lpetrich

The most logical salt content of the blood of a marine animal is that of the surrounding seawater, and that is indeed the case for most marine invertebrates and for hagfish; they are "osmoconformers".

However, jawed vertebrates are "osmoregulators", having an inorganic-ion concentration about 1/2 or less that of seawater. Meaning that a marine one will tend to lose water to its environment. However, marine jawed fish have some ways of avoiding dying of thirst.

Sharks and coelacanths accumulate urea in their blood, thus making their solute concentration comparable to that of seawater.

Bony fish, however, continually excrete ions with their kidneys and drink seawater to restore the lost water.

This has led to the speculation that jawed vertebrates had evolved in freshwater and have gone back to the sea several times. They had lowered their inorganic-ion content to reduce the load on their kidneys; lower concentration means less water leaking in, and thus less work for the kidneys.

However, they apparently got stuck with this preferred ion concentration; they have not been able to go back to a marine ion concentration. This difficulty, and the different ways of getting around it, suggests evolution in action, rather than the action of some designer(s) who are powerful enough to fix the ion concentration of a marine fish.

Finally, land vertebrates most likely evolved from freshwater fish and not saltwater ones; present-day amphibians cannot live in saltwater.

References:

http://www.auburn.edu/~aliasim/schoo...r/notes_51.htm
http://www.eeb.uconn.edu/Courses/EEB...regulation.htm

scigirl

Oh man, I was ready to post "why it's 0.9%" Heh.

Interesting links - I remember learning about this in physiology. Even our salt content suggests that we evolved from sea creatures.

scigirl

lpetrich

scigirl:
Oh man, I was ready to post "why it's 0.9%" Heh.

???

There is evidence of variations of ocean chemistry, which may mean variations in overall solute content. That page discussed Mg and Ca concentrations, but not the more-abundant Na and Cl. So lower salt content may be some vestige of the early-Paleozoic ocean instead of an adaptation to freshwater.

And it is curious that jawed vertebrates should require some specific blood salt concentration -- is that necessary for certain molecular mechanisms to work properly?

Interesting links - I remember learning about this in physiology. Even our salt content suggests that we evolved from sea creatures.

If one goes far back enough, certainly. Along the way was likely some freshwater fish, because land vertebrates are better adapted to freshwater than to present-day seawater. No amphibian is capable of surviving in saltwater, and some seabirds have special glands at their eyes for excreting salt.

pz

Given that our salt concentration is roughly 300 mOsmolar, and that seawater is roughly 1000 mOsm, I'd say that argues that our vertebrate ancestors were not marine.

Quetzal

This is simply a question, and pure speculation to boot. Is there any particular reason why land vertebrates couldn't simply have seen their ion content change over time? If we have 350-400 million years of land-based vertebrate evolution in our ancestors, all done in the absence of saltwater, is there any particular physiological reason why any particular concentration of ions in the putative water-land transitional would have been retained?

lpetrich

pz, I think you mean our aquatic ones.

I brought this subject up because I had read about it sometime in my childhood/adolescence, and I wanted to follow up on it.

Here is further evidence:

Such fish-to-amphibian transitional forms as Acanthostega and Ichthyostega are known mostly from freshwater deposits; here is a site with nice drawings on them.

Here's [http://users.tamuk.edu/kfjab02/Biolo...b3405_ch04.htm]another site on vertebrate osmoregulation[/url]; it proposes that teleost fish (nearly all present-day bony fish) originated in freshwater and swam to the oceans in the Triassic.

I wonder how creationists propose to explain the inorganic-ion-concentration mismatch of marine jawed vertebrates -- one that requires continual excretion of these ions to maintain. It would be easier to create them fully adapted to seawater concentrations; such adaptation is already the case for hagfish and most marine invertebrates.

But I've seen some creationists huff and puff that I am presuming that maximum efficiency is one of the creator's goals. Yet it is implicit in their praise of high-quality adaptations.

Qinopio

In the site (with nice drawings) that you linked, lpetrich, I'm curious as to how the authors/sources were able to find the shape of these creatures' bodies from only a bone or two in some cases.

I can see the reasoning behind them finding a bone that was in between two other known species' corresponding bones, and then saying that the new bone was likely from a transitional form. Maybe I'm missing something, but the level of reconstruction here seems to be fairly speculative.

MrDarwin

Just a quibble--what we call "amphibians" that are alive today are almost certainly a monophyletic group (unlike "reptiles") but are very different from the first tetrapods that crawled out of the water, which are usually also called amphibians. It is one of my pet peeves that these original tetrapods are called amphibians.

It is very dangerous to assume an ancestral state based on the characteristics of the surviving members of a group. Modern amphibians may well have evolved from freshwater (or even fully terrestrial) tetrapods and it would have no bearing on the original state of tetrapods unless one can demonstrate a reasonably direct link between the two groups (and in the case of modern amphibians, their early history is hazy at best).

The best clues to the habitats and lifestyles of early tetrapods comes from the fossils themselves and the sediments they are found in--and as far as I know, all are found associated with freshwater sediments.

Undercurrent

The key words here are "in some cases". In some cases we only have a bone or two, in other cases (Ichtyostega and Acanthostega) we have more complete sets of remains. The notion that paleontologists routinely make grand reconstrcutions of creatures based only on single bones and try to hide the state of the actual evidence behind a pretty reconstruction is one of the great creationist lies.

Coragyps

Jennifer Clack's book Gaining Ground, which I've plugged before around here, gets into pretty good detail on just how much material is known for these and a wide selection of other critters at the tetrapod invasion of land. She readily admits that some are speculative in parts, but others are pretty complete for 300,000,000 years ago.