Well, we can't--at least, not very well. I suppose we can eat it in small amounts, but sand itself is one of the most nutrient-poor substances imaginable. Sand is, after all, ground up bits of rock. It would provide certain minerals, depending on the type of rock, but none of the essential macronutrients--carbohydrates, proteins, and lipids--that animals need to survive.
When I was a kid I thought that sand dollars were called sand dollars because I'd find their broken tests on sandy beaches. I knew they lived in sand, hence the name. As I started studying marine invertebrates in college I learned that sand dollars don't just live in the sand; they also eat sand. In addition to organic matter, usually in the form of detritus, sand dollars eat sand to create ballast. This makes them heavy and keeps them from being picked up and carried away by waves. It is also why, if you come across an intact sand dollars test and break it open, sand will fall out of it.
I have a batch of recently settled Dendraster excentricus, the common sand dollar in northern California. They began metamorphosing only 30 days post-fertilization. As the larvae settled and transformed into tiny sand dollars, I decided to try to figure out what to feed them. These animals aren't grown commercially and there doesn't seem to be a definitive answer on how to raise them. One of the suggestions I got was "Well, we know they eat sand, so feed them sand."
Which is what I did. The first time I just sprinkled a bit of sand in the dish with the juvenile sand dollars. Then I looked under the microscope to see that the sand grains were about 10 times the size of the animals. Oops. But the sand dollars didn't look unhappy so I let them be. I decided that they also needed something organic to eat so I ground up a small piece of Ulva and dropped some of the resulting slurry on them.
The second time I offered sand to the sand dollars I ground it up in a mortar and pestle that I scrounged from the lab next door. Let me tell you, grinding sand makes a sound that is every bit as horrible as you imagine. At least it produced smaller particles that the sand dollars might be able to eat. I continued to offer Ulva mush in addition to the fine sand. If they end up eating either sand or Ulva, I can provide that pretty easily. The question is, how do I know whether or not they're eating?
How many sand dollars can you find in the above photo? They are exactly the same color as the sand. I don't have real proof that these little guys are eating sand; even their poops would look like the sand. The animals do tend to clear the space in their immediate vicinity, but I think that might be due to the action of the tube feet and spines rather than consumption of either sand or Ulva. In this video clip you will see that the sand dollars are very active, even though all the motion doesn't seem directed the way it does in urchins at this stage.
They do a lot of waving around, but don't actually walk. They do, however, seem to like being tilted up a bit, similar to the way adult sand dollars position themselves when in calm water:
I do have circumstantial evidence that my sand dollars are eating something. The first ones metamorphosed at 30 days post-fertilization. Today is day 51 post-fertilization, which means some of the animals have been post-larvae almost as long as they were larvae. I know it takes about a week for newly metamorphosed sea urchins to form their new guts and begin feeding, and I assume it's the same for sand dollars. In fact, because these sand dollars raced through larval development so quickly I expected their juvenile mouths to break through quickly as well. If this were the case, then these animals should have had complete and functional guts for almost two weeks now. The fact that they're not dead or dying makes me think that they have to be eating.
Call it a hunch, call it intuition, call it wishful thinking. I'm not sure how they're doing it, but I think they're fine. Next week I hope I can find time to measure them.