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So I'm studying oceanography and meteorology right now, and a point made in my ocean text had me puzzled. It mentions that cartiligenous fishes are isoosmotic with their ocean environment, but that bony fishes are constantly consuming seawater and pumping out ions to maintain a relatively low internal osmolarity. Talk about effort! It made me wonder if anyone has researched a potential evolutionary path for ocean-living bony fishes.

Outside of any evidence at all, the following would seem most logical to me: All primitive fishes (and I suppose any ocean-borne organism) initially developed isoosmotically with their environment. As ocean-borne fish migrated over time into fresher and fresher water, they adapted isoosmotically to their new environment. Then, as these now fresh-water fishes began to migrate back into the ocean, they adjusted to the now high salt concentration in their new environment by acquiring the ability to pump ions. This path would mean that bony ocean fishes are descended from fresh-water ancestors.

The simpler path, of course, involves ocean-borne fishes developing the pumps directly as a means to alter their internal osmolarity because it conferred some advantage to them. But what sort of selective pressure would result in this change is beyond me, and it would be very strange for the dramatic pumping of ions to come about without some selective pressure. I feel like this explanation is simpler only on a superficial level. I'm not sure which idea Occam's razor would discriminate against, and in the absence of anything in the way of additional facts, I'm at a loss.

I'm hoping some of you fishy folks can shed light on this for me.

Thanks,
Shawn

Check out this page on Google Books:

http://books.google.com/books?id=5GamlT7UCHYC&lpg=PA42&ots=BkHmX4XQTU&dq=ostracoderms%20osmoregulation&pg=PA42#v=onepage

It seems the situation is a little more complicated than you've described, but the answer to your question of selective pressure seems to be just speculation that it may have had to do with anadromous breeding.

I would also suppose that osmoregulation could have evolved in fish that lived near brackish water, especially places where rivers dump large volumes of nutrient-rich fresh water into the ocean and subsidize huge plankton blooms (a great food resource for fish) in a region with highly variable salinity.

Shawn,

Here's one (fairly straightforward) explanation:
http://www.ca.uky.edu/wkrec/VertebrateFishEvolution.pdf

Thanks, Gonzo - interesting stuff. I'd like to read what Jason intended to link to, as well.

-Shawn

Okay Shawn, I fixed it. It's a little bug in this site, apparently -- the code that parses links was messing up some of the capitalization in a case-sensitive address, so you have to just copy and paste it.

Here it is again: http://books.google.com/books?id=5GamlT7UCHYC&lpg=PA42&ots=BkHmX4XQTU&dq=ostracoderms%20osmoregulation&pg=PA42#v=onepage

Fascinating link Lloyd - particularly interesting to learn that all vertebrates have the same salt content in their blood. The possible explanation for this and how the oceans got salty is very cool.

Kurt

BTW - Never seen it mentioned before, but it seems that the increasing salinity of our oceans is a greater threat to life than the sun running out of gas (give or take a few hundred million years or so) :).

Thanks, Jason. Interesting stuff.

-Shawn