Biologists - tell me about anhydrobiosis and diapause

This is a sort of spin-off question aroused by my recent thread about ephemeral crustaceans (but sufficiently different an emphasis to warrant a new thread I hope).

As I understand it, anhydrobiosis is a state of suspended animation that many kinds of seeds undergo between ripening and germination; diapause is a similar idea, but happens to certain types of animal eggs (or encysted embryos - although I’m not entirely sure why that wouldn’t be called an egg).

My question is this: during anhydrobiosis and/or diapause, is there any metabolism going on? Is it the case that the organism is still alive in there but just living really economically, or can some organisms ‘wind up’ to a position where metabolism can totally cease, ready to start again when conditions permit?

This is NOT something I’m an expert on, nor do I have conprehensive information.

The term “diapause” covers a pretty wide range of physiological effects, which are different in different organisms. It often refers to a state of developmental suspension, rather than metabolic.

For instance, the nematode (small worm) C. elegans normally goes through four larval stages before becoming an adult. When starved, they can enter diapause in their second larval stage, at which point they become known as “dauers”. Dauer worms do have highly effecient metabolisms, but they aren’t really in suspension - they crawl around much faster and farther than non-dauers, and will rear up from the surface to look for new food sources. If they are fed any time within the next six months or so, they will resume development and become normal adults.

Similarly, a pregnant mouse will put her embryos into diapause if she is nursing a litter. These embryos will stay arrested at the blastocyst stage (about 30 cells) for at least a month (until the litter is weaned), then implant and develop normally when the mother’s body cues them to. It’s unclear how much metabolism is going on in the embryos during this time - the cells aren’t dividing - but probably cell metabolism is pretty normal. Embryos can be removed at this point and grown in culture, which artificially removes the cell division block. These cell are effectively still in diapause in that they do not progress developmentally until cued, but they divide wildly. These are the “embryonic stem cells” that have triggered so much political debate.

These examples are pretty much the opposite end of the spectrum from what you were looking for - diapause with little or no metabolic function. Sadly, I don’t know much about that issue.