Re: Lungs and air bladders Re: Water Walking Aquatic Precursor to Apes

On Friday, December 16, 2011 3:17:16 PM UTC-5, pnyikos wrote:
On Dec 16, 8:37 am, Walter Bushell
wrote:
In article <jceh75$7j...@xxxxxxxxxxxxxxxxx>,
 nos...@xxxxxxxxxx (Paul Ciszek) wrote:

In article
<1ba8be4f-0ea8-4986-80de-699e62567...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Darwin123  <drosen0...@xxxxxxxxx> wrote:
  Several large families of fish have open air bladders. Open air
bladders are supposedly homologous to our lungs. That is, our lungs
are modified versions of open air bladders.

This is a new one on me. I've only read theories that said the
reverse:

Wiki suggests the reverse, that all living bony fish are descended from
an ancestor that had a primitive lung,
The open air bladder is sometimes called a "lung" in the popular literature. However, this is not formally correct. Air-filled sacs in fish are homologous to lungs, but not properly called a lung. An air filled sac that is open to the pharynx is referred to as a physostomous swim bladder. An air filled sac that is closed to the pharynx is referred to as a physoclistous swim bladder.
Gould referred to physostomous swim bladders as lungs. He claimed that fish evolved from an ancestor with a "lung" used for breathing air. This lung later was modified to act as a flotation device. He implied that today this lung is only found in lungfish. Gould oversimplified the situation.
Gould forgot to mention that there are many freshwater fish that still use their bladders to breathe air and as flotation devices. Almost all marine fish have a physoclistous swim bladder, committed to use as a flotation device.
Freshwater species that still use the bladder for breathing air are quite common. Catfish, carp and gar have a physostomous swim bladder. I would go so far as to say that the flotation function is secondary in these fish.
The physostomous swim bladder is not vestigial. The reason it is so common in freshwater fish is that many freshwater environments are anoxic (no oxygen). Some places are seasonally anoxic (no oxygen).
Even the domesticated carp (i.e., goldfish) has an open air bladder. That is one of the things that made it so easy to domesticate. Look at a goldfish tank without and aerator. On warm days, the gold fish comes to the surface regularly to gulp air. Warm water does not contain much oxygen. Therefore, the goldfish has to rely mostly on the atmosphere for oxygen.

Or two lungs? *Polypterus*, a primitive ray-finned fish, has two
lungs, as do the African lungfish *Protopterus* and the South American
*Lepidosiren*. The Australian Lungfish *Neoceratodus/Epiceratodus*
has only one, and is a facultative air-breather while the other two
lungfish are obligate air-breathers. {I've read that an African
lungfish died when it was put into a tank that did not give it access
to air.}
Sturgeons also have a pair of physostomous swim bladders. I don't know why they aren't mentioned together with lungfish. They are also primitive fish.
I think it is because the splitting of the physostomous swim bladder occurred independently in sturgeon and other fish.


and some lineages turned that
lung into a swim bladder because they needed a balast tank more than
they needed a lung.
The phrase "swim bladder" is used for both open and closed air-filled sacs. This is because a physostomous swim bladder can serve both functions. It serves both as a means of accessing atmospheric oxygen, and as a floatation device. However, marine fish have found a bigger need for a flotation device then as a means of alternate respiration. The swim bladders became specialized for flotation.
Some fresh water fish can access the atmosphere without using a physostomous swim bladder. Many fish have a modified gill, which is called a branchial organ. The gill aperture gets jammed with blood vessels, so that it absorbs air. The electric eel has some extra capillaries in its pharyngeal cavity. Naturally, these fish have a physoclistous swim bladder. The function of the swim bladder for absorbing oxygen became redundant.

*Latimeria* turned it into an oil-filled tank, sort of like in a
bathyscaphe [sp?].

Ah, but perhaps the swim bladders were later modified to be lungs, in
some cases.
Most scientists now think that the physostomous swim bladder evolved into a physoclistous swim bladder. However, you are conjecturing that a physoclistous swim bladder may sometimes have evolved into a physostomous swim bladder. It is possible, but I doubt it.
The reason that I doubt it is because the physostomous swim bladder is more general in function. The physostomous swim bladder serves as both a means of respiration and as a flotation device. Therefore, one can easily envision a series of small steps where the physostomous swim bladder loses the respiration function. However, the physoclistous swim bladder only serves as a flotation device. It can't be used for both. Therefore, there are no small steps that turn physoclistous to physostomous.


Are there any promising candidates for this evolution in the opposite
direction?
This isn't exactly what you asked. However, there are cases where some other organ developed into a "lung". The original "lung" (i.e., open bladder) then became a flotation device.
There are many freshwater fish that developed alternate ways of breathing air that didn't require a swim bladder. So they developed another organ that acted as a lung. The open swim bladder evolved into a closed swim bladder.
Fish near the surface have physoclistous swim bladders that fill up on nitrogen and carbon dioxide. However, some abysmal fish have an physoclistous swim bladder that contains almost pure oxygen, rather than carbon dioxide or nitrogen. Apparently, these fish are using their bladders as a mean of storing oxygen for an emergency. This can be construed as being a respiration related function. So maybe it is on the way.

I respect much of Gould's work. However, sometimes he is too condescending. I find that I have to check many of his claims. I bought some cheap textbooks on comparative anatomy. Cross reading the textbooks helped me understand evolution better.
.

Relevant Pages

  • Re: Lungs and air bladders Re: Water Walking Aquatic Precursor to Apes
    ... are modified versions of open air bladders. ... fish are homologous to lungs, but not properly called a lung. ... pharynx is referred to as a physoclistous swim bladder. ...
    (sci.bio.paleontology)
  • Re: Low carbohydrate diet necessary when taking antibiotics?
    ... Frank de Groot wrote: ... > bladder, lungs and and nothing cured it yet. ... > Lufenuron ALMOST cured it until resistance appeared. ...
    (sci.med.diseases.lyme)
  • Re: Rear shock & nitrogen pressure
    ... While the one bladder I worked with was pretty thick, ... Doofus pulls an air hose off a compressor, ... Visiting Guitar Center I discovered the process is 1) spend hundreds, ... My septic system has an air pump. ...
    (rec.motorcycles.dirt)
  • Re: Rear shock & nitrogen pressure
    ... Before pressurizing the shock, the air bladder is already full of room ... All the added nitrogen does is increase the pressure. ...
    (rec.motorcycles.dirt)
  • Re: The case for ABX
    ... >>> crawled out of the mud on spiny fins, and belched rank ... >>> air from a bladder. ...
    (rec.audio.opinion)