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Re: Coelacanth article in current NatGeo
As some additional food for thought with regards to fin evolution, consider
that free-swimming organisms have a challenge in the sense that rapid starts
are best initiated using large fluid forces at a low L:D ratio ("drag-based"
locomotion) while sustained swimming tends to be most efficient and rapid using
moderate to large fluid forces at a high L:D ratio ("lift-based" locomotion).
Fleshy fins are one potential solution to this problem, as they can be used as
either a drag-based paddle or a lift-based hydrofoil depending on the
orientation of the limb element.
An alternative is to have compliant fins, but a flexible body that can bend
sufficiently to serve as a drag-based paddle during fast starts. The latter
solution is what ray-finned fish (especially teleosts) generally use, with a
few exceptions. The fast-start mechanic is of such importance that up to 75%
of the muscle mass in some taxa is devoted almost exclusively to fast-start
usage, acting as payload the rest of the time. Of course, in teleosts
especially, this involves a major reorganization - not only are the fins
compliant, but the pelvic and pectoral girdles are shoved up behind the head,
making most of the length of the animal effectively a hypertrophied "tail"
involved in propulsion. The compliance of the ray-fins is then useful for
other aspects of control and propulsion. Note than in secondarily stiff-bodied
fish, such as surgeonfish and triggerfish, the pectoral fins have become
secondarily fleshy, and are used in an aquaflying manner for primary propulsion.
Assistant Professor of Biology
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On Feb 24, 2011, at 1:00 PM, Augusto Haro wrote:
> I think Choo is right. Chondrichthyans also have fleshy fins, thus
> suggesting fleshy fins came first. Thus, the reason by which fleshy
> fins were good may reduce to the reason by which fins are good. Now,
> it may be asked why ray fins are more adaptive, if they persisted
> because of intrinsic adaptive superiority and not because of luck at
> the mass extinction (or because of other improvement in
> 2011/2/24 Erik Boehm <email@example.com>:
>>>> Maybe my understanding of
>>> their diversity is wrong, but it seems odd
>>>> that at one point there were so many fleshy
>>> finned fish, and now
>>>> there are almost none.
>>> A couple of mass extinctions.
>> Of course, but prior to those, the lobe fins were not restricted to
>> bottom/river dwellers correct? if so, what purpose did those fleshy fins
>> Its my understanding the basal lobe fins did not live in rivers, or along
>> the sea floor, where such fins may have been useful for holding the fish
>> against a surface. What was the original use?