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Urechis and sea cucumbers are the only aquatic animals that exhibit that type of respiration, that lung type of respiration rather that a gill surface, because the energetic cost of moving fluid in and out of your respiratory structure is quite high. We believe that there are other aspects of the inflation of the hind gut ... for instance the positioning of the animal in its burrow, though the maintenance of the hydrostatic skeleton of the animal. Again, this animal is a classic model for demonstration of a hydrostatic skeleton. So it quite interesting in that avenue as well.

The reason my laboratory group is interested in Urechis, and that we turned to studying Urechis about ten years ago after I'd spent a great deal of time working with hydrothermal vent organisms, was that obviously it is much easier to collect and work with. It is also exposed to some of the same environmental challenges that hydrothermal vent and seep animals are. That is principally, the exposure to toxic hydrogen sulfide gas.

In the marine mud flat environment, when the tide goes out, burrow water becomes stagnant and there is a lot of bacterial activity in the mud flat. It is very organically enriched, and the bacteria present in the mud are sulfide producing bacteria. So, they metabolize and if you walk out on the mud flat at low tide, it stinks like hydrogen sulfide gas, and that's why. But these animals, by nature of their habitat, this burrow habitat, are stuck in this pool of water where hydrogen sulfide levels rise and oxygen levels drop and it creates quite an interesting microhabitat.

We have looked at a variety of mechanisms for sulfide tolerance and detoxification in these animals and I'm just going to summarize these very quickly for you. First of all, if we look at the internal space of the animal, the coelomic pool and the contents there.....if you cut open an animal here along the dorsal surface, coelomic fluid spills out into the pan. The coelomic fluid is rich in heme compounds. Also here you can see, this is the digestive tract and this is that thin walled hind gut I mentioned. These are gonads. Often you will open up an animal, spill the coelomic fluid and see that it is very rich in what looks like red hemoglobin in the coelomic fluid. And then sometimes you will open up an animal and find this very dark color to their internal body fluids. The reason it's dark is that the heme compound in this case is a slightly altered molecule. It is what we term hematin. It is similar to hemoglobin, although it is not associated with a protein component.. There is no protein, no globin. And the difference. and this makes the difference in the color, is that the iron is in the ferric state. It has another charge on it. It is not functional for oxygen binding, but it is very functional for sulfide detoxification. It is a catalyst, a very dramatic catalyst of sulfide oxidation.

So what we believe is happening in the internal space, is that as sulfide permeates through the body wall and hind gut and enters into the coelomic compartment, the hemoglobin or hematin rapidly oxidizes it to non-toxic forms of hydrogen sulfide, thiosulfate is the principle end-product of sulfide detoxification, in this case.

We have also looked at the body wall and hind gut epithelial surfaces. If you go up in magnification and look at just a very small section of the body wall at the transmission electron micrograph level, there are a number of interesting components of the body wall that we believe play an interesting role. They work together to allow the animal to tolerate and detoxify sulfide. There are interestingly, some symbiotic bacteria in the cuticle in the body wall of this animal. They are there in very diffuse concentrations, so it is impossible to get them out and culture them, and characterize them. They probably are sulfide oxidizing bacteria, but even if they are, they are playing a fairly small role in the overall metabolism of the animal because they are not very plentiful.

Deeper in the body wall that you can't see right here, there are unusual lysosomes that are present in the epithelial cells. They have been termed sulfide oxidizing bodies, or S.O.Bs. They are very difficult to work with, so the name is perfect. And we believe that these lysosomes are involved once again in the catalysis of sulfide to non-toxic compounds. So, basically they are playing a role in sulfide detoxification by transforming sulfide into a non-toxic compound.

So the overall scheme of tolerance in Urechis is summarized in this slide. Again, because these animals don't have internal symbiants that are utilizing sulfide for their metabolism, the issue here, the interest for us, is how they detoxify sulfide and how they manage to tolerate sulfide in their environment. And, they do it in a number of ways. If we take this intact animal, take a piece of its body wall, coelom and hind-gut and blow it up over here, we have the thick muscle of the body wall, the outside water bathing the outside of the animal, the coelomic fluid here, the thin walled hind gut, and then the water that is inside the hind gut of the animal.

Hydrogen sulfide is present on both sides of the animal. It is certainly present in the burrow water that is bathing the outside of the animal. It is also present in the water that is in the hind gut because there is free exchange going on with the burrow water here. So, sulfide is basically surrounding all tissues in this animal. It easily permeates through. This tissue is very permeable to hydrogen sulfide. We have shown that. In the case of the body wall, we think what is happening is that hydrogen sulfide comes rapidly into the body wall, it is oxidized with the help of specialized mitochondria and these SOB organelles and the hydrogen sulfide is chemically transformed to a non-toxic form of hydrogen sulfide, thiosulfate.

Similarly, hydrogen sulfide entering across the hind gut into the coelomic fluid or even through the body wall into the coelomic fluid rapidly comes in contact with the hematin and hemoglobin. Again is catalyzed with the help of oxygen to a non-toxic compounds such as thiosulfate. Thiosulfate is easily permeated out of the body, excreted from the body.

Conclusion

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