still
extremely effective especially in smaller body sizes.

Still the notochord is not a backbone (a vertebral column).  The
vertrebra (plus skull)  were protective structures enclosing the
central nervous system and, originally, the dorsal aorta.  Only later
in evolutionary terms did the vertebra replace the notochord as the
main structural and support element maintaining the body shape. You
can't consider "modern" vertebrates since everything from most fish on
up have bone, a radical change in the ability to build skeletons.  To
see the change from relying on notochord to using the vertebral
skeleton, you have to look at modern lampreys and hagfish (who retain
a notochord) and sharks and other cartilaginous fish (whose notochord
remains but in greatly reduced form).  The change might be due to
different methods of constructiion.  The notochord is all one piece
and relies on flexibility to function.  The vertebra are constructed
segmentally, one in each segment or myotome of the developing chordate
embryo.  The separate pieces could each become relatively rigid, hence
very strong, and yet retain flexibility in the way they connect
together.  In this way, the vertebral column was a much better
skeletal structure than the notochord. 

Note: we still have remnants of the notochord in the soft, squishy
centers of our intervertebral disks.