Here's a mystery photo taken under a microscope today. Do you have a guess about what type of animal it is?
Here are two more clues:
In the photos above, you can see different types of spines, a madreporite (the darker, olive-green central disc), and long tubefeet. These characteristics tell you this is an echinoderm (the group that includes sea stars, sea urchins, sand dollars, sea cucumbers, and crinoids).
The entire animal is shown below. This is a very small live sand dollar; it was only ~8 mm across!
Underneath, sand dollars can look very fuzzy:
The close-up below shows the long spines on the oral surface (underneath). You can see how the spines are attached to small domed tubercles (bumps) and how they're movable — they rotate and assist in burrowing and the manipulation of food particles. Close inspection also reveals small, dark red pigment spots.
I was curious about whether you could see the petaloids, or the petal-like structures visible on sand dollar skeletons (review post from September). So I turned off the room lights and used a light below the sand dollar.
With this experimental lighting, I was excited to see the petaloids radiating out from the central madreporite.
I found this very small sand dollar washed up on Doran Beach. When I discovered it, I wasn't sure if it was alive. I placed it in a large clam shell with some sand and sea water to find out. This is what happened:
Very quickly, it burrowed beneath the sand.
In some ways, I feel like I've lived with sand dollars all of my life. When we were kids we would head out to sand bars (in Massachusetts) to search for their beautiful skeletons. And occasionally I've encountered live individuals. But until today, I'd never looked at a live sand dollar under a microscope, and all I could say was, "Wow!"