species of the day: Champsocephalus gunnari

[eub]

Chosen semi-arbitrarily as a representative of the Antarctic icefish family, Channichthyidae, who lack hemoglobin. "Semi-" because it goes some one better in lacking myoglobin, too.

"Preliminary observations on the heart of the haemoglobinless Antarctic fish Champsocephalus gunnari, Lonnberg":

The heart of the haemoglobinless and myoglobinless "icefish" Champsocephalus gunnari shows interesting morphometric and histological features.

"Variable expression of myoglobin among the hemoglobinless Antarctic icefishes":

Of the eight icefish species examined, three lack Mb polypeptide in heart ventricle, although one of these expresses the Mb mRNA. All species of icefish retain the Mb gene in their genomic DNA. Based on phylogeny of the icefishes, loss of Mb expression has occurred independently at least three times and by at least two distinct molecular mechanisms during speciation of the family.
[...]
Pale coloration of icefish tissues has led most researchers to assume that Mb is not present in icefish oxidative muscles (3-7). However, we collected three channichthyid species off the Antarctic Peninsula, Chionodraco rastrospinosus, Pseudochaenichthys georgianus, and Chaenodraco wilsoni, that displayed distinctly rose-colored hearts.

(Why do they lack hemoglobin? Water's unaided gas solubility is better down there, but clearly not as good as they'd like. I think (after changing my mind) that this is saying that hemoglobin delivers oxygen poorly at low temperatures, but is it worse than nothing, or just not worth the synthesis?)

Comments

[brokengoose.livejournal.com]

I think (after changing my mind) that this is saying that hemoglobin delivers oxygen poorly at low temperatures, but is it worse than nothing, or just not worth the synthesis?

At first guess, I'd blame random mutation. Some were born without the ability to process hemoglobin, and that mattered so little in that environment that they survived without problems. Perhaps it even went along with an unrelated change that was a significant advantage.

[bhudson.livejournal.com]

Two questions:
(1) what is myoglobin about? I thought it was pretty critical to having muscles, but clearly I'm wrong.
(2) how do the fish transfer oxygen around? or are you saying they just absorb it through the skin and it happens to diffuse well enough (like insects do)?

[eub.livejournal.com]

Myoglobin: I don't know.

I'm assuming they still have gills and all of the usual plumbing, just that their blood doesn't involve hemoglobin.

[bhudson.livejournal.com]

Even though I *work* on (simulated) red blood cells, I am stunningly ignorant as to how they work.

[aquaeri.livejournal.com]

I think myoglobin is basically a secondary, muscular oxygen store. I remember whales have an enormous amount of it, allowing them to go for half-hour dives.

[aquaeri.livejournal.com]

It wouldn't surprise me if the haemoglobin might turn out to be worse than useless at lower temperatures.

As it is, us mammals have several different hemoglobins, used at different stages in fetal development, because the hemoglobin of the fetus needs to be able to "steal" oxygen from the mother's hemoglobin (rather than directly from air), and return CO2 to her.

[eub.livejournal.com]

Whoa. Cool. The fetal hemoglobin has higher O2 affinity, so, hm, so, let me think about the partition sequence here. At first thought it seems like the high-affinity fetal hemoglobin would be a wash, since the partition coefficient for fetal blood to fetal tissue goes down as the one for adult blood to fetal blood goes up, I'd think. Maybe it's something to do with making use of the small volume of the fetal-blood compartment, or maybe it's about the kinetics instead of the equilibrium? My intuition is rusty.

Embryonic hemoglobin, then fetal. Huh.

(Relatedly, my dad did point out that results on small-mammal hemoglobin don't necessarily generalize to counterfactual icefish hemoglobin.)

[eub.livejournal.com]

Wikipedia explains "by the time blood reaches the capillaries of the placenta, its oxygen saturation has decreased considerably". So, I guess that, for example, if adults lived in a lower-oxygen atmosphere, we'd want higher-affinity hemoglobin, and I guess once I understand why I'll understand the fetal situation.

[aquaeri.livejournal.com]

I don't understand the physics at all, I just know the genes, and how they turn on in sequence. And how the blood of the mother doesn't cross to the fetus, so the oxygen has to get over there somehow.