Thursday, March 5, 2009

Green eggs and jelly.

I have been reading about salamanders lately. I came across a paper I had not seen before. It addressed a question that hadn’t occurred to me, but should have, I think.

You see, salamanders of the genus Ambystoma, which should be breeding in a pond near you, soon if not already—if you live in temperate North America—lay eggs in the water in masses of jelly attached to something, often a submerged stick. The jelly masses absorb water, becoming quite large in some species such as A. gracile, the Northwestern Salamander, which is one of two species found in southwestern BC. Well, here’s the question: How do eggs in the middle of a large mass of jelly undergo the gas exchange necessary for development? Where do they get the O2? Diffusion of dissolved oxygen through centimetres of glorp must take an awfully long time.

Northwestern Salamander eggs, recently deposited. Cypress Mtn, BC.

Well, a clue, perhaps. As early as the late 1880s it had been noted that the originally clear, gelatinous eggs of these salamanders would turn green to the point of obscuring the developing embryos. The culprit was a single-celled green alga that lived within the egg envelope. The alga was named Oophila amblystomatis (egg loving, of Ambystoma; note: there was at some point a typographical error, confusing Ambystoma and Amblystoma). The presence of the alga did not harm the eggs. Could the alga be providing O2 to the embryos, and accepting CO2 and nitrogenous waste?

A short answer from Wikipedia:

"Oophila amblystomatis, commonly known as chlamydomonad algae or salamander algae, is a species of single-celled alga. The Latin specific name means "loves salamander eggs". It does not occur anywhere in nature other than in the eggs of a few amphibians, such as those of the spotted salamander, Ambystoma maculatum. The alga can invade and grow within an egg's jelly. Once inside, it metabolizes the carbon dioxide produced by the embryo and provides it with oxygen as a result of photosynthesis. "


Older eggs, greening up. Rice Lake, North Vancouver.

A longer answer is that this seems to be true, but it turns out to be somewhat complicated, especially for the eggs farthest from the edges of the jelly blob. These are exposed to adequate or excess levels of oxygen during daylight, while the alga is photosynthesizing, but experience hypoxia (low O2) at night when the algae are using O2.

Here is the paper that applies. A more recent paper giving additional detail and cited by the brief Wikipedia account is here. (Or see Ref list below.)

A lot goes on in a jelly blob. If you’re a developing embryo near the edge, your gas exchange situation may be better, but your chances of being eaten by a Rough-skinned Newt are increased. In the middle, you may not be eaten, but you are subject to wild swings in oxygen levels, which may compromise your development. Hedge your bets, sit in the blue seats. *

*In the hockey shrine of Maple Leaf Gardens, after about 1970, there were five colours of seats, arranged in concentric rings. Starting closest to the ice, the colours went gold, red, blue, green, grey.

Addition: For an eye-popping look at the inside of a salamander egg mass, see this astounding image by David Blevins. Your local pond is a sci-fi universe, with larval aliens in suspended animation.


Refs:

Pinder A.W. and S. C. Friet. 1994. Oxygen transport in egg masses of the amphibians Rana sylvatica and Ambystoma maculatum: convection, diffusion and oxygen production by algae. The Journal of Experimental Biology 197, 17–30.


Valls, J. H. and N. E. Mills. 2007. Intermittent hypoxia in eggs of Ambystoma maculatum: embryonic developmentand egg capsule conductance. The Journal of Experimental Biology 210, 2430-2435.

5 comments:

David Blevins said...

Great story, thanks! I should have wondered that too. Especially since last year at this time I was making this photo (http://www.blevinsphoto.com/spottedsalamandereggs.htm) and wondering why the older egg masses were green.

Hugh said...

David, I should have remembered your amazing photo! I added a link to it.

themanicgardener said...

Oh, those rough-skinned newts--they strike terror to the heart of my salamander-eggy soul. But seriously, that's an amazing arrangement for providing oxygen. Nature is endlessly inventive.
--Kate

KaHolly said...

After Kate's clever comment, what is there to say but thanks for sharing such interesting info. I just took for granted that the egg masses turned green because they'd been in the water so long and were covered with algea. But, of course, there is a reason, a connection! A good reminder to me to be more curious. Great photos and text, interspersed with your incredible wit.

beetlesinthebush said...

Very interesting post, and David's picture is astounding.
regards--ted