We found one of these last week, and I brought it home to add to the aquarium, where it settled in happily, burrowing slowly under the sand.
I fished it out two days later, to see if it had survived the transfer. It had; it was trying to walk out of my hand, on its spine "feet".
|Top side. One edge is broken.|
Under the light, it wasn't really black. And where the edge is regrowing, the spines had a pink tinge to them. When I had a photo, I checked the coloring; the main colour is muted down so it looks brown or black, but when I saturated the photo, the spines were all red.
|Detail, central "flower".|
I looked at the spines under my little microscope. They're beautiful, transparent as glass, but with dotted lines running the length of each spine. There are two types; long, tapered pillars, and between them, short, thin hairs with a knob on top. And all of them, all the time, are swaying in all directions.
They are far too small for my camera, but I found a photo on the web that is more or less what I saw.
|Microphotograph from the University of Queensland, AU. I've lightened it up for better visibility.|
Back in the tank, the spines went to work, sliding the sand dollar over and under the sand until it disappeared from view.
For a flat plate, moving on microscopic fur, the dollar manages to cover a lot of territory. Here's a time-lapse video of one in California, from mrjustin5 on YouTube.
This was about 25 min(ute)s of footage condensed into 1 min 30 s.A mini-Roomba! All it needs is a mini-kitten.
The sand dollar can live up to 10 years. This one is about 3 inches across, half the size of the largest I've seen, so it's probably still young. I hope it does well in the tank, away from its hungriest predators, starfish and gulls. Hermit crabs will eat them, too, so I'll watch for that and remove it if necessary.